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Projection Effects in Merger Dating for Illustris TNG Shell Galaxies
Authors:
Tuğba Erol,
Mustafa Kürşad Yıldız,
Ivana Ebrová,
Michal Bílek
Abstract:
Stellar shells are low-surface-brightness structures that typically appear as concentric arcs. They are observed in many giant elliptical and lenticular galaxies, as well as in some spiral and dwarf galaxies. They presumably result from minor and intermediate close-to-radial mergers of galaxies. An essential factor in determining the merger time are the distances of shells from the centre of the g…
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Stellar shells are low-surface-brightness structures that typically appear as concentric arcs. They are observed in many giant elliptical and lenticular galaxies, as well as in some spiral and dwarf galaxies. They presumably result from minor and intermediate close-to-radial mergers of galaxies. An essential factor in determining the merger time are the distances of shells from the centre of the galaxy. Consequently, estimates of the merger time can be significantly affected if some shells remain undetected due to projection effects. In this study, we present the first systematic investigation of how measured shell radii depend on the orientation of a galaxy relative to the observer. Using the Illustris TNG50 simulation, we examine shell galaxies from nine selected lines of sight and measure the shell radii. We model shell evolution and calculate the merger time accordingly for each viewing angle and quantify the impact of orientation on the inferred merger age. Our results indicate that the line of sight can have a substantial effect on merger-time estimates derived from shell radii.
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Submitted 3 August, 2026;
originally announced August 2026.
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A correlation predicting galaxies without dark matter
Authors:
Michal Bílek
Abstract:
The standard theory of galaxy formation predicts that all galaxies should contain dark matter, yet a handful of recently discovered galaxies appear to lack it, challenging our understanding of galaxy formation. We investigate whether such dark-matter deficient objects can be identified from their baryonic properties alone, analogously to the radial-acceleration relation, which tightly links baryon…
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The standard theory of galaxy formation predicts that all galaxies should contain dark matter, yet a handful of recently discovered galaxies appear to lack it, challenging our understanding of galaxy formation. We investigate whether such dark-matter deficient objects can be identified from their baryonic properties alone, analogously to the radial-acceleration relation, which tightly links baryon and dark matter distributions in spiral galaxies. Using a sample of ultra-diffuse and dwarf spheroidal galaxies -- systems whose baryonic properties resemble those of the confirmed dark-matter-deficient galaxies -- we systematically search for a formula to predict baryonic fractions from stellar mass, effective radius, distance to the host, and the host's baryonic mass. We find that baryonic fraction correlates most strongly with the gravitational acceleration expected from baryons alone, $a_\mathrm{bar}$, or equivalently, with mean surface brightness, following an approximately $a_\mathrm{bar}^{-1}$ dependence. This scaling resembles the radial-acceleration relation but differs in functional form and applies to a different galaxy population. Strikingly, the dark-matter-deficient galaxies occupy the extreme end of the correlation. This suggests that they result from standard formation processes operating at unusual intensities rather than from exotic mechanisms. Importantly, the correlation predicts that all ultra-diffuse galaxies brighter than approximately 25 mag arcsec$^{-2}$ in the $g$-band should have very low dark matter content, offering a straightforward observational criterion for identifying these rare objects.
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Submitted 11 May, 2026;
originally announced May 2026.
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Deviations from the radial acceleration relation in the central galaxies of clusters, subclusters, and groups
Authors:
Michal Bílek,
Florent Renaud,
Srdjan Samurović
Abstract:
Most galaxies closely follow the radial acceleration relation (RAR), which tightly links the observed accelerations to those predicted by Newtonian gravity from visible baryonic matter. Galaxy clusters, however, deviate from this relation. Several explanations have been proposed. Some of them predict that even some galaxies in clusters should deviate, but this hypothesis remains largely untested.…
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Most galaxies closely follow the radial acceleration relation (RAR), which tightly links the observed accelerations to those predicted by Newtonian gravity from visible baryonic matter. Galaxy clusters, however, deviate from this relation. Several explanations have been proposed. Some of them predict that even some galaxies in clusters should deviate, but this hypothesis remains largely untested. We test it here by analyzing acceleration profiles for 17 early-type galaxies, derived from Jeans modeling of their globular cluster systems in our older work. Our sample spans central galaxies in clusters and groups, non-central galaxies, isolated ones, and-uniquelly for this paper-centrals in galactic subclusters, which are smaller clusters being accreted by larger ones. We compare these profiles to the standard RAR for non-cluster galaxies and its counterpart for clusters. We find that isolated and non-central galaxies adhere to the standard RAR. In contrast, central galaxies of clusters, subclusters, and groups exhibit enhanced accelerations in most cases, tracing instead the cluster acceleration behavior either partly or fully. The radius at which divergence from the standard RAR begins tends to decrease with increasing group mass. These findings imply that if cluster fields depart from the standard RAR due to undetected material, it must be dynamically cold and collisionless, such as non-baryonic cold dark matter, but also compact clouds of cold gas.
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Submitted 24 March, 2026;
originally announced March 2026.
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Discovery of a galaxy associated with the HI cloud FAST J0139+4328
Authors:
Ana Mitrašinović,
Marko Grozdanović,
Ana Lalović,
Milena Jovanović,
Michal Bilek,
Nataša Pavlov,
Alexei V. Moiseev,
Dmitry V. Oparin
Abstract:
The search for ``dark galaxies,'' a key prediction of the lambda cold dark matter, has yielded few viable candidates. Recently, FAST J0139+4328 was reported as the first isolated dark galaxy in the nearby universe, based on a neutral hydrogen (HI) detection and a non-detection in the Pan-STARRS1 survey. To verify the nature of this candidate, we obtained deep optical imaging, using the…
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The search for ``dark galaxies,'' a key prediction of the lambda cold dark matter, has yielded few viable candidates. Recently, FAST J0139+4328 was reported as the first isolated dark galaxy in the nearby universe, based on a neutral hydrogen (HI) detection and a non-detection in the Pan-STARRS1 survey. To verify the nature of this candidate, we obtained deep optical imaging, using the $1.4\,\mathrm{m}$ \textit{Milanković} and $0.6\,\mathrm{m}$ \textit{Nedeljković} telescopes, and spectroscopic follow-up of the field. We report the unambiguous discovery of a low-surface-brightness (LSB) optical counterpart at the location of the HI cloud. Furthermore, the detection of H$α$ emission via the $6\,\mathrm{m}$ Big Telescope Alt-Azimuthal (BTA) confirms that the stellar system lies at a redshift consistent with the HI source, establishing their physical association. Through detailed photometry and employing color-dependent mass-to-light scaling relations, we derive a total stellar mass of $M_\star = (7.2 \pm 3.7) \times 10^6\, M_{\odot}$, about an order of magnitude higher than the previously estimated upper limit. Using the literature HI mass, this implies a gas-to-stellar mass ratio of $M_{\mathrm{HI}} / M_{\star} = 11.5 \pm 6.4$. Our findings demonstrate that FAST J0139+4328 is not a dark galaxy but an extremely gas-rich LSB dwarf galaxy, whose stellar component was simply below the detection limit of the Pan-STARRS1 survey. This reclassification resolves the status of this prominent dark galaxy candidate and underscores the necessity of deep optical follow-up to classify faint HI-selected systems.
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Submitted 31 December, 2025;
originally announced December 2025.
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Photometric stellar masses for galaxies in DESI Legacy Imaging Surveys
Authors:
Ivana Ebrová,
Michal Bílek,
Jiří Eliášek
Abstract:
In many areas of extragalactic astrophysics, we need to convert the luminosity of a galaxy into its stellar mass. In this work, we aim to find a simple and effective formula to estimate the stellar mass from the images of galaxies delivered by the currently popular DESI Legacy Imaging Surveys. This survey provides an unsurpassed combination of a deep imaging with an extensive sky coverage in up to…
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In many areas of extragalactic astrophysics, we need to convert the luminosity of a galaxy into its stellar mass. In this work, we aim to find a simple and effective formula to estimate the stellar mass from the images of galaxies delivered by the currently popular DESI Legacy Imaging Surveys. This survey provides an unsurpassed combination of a deep imaging with an extensive sky coverage in up to four photometric bands. We calibrated the sought formula against a sample of local galaxies observed by the Spitzer Survey of Stellar Structure in Galaxies (S$^4$G) that was directly dedicated to measure the stellar masses. For the absolute magnitudes $M_g$ and $M_r$ of a galaxy in the Legacy Surveys $g$ and $r$ bands, we find that the stellar masses can be estimated as $0.673M_g - 1.108M_r + 0.996$ with the scatter of 25\%. Employing more complex functions does not improve the estimate appreciably, even after including the galaxy ellipticity, Sérsic index, or the magnitudes in different Legacy Surveys bands. Generally, measurements in $r$ band were the most helpful ones, while adding $z$-band measurements did not improve the mass estimate much. We provide a Python-based script \texttt{photomass\_ls.py} to automatically download images of any galaxy from the Legacy Surveys database, create image masks, generate GALFIT input files with well-assessed initial values, perform the GALFIT photometry, and calculate the stellar mass estimate. Additionally, we tuned another version of the formula to the magnitudes provided by the Siena Galaxy Atlas 2020 (SGA-2020) with a scatter of 29\%. For both\,--\,our default and SGA-2020 formula, we offer two alternatives derived from different calibrations of S$^4$G masses that were based on different methods and assumptions.
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Submitted 3 December, 2025; v1 submitted 2 October, 2025;
originally announced October 2025.
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An LBT view of the co-rotating group of galaxies around NGC 2750: Deep imaging and new satellite candidates
Authors:
S. Taibi,
M. S. Pawlowski,
O. Müller,
M. Bílek,
M. P. Júlio,
K. J. Kanehisa,
M. Jovanović,
A. Lalović,
S. Samurović
Abstract:
Some galaxies such as the Milky Way and Andromeda display coherently rotating satellite planes, posing tensions with cosmological simulations. NGC 2750 has emerged as an additional candidate system hosting a co-rotating group of galaxies. We aim to assess the presence of a coherent satellite plane around NGC 2750 by identifying new candidate dwarf galaxies and low surface brightness features. We c…
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Some galaxies such as the Milky Way and Andromeda display coherently rotating satellite planes, posing tensions with cosmological simulations. NGC 2750 has emerged as an additional candidate system hosting a co-rotating group of galaxies. We aim to assess the presence of a coherent satellite plane around NGC 2750 by identifying new candidate dwarf galaxies and low surface brightness features. We conducted deep, wide-field photometric observations of NGC 2750 using the Large Binocular Telescope in the g- and r-bands. Standard data reduction techniques were applied to enhance the detection of low-surface-brightness features down to about 31 mag/arcsec^2 in r. Our observations led to the discovery of six new candidate dwarf galaxies, including one with properties consistent with an ultra-diffuse galaxy. We also identified tidal features around NGC 2750, indicating past interactions with nearby satellites. The spatial distribution of satellites suggests a moderate flattening, further supported by the newly identified candidates. Follow-up spectroscopic measurements will be critical in confirming or challenging the strong kinematic coherence observed previously. The luminosity function of NGC 2750 reveals an excess of bright satellites compared to similar systems, adding to the growing tension between observed satellite populations and cosmological simulations.
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Submitted 23 June, 2025;
originally announced June 2025.
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Fornax dwarf spheroidal in MOND: its formation and the survival of its globular clusters
Authors:
Michal Bílek,
Hongsheng Zhao
Abstract:
The Fornax dwarf spheroidal galaxy has five massive globular clusters (GCs). They are often used for testing different dark matter and modified gravity theories, because it is difficult to reconcile their old stellar ages with the short time they need to settle in the center of the galaxy due to dynamical friction. Using high resolution $N$-body simulations with the Phantom of Ramses code, we inve…
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The Fornax dwarf spheroidal galaxy has five massive globular clusters (GCs). They are often used for testing different dark matter and modified gravity theories, because it is difficult to reconcile their old stellar ages with the short time they need to settle in the center of the galaxy due to dynamical friction. Using high resolution $N$-body simulations with the Phantom of Ramses code, we investigate whether the GCs of Fornax can be reconciled with the modified Newtonian dynamics (MOND), namely its QUMOND formulation. Observational data interpreted in MOND indicate that Fornax is a tidal dwarf galaxy formed at redshift $z=0.9$ in a flyby of the Milky Way (MW) and Andromeda galaxies, and that its GCs were initially massive star clusters in the disk of the MW. This helps us to set up and interpret the simulations. In the simulations, a point-mass GC orbits Fornax, and they both orbit the MW. When we ran multiple simulations with varying initial conditions for the GC, we found a 20% probability of Fornax being observed with five unsunk GCs. The unsunk GCs have the observed radial distribution. Moreover, we found: 1) In MOND, Fornax has an orbit around the MW such that the pericenters coincide with the observed peaks in the star formation history of Fornax; 2) The simulations reproduce the observed ``diffuse stellar halo'' of Fornax; 3) The simulations predict that Fornax has a stellar stream, which could be detectable in the existing data. 4) An extra simulation shows that if Fornax was initially a rotating disky tidal dwarf galaxy, the gravitational influence of the MW would be able to transform it into a nonrotating spheroidal. 5) Sometimes Phantom of Ramses does not conserve angular momentum. This makes the GC sink too fast if it is simulated as an $N$-body object.
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Submitted 18 February, 2026; v1 submitted 20 May, 2025;
originally announced May 2025.
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Low Surface Brightness structures from annotated deep CFHT images: effects of the host galaxy's properties and environment
Authors:
Elisabeth Sola,
Pierre-Alain Duc,
Mathias Urbano,
Felix Richards,
Adeline Paiement,
Michal Bílek,
Mustafa K. Yıldız,
Alessandro Boselli,
Patrick Côté,
Jean-Charles Cuillandre,
Laura Ferrarese,
Stephen Gwyn,
Olivier Marchal,
Alan W. McConnachie,
Matthieu Baumann,
Thomas Boch,
Florence Durret,
Matteo Fossati,
Rebecca Habas,
Francine Marleau,
Oliver Müller,
Mélina Poulain,
Vasily Belokurov
Abstract:
Hierarchical galactic evolution models predict that mergers drive galaxy growth, producing low surface brightness (LSB) tidal features that trace galaxies' late assembly. These faint structures encode information about past mergers and are sensitive to the properties and environment of the host galaxy. We investigated the relationships between LSB features and their hosts in a sample of 475 nearby…
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Hierarchical galactic evolution models predict that mergers drive galaxy growth, producing low surface brightness (LSB) tidal features that trace galaxies' late assembly. These faint structures encode information about past mergers and are sensitive to the properties and environment of the host galaxy. We investigated the relationships between LSB features and their hosts in a sample of 475 nearby massive galaxies spanning diverse environments (field, groups, Virgo cluster) using deep optical imaging from the Canada-France-Hawaii Telescope (MATLAS, UNIONS/CFIS, VESTIGE, NGVS). Using Jafar, an online annotation tool, we manually annotated tidal features and extended stellar haloes, including 199 tidal tails and 100 streams. Geometric and photometric measurements were extracted to analyse their dependence on galaxy mass, environment, and internal kinematics. At our surface brightness limit of 29 mag$\,$arcsec$^{-2}$, tidal features and stellar haloes contribute 2% and 10% of total galaxy luminosity, respectively. Tidal features are detected in 36% of galaxies, with none fainter than 27.8 mag$\,$arcsec$^{-2}$. The most massive galaxies are twice as likely to host tidal debris, and for early-type galaxies their halos are twice as luminous as those in lower-mass systems, a trend not observed in late-type galaxies. Although small-scale interactions increase the frequency of tidal features, the large-scale environment does not influence it. An anticorrelation between this frequency and rotational support is found, but may reflect the mass-driven effect. We release our database of annotated features for deep learning applications. Our findings confirm that galaxy mass is the dominant factor influencing tidal feature prevalence, consistent with hierarchical formation models.
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Submitted 8 July, 2025; v1 submitted 24 March, 2025;
originally announced March 2025.
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X-ray observations of Blueberry galaxies
Authors:
B. Adamcová,
J. Svoboda,
E. Kyritsis,
K. Kouroumpatzakis,
A. Zezas,
P. G. Boorman,
A. Borkar,
M. Bílek,
M. Clavel,
P. -O. Petrucci
Abstract:
Compact star-forming galaxies were dominant galaxy types in the early Universe. Blueberry galaxies (BBs) represent their local analogues being very compact and having intensive star formation. Motivated by high X-ray emission recently found in other analogical dwarf galaxies, called Green Peas, we probe into the X-ray properties of BBs to determine if their X-ray emission is consistent with the em…
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Compact star-forming galaxies were dominant galaxy types in the early Universe. Blueberry galaxies (BBs) represent their local analogues being very compact and having intensive star formation. Motivated by high X-ray emission recently found in other analogical dwarf galaxies, called Green Peas, we probe into the X-ray properties of BBs to determine if their X-ray emission is consistent with the empirical laws for star-forming galaxies. We performed the first X-ray observations of a small sample of BBs with the XMM-Newton satellite. Spectral analysis for detected sources and upper limits measured via Bayesian-based analysis for very low-count measurements were used to determine the X-ray properties of our galaxy sample. Clear detection was obtained only for 2 sources, with one source exhibiting an enhanced X-ray luminosity to the scaling relations. For the remaining 5 sources, only an upper limit was constrained, suggesting BBs to be rather underluminous as a whole. Our analysis shows that the large scatter cannot be easily explained by the stochasticity effects. While the bright source is above and inconsistent at almost the 99% confidence level, the upper limits of the two sources are below the expected distribution. These results indicate that the empirical relations between the star formation rate, metallicity, and X-ray luminosity might not hold for BBs with uniquely high specific star formation rates. One possible explanation could be that the BBs may not be old enough to have a significant X-ray binary population. The high luminosity of the only bright source can be then caused by an additional X-ray source, such as a hidden active galactic nucleus or more extreme ultraluminous X-ray sources.
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Submitted 24 August, 2024;
originally announced August 2024.
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Peculiar dark matter halos inferred from gravitational lensing as a manifestation of modified gravity
Authors:
Michal Bílek
Abstract:
If modified gravity holds, but the weak lensing analysis is done in the standard way, one finds that dark matter halos have peculiar shapes, not following the standard Navarro-Frenk-White profiles, and are fully predictable from the distribution of baryons. Here we study in detail the distribution of the apparent dark matter around point masses, which approximate galaxies and galaxy clusters, and…
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If modified gravity holds, but the weak lensing analysis is done in the standard way, one finds that dark matter halos have peculiar shapes, not following the standard Navarro-Frenk-White profiles, and are fully predictable from the distribution of baryons. Here we study in detail the distribution of the apparent dark matter around point masses, which approximate galaxies and galaxy clusters, and their pairs for the QUMOND MOND gravity, taking an external gravitational acceleration $g_e$ into account. At large radii, the apparent halo of a point mass $M$ is shifted against the direction of the external field. When averaged over all lines-of-sight, the halo has a hollow center, and denoting the by $a_0$ the MOND acceleration constant, its density behaves like $ρ(r)=\sqrt{Ma_0/G}/(4πr^2)$ between the galacticentric radii $\sqrt{GM/a_0}$ and $\sqrt{GMa_0}/g_e$, and like $ρ\propto r^{-7}G^2M^3a_0^3/g_e^5$ further away. Between a pair of point masses, there is a region of a negative apparent dark matter density, whose mass can exceed the baryonic mass of the system. The density of the combined dark matter halo is not a sum of the densities of the halos of the individual points. The density has a singularity near the zero-acceleration point, but remains finite in projection. We compute maps of the surface density and the lensing shear for several configurations of the problem, and derive formulas to scale them to further configurations. In general, for a large subset of MOND theories in their weak field regime, for any configuration of the baryonic mass $M$ with the characteristic size of $d$, the total lensing density scales as $ρ({\vec{x}})=\sqrt{Ma_0/G}d^{-2}f\left(\vecα,\vec{x}/d,g_ed/\sqrt{GMa_0}\right)$, where the vector $\vecα$ describes the geometry of the system. Distinguishing between QUMOND and cold dark matter seems possible with the existing instruments.
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Submitted 18 February, 2026; v1 submitted 5 August, 2024;
originally announced August 2024.
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A closer look at the extended edge-on low-surface brightness galaxies
Authors:
Anna S. Saburova,
Damir Gasymov,
Evgenii V. Rubtsov,
Igor V. Chilingarian,
Sviatoslav Borisov,
Ivan Gerasimov,
Fedor Kolganov,
Anastasia V. Kasparova,
Roman I. Uklein,
Michal Bílek,
Kirill A. Grishin,
Anatoly Zasov,
Mariia Demianenko,
Ivan Yu. Katkov,
Ana Lalović,
Srdjan Samurović
Abstract:
To understand the origin of extended disks of low-surface brightness (LSB) galaxies, we studied in detail 4 such systems with large disks seen edge-on. Two of them are edge-on giant LSB galaxies (gLSBGs) recently identified by our team. The edge-on orientation of these systems boosts their surface brightnesses that provided an opportunity to characterize stellar populations spectroscopically and y…
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To understand the origin of extended disks of low-surface brightness (LSB) galaxies, we studied in detail 4 such systems with large disks seen edge-on. Two of them are edge-on giant LSB galaxies (gLSBGs) recently identified by our team. The edge-on orientation of these systems boosts their surface brightnesses that provided an opportunity to characterize stellar populations spectroscopically and yielded the first such measurements for edge-on gLSBGs. We collected deep images of one galaxy using the 1.4-m Milanković Telescope which we combined with the archival Subaru Hyper Suprime-Cam and DESI Legacy Surveys data available for the three other systems, and measured the structural parameters of the disks. We acquired deep long-slit spectra with the Russian 6-meter telescope and the 10-m Keck II telescope and estimated stellar population properties in the high- and low-surface brightness regions as well as the gas-phase metallicity distribution. The gas metallicity gradients are shallow to flat in the range between 0 and -0.03 dex per exponential disk scale length, which is consistent with the extrapolation of the gradient -- scale length relation for smaller disk galaxies. Our estimates of stellar velocity dispersion in the LSB disks as well as the relative thickness of the disks indicate the dynamical overheating. Our observations favor mergers as the essential stage in the formation scenario for massive LSB galaxies.
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Submitted 24 July, 2024;
originally announced July 2024.
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Globular cluster orbital decay in dwarf galaxies with MOND and CDM: Impact of supernova feedback
Authors:
M. Bílek,
F. Combes,
S. T. Nagesh,
M. Hilker
Abstract:
Dynamical friction works very differently for Newtonian gravity with dark matter and in modified Newtonian dynamics (MOND). While the absence of dark matter considerably reduces the friction in major galaxy mergers, analytic calculations indicate the opposite for very small perturbations, such as globular clusters (GCs) sinking in dwarf galaxies. Here, we study the decay of GCs in isolated gas-ric…
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Dynamical friction works very differently for Newtonian gravity with dark matter and in modified Newtonian dynamics (MOND). While the absence of dark matter considerably reduces the friction in major galaxy mergers, analytic calculations indicate the opposite for very small perturbations, such as globular clusters (GCs) sinking in dwarf galaxies. Here, we study the decay of GCs in isolated gas-rich dwarf galaxies using simulations with the Phantom of Ramses code, which enables both the Newtonian and the QUMOND MOND gravity. We modeled the GCs as point masses, and we simulated the full hydrodynamics, with star formation and supernovae feedback. We explored whether the fluctuations in gravitational potential caused by the supernovae can prevent GCs from sinking toward the nucleus. For GCs of typical mass or lighter, we find that this indeed works in both Newtonian and MOND simulations. The GC can even make a random walk. However, we find that supernovae cannot prevent massive GCs ($M\geq 4\times10^5\,M_\odot$) from sinking in MOND. The resulting object looks similar to a galaxy with an offset core, which embeds the sunk GC. The problem is much milder in the Newtonian simulations. This result thus favors Newtonian over QUMOND gravity, but we note that it relies on the correctness of the difficult modeling of baryonic feedback. We propose that the fluctuations in the gravitational potential could be responsible for the thickness of the stellar disks of dwarf galaxies and that strong supernova winds in modified gravity can transform dwarf galaxies into ultra-diffuse galaxies.
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Submitted 15 July, 2024;
originally announced July 2024.
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Simulations of cluster ultra-diffuse galaxies in MOND
Authors:
Srikanth T. Nagesh,
Jonathan Freundlich,
Benoit Famaey,
Michal Bílek,
Graeme Candlish,
Rodrigo Ibata,
Oliver Müller
Abstract:
Ultra-diffuse galaxies (UDGs) in the Coma cluster have velocity dispersion profiles that are in full agreement with the predictions of Modified Newtonian Dynamics (MOND) in isolation. However, the external field effect (EFE) from the cluster seriously deteriorates this agreement. It has been suggested that this could be related to the fact that UDGs are out-of-equilibrium objects whose stars have…
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Ultra-diffuse galaxies (UDGs) in the Coma cluster have velocity dispersion profiles that are in full agreement with the predictions of Modified Newtonian Dynamics (MOND) in isolation. However, the external field effect (EFE) from the cluster seriously deteriorates this agreement. It has been suggested that this could be related to the fact that UDGs are out-of-equilibrium objects whose stars have been heated by the cluster tides or that they recently fell onto the cluster on radial orbits, such that their velocity dispersion may not reflect the EFE at their instantaneous distance from the cluster center. Here, we simulate UDGs within the Coma cluster in MOND, using the Phantom of Ramses (\textsc{por}) code, and show that if UDGs are initially at equilibrium within the cluster, tides are not sufficient to increase their velocity dispersions to values as high as the observed ones. On the other hand, if they are on a first radial infall onto the cluster, they can keep high velocity dispersions without being destroyed until their first pericentric passage. We conclude that, without alterations such as a screening of the EFE in galaxy clusters or much higher baryonic masses than currently estimated, in the MOND context UDGs must be out-of-equilibrium objects on their first infall onto the cluster.
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Submitted 3 July, 2024;
originally announced July 2024.
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Why do different early-type-galaxies have different amounts of rotational support?
Authors:
Michal Bilek,
Pierre-Alain Duc,
Elisabeth Sola
Abstract:
Early-type galaxies (ETGs, i.e. elliptical and lenticular galaxies) differ in their amount of rotational support -- some are purely supported by velocity dispersion, while others show pronounced ordered rotation. Cosmological hydrodynamical simulations show that the progenitors of all ETGs were first rotating quickly, but then mergers decreased their rotational support. In the presented work, we s…
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Early-type galaxies (ETGs, i.e. elliptical and lenticular galaxies) differ in their amount of rotational support -- some are purely supported by velocity dispersion, while others show pronounced ordered rotation. Cosmological hydrodynamical simulations show that the progenitors of all ETGs were first rotating quickly, but then mergers decreased their rotational support. In the presented work, we studied this process using an observational archaeological approach. Namely, we inspected the correlations of 23 merger-sensitive characteristics of local ETGs with a parameter quantifying the rotational support. We used a volume-limited sample of local ETGs, that are not in galaxy clusters, from the MATLAS survey. We found, for example, that slowly rotating galaxies have tidal features and kinematically distinct components more often and have lower metallicities. We sought for mutual interpretation of the correlations among all 23 quantities, together with literature results on high-redshift massive galaxies. There seems to be only one interpretation possible: on average, ETGs lose their rotational support through multiple minor wet mergers happening at the redshifts above about two.
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Submitted 7 May, 2024;
originally announced May 2024.
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Deep imaging with Milanković telescope: Linking merger history to kinematics of elliptical galaxies
Authors:
Ivana Ebrová,
Michal Bílek,
Ana Lalović,
Mustafa K. Yıldız,
Pierre-Alain Duc,
Martin Mašek,
Michael Prouza
Abstract:
Kinematical and morphological features observed in early-type galaxies provide valuable insights into the evolution of their hosts. We studied the origin of prolate rotation (i.e., rotation around the long axis) in Illustris large-scale cosmological hydrodynamical simulations. We found that basically all the simulated massive prolate rotators were created in relatively recent major mergers of gala…
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Kinematical and morphological features observed in early-type galaxies provide valuable insights into the evolution of their hosts. We studied the origin of prolate rotation (i.e., rotation around the long axis) in Illustris large-scale cosmological hydrodynamical simulations. We found that basically all the simulated massive prolate rotators were created in relatively recent major mergers of galaxies. Such mergers are expected to produce tidal features such as tails, shells, asymmetric stellar halos. We investigated deep optical images of prolate rotators, including newly obtained Milanković data, revealing signs of galaxy interaction in all of them. This correlation proves to be statistically very significant when compared with a general sample of early-type galaxies from the MATLAS deep imaging survey. In an ongoing project, we use Milanković to assemble deep images of the complete sample of all known nearby massive prolate rotators. Additionally, we searched these data for asteroids to improve the accuracy of trajectories and even discover one previously unknown main-belt asteroid. The most frequent tidal features among the prolate rotators happen to be shells. We developed methods to calculate the probable time of the merger from optical images. This will allow us to compare the merger history of the sample with predictions from Illustris. Our plan is to expand these methods to even larger samples of shell galaxies supplied by upcoming large surveys like LSST at Rubin Observatory. This will provide an unprecedented amount of statistically significant data on the recent merger history of our Universe and allow extensive investigation of the impact of mergers to a wide range of other astrophysical phenomena.
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Submitted 27 August, 2024; v1 submitted 9 December, 2023;
originally announced December 2023.
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The galactic acceleration scale is imprinted on globular cluster systems of early-type galaxies of most masses and on red and blue globular cluster subpopulations
Authors:
Michal Bílek,
Michael Hilker,
Florent Renaud,
Tom Richtler,
Avinash Chaturvedi,
Srdjan Samurović
Abstract:
Context. Globular clusters (GCs) carry information about the formation histories and gravitational fields of their host galaxies. Bílek et al. (2019, BSR19 hereafter) reported that the radial profiles of the volume number density of GCs in GC systems (GCSs) follow broken power laws, while the breaks occur approximately at the a0 radii. These are the radii at which the gravitational fields of the g…
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Context. Globular clusters (GCs) carry information about the formation histories and gravitational fields of their host galaxies. Bílek et al. (2019, BSR19 hereafter) reported that the radial profiles of the volume number density of GCs in GC systems (GCSs) follow broken power laws, while the breaks occur approximately at the a0 radii. These are the radii at which the gravitational fields of the galaxies equal the galactic acceleration scale $a_0 = 1.2\times 10^{-10}$ms$^{-2}$ known from the radial acceleration relation or the MOND theory of modified dynamics.
Aims. Our main goals here are to explore whether the results of BSR19 hold true for galaxies of a wider mass range and for the red and blue GC subpopulations.
Methods. We exploited catalogs of photometric GC candidates in the Fornax galaxy cluster based on ground and space observations and a new catalog of spectroscopic GCs of NGC 1399, the central galaxy of the cluster. For every galaxy, we obtained the parameters of the broken power-law density by fitting the on-sky distribution of the GC candidates, while allowing for a constant density of contaminants. The logarithmic stellar masses of our galaxy sample span 8.0 - 11.4$M_\odot$.
Results. All investigated GCSs with a sufficient number of members show broken power-law density profiles. This holds true for the total GC population and the blue and red subpopulations. The inner and outer slopes and the break radii agree well for the different GC populations. The break radii agree with the a0 radii typically within a factor of two for all GC color subpopulations. The outer slopes correlate better with the a0 radii than with the galactic stellar masses. The break radii of NGC 1399 vary in azimuth, such that they are greater toward and against the interacting neighbor galaxy NGC 1404.
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Submitted 20 September, 2023; v1 submitted 16 August, 2023;
originally announced August 2023.
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Do old globular clusters in low mass galaxies disprove modified gravity?
Authors:
Michal Bílek,
Hongsheng Zhao,
Benoit Famaey,
Srikanth T. Nagesh,
Françoise Combes,
Oliver Müller,
Michael Hilker,
Pavel Kroupa,
Rodrigo Ibata
Abstract:
The controversy "dark matter vs. modified gravity" constitutes a major topic of discussion. It was proposed that dynamical friction could be used to discriminate between the two alternatives. Analytic calculations indicate that, with modified gravity, globular clusters (GCs) of low-mass galaxies experience much stronger dynamical friction than in the equivalent system with Newtonian gravity and da…
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The controversy "dark matter vs. modified gravity" constitutes a major topic of discussion. It was proposed that dynamical friction could be used to discriminate between the two alternatives. Analytic calculations indicate that, with modified gravity, globular clusters (GCs) of low-mass galaxies experience much stronger dynamical friction than in the equivalent system with Newtonian gravity and dark matter. As a result, in modified gravity the old GCs of low mass galaxies should have already settled in the centers of the galaxies. This is not observed. Here we report on our efforts to verify the analytic results by self-consistent simulations with the MOND-type (modified Newtonian dynamics) gravity. The core stalling mechanism, that was not considered in the analytic calculations, prevents GCs to settle in centers of ultra-diffuse galaxies. For isolated dwarf galaxies, which are gas-rich objects, supernova explosions prevent the GCs from settling.
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Submitted 5 July, 2023;
originally announced July 2023.
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Imprint of the galactic acceleration scale on globular cluster systems: Galaxies in the Fornax Cluster
Authors:
Michal Bílek,
Michael Hilker,
Florent Renaud,
Tom Richtler,
Avinash Chaturvedi,
Srdjan Samurović
Abstract:
Dark matter is required in galaxies at galactocentric radii that are larger than the $a_0$-radius, which is where the gravitational acceleration generated by baryons of the galaxy equals the constant $a_0=1.2\times 10^{-10}$ms$^{-2}$ known as the galactic acceleration scale. It was found previously for massive early-type galaxies that the radial number-density profiles of their globular cluster (G…
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Dark matter is required in galaxies at galactocentric radii that are larger than the $a_0$-radius, which is where the gravitational acceleration generated by baryons of the galaxy equals the constant $a_0=1.2\times 10^{-10}$ms$^{-2}$ known as the galactic acceleration scale. It was found previously for massive early-type galaxies that the radial number-density profiles of their globular cluster (GC) systems follow broken power laws and the breaks occur at the $a_0$-radii. We have newly analyzed the distribution of GCs around galaxies in the Fornax cluster in existing photometric catalogs. We found that 1) the coincidence between $a_0$-radii and the break radii of globular cluster systems is valid for early-type galaxies of all masses and, 2) this also applies to the red and blue sub-populations of GCs separately.
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Submitted 12 April, 2023;
originally announced April 2023.
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Origin of the differences in rotational support among early-type galaxies: The case of galaxies outside clusters
Authors:
Michal Bílek,
Pierre-Alain Duc,
Elisabeth Sola
Abstract:
Context: Early-type galaxies (ETGs) are divided into slow and fast rotators (FRs and SRs) according to the degree of ordered rotation of their stellar populations. Cosmological hydrodynamical simulations indicate that galaxies form as FRs before their rotational support decreases, usually because of mergers.
Aims: We aimed to investigate this process observationally for galaxies outside of clust…
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Context: Early-type galaxies (ETGs) are divided into slow and fast rotators (FRs and SRs) according to the degree of ordered rotation of their stellar populations. Cosmological hydrodynamical simulations indicate that galaxies form as FRs before their rotational support decreases, usually because of mergers.
Aims: We aimed to investigate this process observationally for galaxies outside of clusters.
Methods: We made use of the fact that different merger types leave different traces that have different lifetimes. We statistically analyzed multiple characteristics of galaxies that are expected to be influenced by mergers, such as tidal features, kinematically distinct cores, and stellar ages. They were taken from the MATLAS and ATLAS$^\mathrm{3D}$ databases. Through multilinear regression we identified the quantities that, at a fixed mass and environmental density of the galaxy, significantly correlate with a measure of the ordered rotation of the galaxy, $λ_{R_e}^N$.
Results: We found a negative correlation of the rotational support with the occurrence of tidal disturbances and kinematic substructures, and a positive correlation with metallicity and metallicity gradients. For massive galaxies, the rotational support correlates negatively with the abundance of alpha elements, and for the galaxies in low-density environments, it correlates negatively with the central photometric cuspiness. These and additional literature observational constraints are explained the easiest if the mergers that decreased the rotational support of ETGs were typically minor, wet, and happening at $z\approx 2$. They did not form the currently observed tidal features. The observed frequency of tidal features implies a merging rate of 0.07-0.2 per Gyr. This is insufficient to explain the observed growth of the radii of ETGs with redshift by mergers.
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Submitted 15 November, 2022; v1 submitted 5 October, 2022;
originally announced October 2022.
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Characterization of Low Surface Brightness structures in annotated deep images
Authors:
Elisabeth Sola,
Pierre-Alain Duc,
Felix Richards,
Adeline Paiement,
Mathias Urbano,
Julie Klehammer,
Michal Bílek,
Jean-Charles Cuillandre,
Stephen Gwyn,
Alan McConnachie
Abstract:
The characterization of Low Surface Brightness (LSB) stellar structures around galaxies such as tidal debris of on-going or past collisions is essential to constrain models of galactic evolution. Our goal is to obtain quantitative measurements of LSB structures identified in deep images of samples consisting of hundreds of galaxies. We developed an online annotation tool that enables contributors…
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The characterization of Low Surface Brightness (LSB) stellar structures around galaxies such as tidal debris of on-going or past collisions is essential to constrain models of galactic evolution. Our goal is to obtain quantitative measurements of LSB structures identified in deep images of samples consisting of hundreds of galaxies. We developed an online annotation tool that enables contributors to delineate the shapes of diffuse extended stellar structures, as well as artefacts or foreground structures. All parameters are automatically stored in a database which may be queried to retrieve quantitative measurements. We annotated LSB structures around 352 nearby massive galaxies with deep images obtained with the CFHT as part of two large programs: MATLAS and UNIONS/CFIS. Each LSB structure was delineated and labeled according to its likely nature: stellar shells, streams associated to a disrupted satellite, tails formed in major mergers, ghost reflections or cirrus. From our database containing 8441 annotations, the area, size, median surface brightness and distance to the host of 228 structures were computed. The results confirm the fact that tidal structures defined as streams are thinner than tails, as expected by numerical simulations. In addition, tidal tails appear to exhibit a higher surface brightness than streams (by about 1 mag), which may be related to different survival times for the two types of collisional debris. We did not detect any tidal feature fainter than 27.5 mag.arcsec$^{-2}$, while the nominal surface brightness limits of our surveys range between 28.3 and 29 mag.arcsec$^{-2}$, a difference that needs to be taken into account when estimating the sensitivity of future surveys to identify LSB structures. Our annotation database of observed LSB structures may be used for quantitative analysis and as a training set for machine learning algorithms (abbreviated).
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Submitted 8 March, 2022;
originally announced March 2022.
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Self-repairing high entropy oxides
Authors:
Zongwen Liu,
Pengru Huang,
Lixian Sun,
Yanping Liu,
Jiangtao Qu,
Julie Cairney,
Zhong Zheng,
Zhiming M. Wang,
Naveed A. Khan,
Zhiping Lai,
Li Fu,
Bing Teng,
Cuifeng Zhou,
Hong Zhao,
Fen Xu,
Pan Xiong,
Junwu Zhu,
Peng Yuan,
Kosta Tsoutas,
Behnam Akhavan,
Marcela M. Bilek,
Simon P. Ringer,
Kostya S. Novoselov
Abstract:
All biological organisms, from plants to living creatures, can heal minor wounds and damage. The realization of a similar self-healing capacity in inorganic materials has been a design target for many decades. This would represent a breakthrough in materials engineering, enabling many novel technological applications, since such materials would be able to resist damage caused by electromagnetic ir…
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All biological organisms, from plants to living creatures, can heal minor wounds and damage. The realization of a similar self-healing capacity in inorganic materials has been a design target for many decades. This would represent a breakthrough in materials engineering, enabling many novel technological applications, since such materials would be able to resist damage caused by electromagnetic irradiation and/or mechanical impact. Here we demonstrate that a high-entropy oxide is intrinsically capable of undergoing an autonomous self-repairing process. Transmission electron microscopy revealed that the spinel structure of (AlCoCrCu0.5FeNi)3O4 can regrow and repair itself at the atomic level when damaged. Density functional theory calculations reveal that the extra enthalpy stored in the high entropy material during fabrication can be released to effectively heal macroscopic defects by regrowing into a partially ordered state. This extraordinary self-repairing phenomenon makes this new material highly desirable as a coating, enabling structures used in harsh environments to better withstand damage, such as cosmic irradiation in space, nuclear irradiation in nuclear power facilities, or tribological damage. Most importantly, our results set the general design principles for the synthesis of self-repairing materials.
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Submitted 22 December, 2021;
originally announced December 2021.
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Origin of the spectacular tidal shells of galaxy NGC474
Authors:
Michal Bílek,
Jérémy Fensch,
Ivana Ebrová,
Srikanth T. Nagesh,
Benoit Famaey,
Pierre-Alain Duc,
Pavel Kroupa
Abstract:
The lenticular galaxy NGC474 hosts a rich system of tidal shells and streams, some of which are exceptionally bright. Two teams recently presented spectroscopic observations of the brightest shells. These were the first shell spectra ever observed in integrated starlight. The authors studied the stellar populations of the shell, of the center of the galaxy and of its globular clusters. The precise…
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The lenticular galaxy NGC474 hosts a rich system of tidal shells and streams, some of which are exceptionally bright. Two teams recently presented spectroscopic observations of the brightest shells. These were the first shell spectra ever observed in integrated starlight. The authors studied the stellar populations of the shell, of the center of the galaxy and of its globular clusters. The precise formation scenario for the tidal features of this prominent galaxy however still remained unclear.
Here, we add further clues on their formation from the radii of the shells, and we present a scenario for the formation of the tidal features that seems to be unique and explaining all available data.
Shell radii are analyzed with the shell identification method, and we run self-consistent simulations of the formation of the tidal features. We consider Newtonian as well as MOND gravity.
Observations suggest that the tidal features originate from the accretion of a spiral galaxy. The shell identification method yields that the merging galaxies collided first 1.3Gyr ago and then again 0.9Gyr ago, thereby forming the shells in two generations. This would also explain the young ages of stellar populations in the center of the galaxy and the young age of the globular clusters. The analytic models of shell propagation, that underlie the shell identification method, are verified by a simulation. The simulations reproduce well the observed morphology of the tidal features. The accreted spiral likely reached NGC474 nearly radially, in the plane of the sky, from the south, its rotation axis pointing toward us. It should have had a stellar mass of around 1/6 of NGC474, i.e. $10^{9.8}\,M_\odot$. It seems that all tidal features in the galaxy originate from one merger.
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Submitted 14 January, 2022; v1 submitted 29 November, 2021;
originally announced November 2021.
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HI observations of the MATLAS dwarf and ultra-diffuse galaxies
Authors:
Melina Poulain,
Francine R. Marleau,
Rebecca Habas,
Pierre-Alain Duc,
Ruben Sanchez-Janssen,
Patrick R. Durrell,
Sanjaya Paudel,
Oliver Mueller,
Sungsoon Lim,
Michal Bilek,
Jeremy Fensch
Abstract:
The presence of HI gas in galaxies is inextricably linked to their morphology and evolution. This paper aims to understand the HI content of the already identified 2210 dwarfs located in the low-to-moderate density environments of the MATLAS deep imaging survey. We combine the HI observations from the ATLAS$^{3D}$ survey, with the extragalactic HI sources from the ALFALFA survey, to extract the HI…
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The presence of HI gas in galaxies is inextricably linked to their morphology and evolution. This paper aims to understand the HI content of the already identified 2210 dwarfs located in the low-to-moderate density environments of the MATLAS deep imaging survey. We combine the HI observations from the ATLAS$^{3D}$ survey, with the extragalactic HI sources from the ALFALFA survey, to extract the HI line width, velocity and mass of the MATLAS dwarfs. From the 1773 dwarfs in our sample with available HI observations, 8% (145) have an HI line detection. The majority of the dwarfs show irregular morphology, while 29% (42) are ellipticals, the largest sample of HI-bearing dwarf ellipticals (dEs) to date. Of the HI dwarf sample, 2% (3) are ultra-diffuse galaxies (UDGs), 12% have a transition-type morphology, 5% are tidal dwarf candidates, and 10% appear to be disrupted objects. In our optically selected sample, 9.5% of the dEs, 7% of the UDGs and 10% of the classical dwarfs are HI-bearing. The HI-bearing dwarfs have on average bluer colors than the dwarfs without detected HI. We find relations between the stellar and HI masses, gas fraction, color and absolute magnitude consistent with previous studies of dwarfs probing similar masses and environments. For 79% of the dwarfs identified as satellites of massive early-type galaxies, we find that the HI mass increases with the projected distance to the host. Using the HI line width, we estimate dynamical masses and find that 5% (7) of the dwarfs are dark matter deficient.
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Submitted 29 November, 2021;
originally announced November 2021.
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Are disks of satellites comprised of tidal dwarf galaxies?
Authors:
Michal Bílek,
Ingo Thies,
Pavel Kroupa,
Benoit Famaey
Abstract:
It was found that satellites of nearby galaxies can form flattened co-rotating structures called disks of satellites or planes of satellites. Their existence is not expected by the current galaxy formation simulations in the standard dark-matter-based cosmology. On the contrary, modified gravity offers a promising alternative: the objects in the disks of satellites are tidal dwarf galaxies, that i…
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It was found that satellites of nearby galaxies can form flattened co-rotating structures called disks of satellites or planes of satellites. Their existence is not expected by the current galaxy formation simulations in the standard dark-matter-based cosmology. On the contrary, modified gravity offers a promising alternative: the objects in the disks of satellites are tidal dwarf galaxies, that is small galaxies that form from tidal tails of interacting galaxies. After introducing the topic, we review here our work on simulating the formation of the disks of satellites of the Milky Way and Andromeda galaxies. The initial conditions of the simulation were tuned to reproduce the observed positions, velocities and disk orientations of the galaxies. The simulation showed that the galaxies had a close flyby 6.8Gyr ago. One of the tidal tails produced by the Milky Way was captured by Andromeda. It formed a cloud of particles resembling the disk of satellites at Andromeda by its size, orientation, rotation and mass. A hint of a disk of satellites was formed at the Milky Way too. In addition, the encounter induced a warp in the disk of the simulated Milky Way that resembles the real warp by its magnitude and orientation. We present here, for the first time, the proper motions of the members of the disk of satellites of Andromeda predicted by our simulation. Finally, we point out some of the remaining open questions which this hypothesis for the formation of disks of satellites brings up.
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Submitted 9 November, 2021;
originally announced November 2021.
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Ultra diffuse galaxies in the MATLAS low-to-moderate density fields
Authors:
Francine R. Marleau,
Rebecca Habas,
Melina Poulain,
Pierre-Alain Duc,
Oliver Mueller,
Sungsoon Lim,
Patrick R. Durrell,
Ruben Sanchez-Janssen,
Sanjaya Paudel,
Syeda Lammim Ahad,
Abhishek Chougule,
Michal Bilek,
Jeremy Fensch
Abstract:
Recent advances in deep dedicated imaging surveys over the past decade have uncovered a surprisingly large number of extremely faint low surface brightness galaxies with large physical sizes called ultra diffuse galaxies (UDGs) in clusters and, more recently, in lower density environments. As part of the MATLAS survey, a deep imaging large program at the Canada-France-Hawaii Telescope (CFHT), our…
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Recent advances in deep dedicated imaging surveys over the past decade have uncovered a surprisingly large number of extremely faint low surface brightness galaxies with large physical sizes called ultra diffuse galaxies (UDGs) in clusters and, more recently, in lower density environments. As part of the MATLAS survey, a deep imaging large program at the Canada-France-Hawaii Telescope (CFHT), our team has identified 2210 dwarf galaxies, 59 (~3%) of which qualify as UDGs. Averaging over the survey area, we find ~0.4 UDG per square degree. They are found in a range of low to moderate density environments, although 61% of the sample fall within the virial radii of groups. Based on a detailed analysis of their photometric and structural properties, we find that the MATLAS UDGs do not show significant differences from the traditional dwarfs, except from the predefined size and surface brightness cut. Their median color is as red as the one measured in galaxy clusters, albeit with a narrower color range. The majority of the UDGs are visually classified as dwarf ellipticals with log stellar masses of ~6.5-8.7. The fraction of nucleated UDGs (~34%) is roughly the same as the nucleated fraction of the traditional dwarfs. Only five (~8%) UDGs show signs of tidal disruption and only two are tidal dwarf galaxy candidates. A study of globular cluster (GC) candidates selected in the CFHT images finds no evidence of a higher GC specific frequency S_N for UDGs than for classical dwarfs, contrary to what is found in most clusters. The UDG halo-to-stellar mass ratio distribution, as estimated from the GC counts, peaks at roughly the same value as for the traditional dwarfs, but spans the smaller range of ~10-2000. We interpret these results to mean that the large majority of the field-to-group UDGs do not have a different formation scenario than traditional dwarfs.
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Submitted 27 September, 2021;
originally announced September 2021.
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Probing the radial acceleration relation and the strong equivalence principle with the Coma cluster ultra-diffuse galaxies
Authors:
J. Freundlich,
B. Famaey,
P. -A. Oria,
M. Bílek,
O. Müller,
R. Ibata
Abstract:
The tight radial acceleration relation (RAR) obeyed by rotationally supported disk galaxies is one of the most successful a priori predictions of the modified Newtonian dynamics (MOND) paradigm on galaxy scales. Another important consequence of MOND as a classical modification of gravity is that the strong equivalence principle (SEP) -- which requires the dynamics of a small, free-falling, self-gr…
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The tight radial acceleration relation (RAR) obeyed by rotationally supported disk galaxies is one of the most successful a priori predictions of the modified Newtonian dynamics (MOND) paradigm on galaxy scales. Another important consequence of MOND as a classical modification of gravity is that the strong equivalence principle (SEP) -- which requires the dynamics of a small, free-falling, self-gravitating system not to depend on the external gravitational field in which it is embedded -- should be broken. Multiple tentative detections of this so-called external field effect (EFE) of MOND have been made in the past, but the systems that should be most sensitive to it are galaxies with low internal gravitational accelerations residing in galaxy clusters within a strong external field. Here, we show that ultra-diffuse galaxies (UDGs) in the Coma cluster do lie on the RAR, and that their velocity dispersion profiles are in full agreement with isolated MOND predictions, especially when including some degree of radial anisotropy. However, including a breaking of the SEP via the EFE seriously deteriorates this agreement. We discuss various possibilities to explain this within the context of MOND, including a combination of tidal heating and higher baryonic masses. We also speculate that our results could mean that the EFE is screened in cluster UDGs. The fact that this would happen precisely within galaxy clusters, where classical MOND fails, could be especially relevant to the nature of the residual MOND missing mass in clusters of galaxies.
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Submitted 22 November, 2021; v1 submitted 9 September, 2021;
originally announced September 2021.
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Structure and morphology of the MATLAS dwarf galaxies and their central nuclei
Authors:
Melina Poulain,
Francine R. Marleau,
Rebecca Habas,
Pierre-Alain Duc,
Ruben Sanchez-Janssen,
Patrick R. Durrell,
Sanjaya Paudel,
Syeda Lammim Ahad,
Abhishek Chougule,
Oliver Mueller,
Sungsoon Lim,
Michal Bilek,
Jeremy Fensch
Abstract:
We present a photometric study of the dwarf galaxy population in the low to moderate density environments of the MATLAS (Mass Assembly of early-Type gaLAxies with their fine Structures) deep imaging survey. The sample consists of 2210 dwarfs, including 508 nucleated. We define a nucleus as a compact source that is close to the galaxy photocentre (within 0.5 $R_e$) which is also the brightest such…
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We present a photometric study of the dwarf galaxy population in the low to moderate density environments of the MATLAS (Mass Assembly of early-Type gaLAxies with their fine Structures) deep imaging survey. The sample consists of 2210 dwarfs, including 508 nucleated. We define a nucleus as a compact source that is close to the galaxy photocentre (within 0.5 $R_e$) which is also the brightest such source within the galaxy's effective radius. The morphological analysis is performed using a 2D surface brightness profile modelling on the g-band images of both the galaxies and nuclei. Our study reveals that, for similar luminosities, the MATLAS dwarfs show ranges in the distribution of structural properties comparable to cluster (Virgo and Fornax) dwarfs and a range of sizes comparable to the Local Group and Local Volume dwarfs. Colour measurements using the r- and i-band images indicate that the dwarfs in low and moderate density environments are as red as cluster dwarfs on average. The observed similarities between dwarf ellipticals in vastly different environments imply that dEs are not uniquely the product of morphological transformation due to ram-pressure stripping and galaxy harassment in high density environments. We measure that the dwarf nuclei are located predominantly in massive, bright and round dwarfs and observe fewer nuclei in dwarfs with a faint centre and a small size. The colour of the galaxy nucleus shows no clear relation to the colour of the dwarf, in agreement with the migration and wet migration nucleus formation scenarios. The catalogues of the MATLAS dwarfs photometric and structural properties are provided.
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Submitted 16 August, 2021; v1 submitted 20 July, 2021;
originally announced July 2021.
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Evolution of globular-cluster systems of ultra-diffuse galaxies due to dynamical friction in MOND gravity
Authors:
Michal Bílek,
Hongsheng Zhao,
Benoit Famaey,
Oliver Müller,
Pavel Kroupa,
Rodrigo Ibata
Abstract:
(Abridged) Dynamical friction can be used to distinguish Newtonian gravity and modified Newtonian dynamics (MOND) because it works differently in these frameworks. This concept, however, has yet to be explored very much with MOND. Previous simulations showed weaker dynamical friction during major mergers for MOND than for Newtonian gravity with dark matter. Analytic arguments suggest the opposite…
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(Abridged) Dynamical friction can be used to distinguish Newtonian gravity and modified Newtonian dynamics (MOND) because it works differently in these frameworks. This concept, however, has yet to be explored very much with MOND. Previous simulations showed weaker dynamical friction during major mergers for MOND than for Newtonian gravity with dark matter. Analytic arguments suggest the opposite for minor mergers. In this work, we verify the analytic predictions for MOND by high-resolution $N$-body simulations of globular clusters (GCs) moving in isolated ultra-diffuse galaxies (UDGs). We test the MOND analog of the Chandrasekhar formula for the dynamical friction proposed by Sánchez-Salcedo on a single GC. We also explore whether MOND allows GC systems of isolated UDGs to survive without sinking into nuclear star clusters. The simulations are run using the adaptive-mesh-refinement code Phantom of Ramses. The mass resolution is $20\,M_\odot$ and the spatial resolution $50\,$pc. The GCs are modeled as point masses. Simulations including a single GC reveal that, as long as the apocenter of the GC is over about 0.5 effective radii, the Sánchez-Salcedo formula works excellently, with an effective Coulomb logarithm increasing with orbital circularity. Once the GC reaches the central kiloparsec, its sinking virtually stops, likely because of the core stalling mechanism. In simulations with multiple GCs, many of them sink toward the center, but the core stalling effect seems to prevent them from forming a nuclear star cluster. The GC system ends up with a lower velocity dispersion than the stars of the galaxy. By scaling the simulations, we extend these results to most UDG parameters, as long as these UDGs are not external-field dominated.
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Submitted 12 July, 2021;
originally announced July 2021.
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Testing the performance of the Milanković telescope
Authors:
A. Vudragović,
M. Bílek,
O. Müller,
S. Samurović,
M. Jovanović
Abstract:
We have undertaken a multi-band imaging campaign of several galaxies to study their low surface brightness features such as shells and streams. Using the 1.4-m Milanković telescope, we measured the surface brightness limits in various bands depending on the exposure time. Remarkably, within three to four hours of observations with the $L$-filter we reached the surface brightness limit of $μ_g$=28.…
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We have undertaken a multi-band imaging campaign of several galaxies to study their low surface brightness features such as shells and streams. Using the 1.4-m Milanković telescope, we measured the surface brightness limits in various bands depending on the exposure time. Remarkably, within three to four hours of observations with the $L$-filter we reached the surface brightness limit of $μ_g$=28.5-29.0 mag/arcsec$^2$. We have confirmed the faint stream of the elliptical galaxy NGC\,474 discovered with MegaCam. The comparison to other deep photometric surveys has revealed that within few hours of observations we can produce competitive results, showing that the Milanković telescope can be a valuable asset in the study of the low-surface brightness Universe.
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Submitted 22 March, 2021;
originally announced March 2021.
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Ubiquitous signs of interactions in early-type galaxies with prolate rotation
Authors:
Ivana Ebrová,
Michal Bílek,
Ana Vudragović,
Mustafa K. Yıldız,
Pierre-Alain Duc
Abstract:
A small fraction of early-type galaxies (ETGs) show prolate rotation, i.e. they rotate around their long photometric axis. In simulations, certain configurations of galaxy mergers are known to produce this type of rotation. We investigate the association of prolate rotation and signs of galaxy interactions among the observed galaxies. We collected a sample of 19 nearby ETGs with distinct prolate r…
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A small fraction of early-type galaxies (ETGs) show prolate rotation, i.e. they rotate around their long photometric axis. In simulations, certain configurations of galaxy mergers are known to produce this type of rotation. We investigate the association of prolate rotation and signs of galaxy interactions among the observed galaxies. We collected a sample of 19 nearby ETGs with distinct prolate rotation from the literature and inspected their ground-based deep optical images for interaction signs - 18 in archival images and one in a new image obtained with the Milanković telescope. Tidal tails, shells, asymmetric/disturbed stellar halos, or on-going interactions are present in all the 19 prolate rotators. Comparing this with the frequency of tidal disturbance among the general sample of ETGs of a roughly similar mass range and surface-brightness limit, we estimate that the chance probability of such an observation is only 0.00087. We also found a significant overabundance of prolate rotators that are hosting multiple stellar shells. The visible tidal features imply a relatively recent galaxy interaction. That agrees with the Illustris large-scale cosmological hydrodynamical simulation, where prolate rotators are predominantly formed in major mergers during the last 6 Gyr. In the appendix, we present the properties of an additional galaxy, NGC 7052, a prolate rotator for which no deep images are available, but for which an HST image revealed the presence of a prominent shell, which had not been reported before.
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Submitted 11 March, 2021; v1 submitted 8 March, 2021;
originally announced March 2021.
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The MATLAS survey of faint outskirts of bright galaxies
Authors:
Michal Bílek,
Pierre-Alain Duc
Abstract:
Deep imaging, that is imaging capable of capturing very low surface brightness extended objects, is a quickly growing field of extragalactic astronomy. Not only can new types of faint objects be discovered, but deep images of bright galaxies are very valuable, too, since they reveal faint signs of past galaxy collisions, the tidal features. Such "archeological" record can be exploited for investig…
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Deep imaging, that is imaging capable of capturing very low surface brightness extended objects, is a quickly growing field of extragalactic astronomy. Not only can new types of faint objects be discovered, but deep images of bright galaxies are very valuable, too, since they reveal faint signs of past galaxy collisions, the tidal features. Such "archeological" record can be exploited for investigating how galaxies formed. In the MATLAS survey, we obtained extremely deep images of 177 nearby massive elliptical and lenticular galaxies using the 3.5m Canada-France-Hawaii Telescope. In our contribution, we will present the various types of objects and features seen in our images, for example tidal features, faint star-forming regions in otherwise quenched galaxies, or faint dust clouds in our own Galaxy. Finally, we will introduce the deep-imaging efforts at the Milanković Telescope.
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Submitted 6 January, 2021; v1 submitted 1 November, 2020;
originally announced November 2020.
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Deep optical imaging of the dark galaxy candidate AGESVC1 282
Authors:
Michal Bílek,
Oliver Müller,
Ana Vudragović,
Rhys Taylor
Abstract:
The blind HI survey Arecibo Galaxy Environment Survey (AGES) detected several unresolved sources in the Virgo cluster, which do not have optical counterparts in the Sloan Digital Sky Survey. The origin of these dark clouds is unknown. They might be crucial objects since they could be the so-called dark galaxies, that is, the dark matter halos without stellar content that are expected from cosmolog…
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The blind HI survey Arecibo Galaxy Environment Survey (AGES) detected several unresolved sources in the Virgo cluster, which do not have optical counterparts in the Sloan Digital Sky Survey. The origin of these dark clouds is unknown. They might be crucial objects since they could be the so-called dark galaxies, that is, the dark matter halos without stellar content that are expected from cosmological simulations. In order to reveal the nature of the dark clouds, we took a deep optical image of one them, AGESVC1 282, with the newly-commissioned 1.4m Milanković Telescope. After observing it for 10.4h in the $L$-filter, the image reached a surface-brightness limit of about 29.1 mag arcsec$^{-2}$ in $V$. No optical counterpart was detected. We placed an upper limit on the $V$-band luminosity of the object of $1.1\times10^7\,L_\odot$, giving a stellar mass below $1.4\times10^7\,M_\odot$ and a HI-to-stellar mass ratio above 3.1. By inspecting archival HI observations of the surrounding region, we found that none of the standard explanations for optically dark HI clouds fits the available constraints on this object.
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Submitted 23 September, 2020;
originally announced September 2020.
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Census and classification of low-surface-brightness structures in nearby early-type galaxies from the MATLAS survey
Authors:
Michal Bílek,
Pierre-Alain Duc,
Jean-Charles Cuillandre,
Stephen Gwyn,
Michele Cappellari,
David V. Bekaert,
Paolo Bonfini,
Theodoros Bitsakis,
Sanjaya Paudel,
Davor Krajnović,
Patrick R. Durrell,
Francine Marleau
Abstract:
The morphology of galaxies gives essential constraints on the models of galaxy evolution. The morphology of the features in the low-surface-brightness regions of galaxies has not been fully explored yet because of observational difficulties. Here we present the results of our visual inspections of very deep images of a large volume-limited sample of 177 nearby massive early-type galaxies (ETGs) fr…
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The morphology of galaxies gives essential constraints on the models of galaxy evolution. The morphology of the features in the low-surface-brightness regions of galaxies has not been fully explored yet because of observational difficulties. Here we present the results of our visual inspections of very deep images of a large volume-limited sample of 177 nearby massive early-type galaxies (ETGs) from the MATLAS survey. The images reach a surface-brightness limit of $28.5-29$ mag arcsec$^{-2}$ in the $g'$ band. Using a dedicated navigation tool and questionnaire, we looked for structures at the outskirts of the galaxies such as tidal shells, streams, tails, disturbed outer isophotes or peripheral star-forming disks, and simultaneously noted the presence of contaminating sources, such as Galactic cirrus. We also inspected internal sub-structures such as bars and dust lanes. We discuss the reliability of this visual classification investigating the variety of answers made by the participants. We present the incidence of these structures and the trends of the incidence with the mass of the host galaxy and the density of its environment. We find an incidence of shells, stream and tails of approximately 15%, about the same for each category. For galaxies with masses over $10^{11}$ M$_\odot$, the incidence of shells and streams increases about 1.7 times. We also note a strong unexpected anticorrelation of the incidence of Galactic cirrus with the environment density of the target galaxy. Correlations with other properties of the galaxies, and comparisons to model predictions, will be presented in future papers.
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Submitted 1 November, 2020; v1 submitted 27 July, 2020;
originally announced July 2020.
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Revealing the formation mechanism of the shell galaxy NGC474 with MUSE
Authors:
Jeremy Fensch,
Pierre-Alain Duc,
Sungsoon Lim,
Éric Emsellem,
Michal Bílek,
Patrick Durrell,
Chengze Liu,
Éric Peng,
Rory Smith
Abstract:
Stellar shells around galaxies could provide precious insights into their assembly history. However, their formation mechanism remains poorly empirically constrained, in particular the type of galaxy collisions at their origin. We present MUSE@VLT data of the most prominent outer shell of NGC 474, to constrain its formation history. The stellar shell spectrum is clearly detected, with a signal-to-…
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Stellar shells around galaxies could provide precious insights into their assembly history. However, their formation mechanism remains poorly empirically constrained, in particular the type of galaxy collisions at their origin. We present MUSE@VLT data of the most prominent outer shell of NGC 474, to constrain its formation history. The stellar shell spectrum is clearly detected, with a signal-to-noise ratio of around 65 pix$^{-1}$. We use a full spectral fitting method to determine the line-of-sight velocity and the age and metallicity of the shell and associated point-like sources within the MUSE field of view. We detect six GC candidates and eight PN candidates which are all kinematically associated to the stellar shell. We show that the shell has an intermediate metallicity, [M/H] = $-0.83^{+0.12}_{-0.12}$ and a possible $α$-enrichment, [$α$/Fe] ~ 0.3. Assuming the material of the shell comes from a lower mass companion, and that the latter had no initial metallicity gradient, such a stellar metallicity would constrain the mass of the progenitor to be around 7.4 x 10^8 M$_\odot$, implying a merger mass ratio of about 1:100. However our census of PNs and earlier photometry of the shell would suggest a much higher ratio, around 1:20. Given the uncertainties, this difference is significant only at the ~1sigma level. We discuss the characteristics of the progenitor, in particular whether the progenitor could also be composed of stars from the low metallicity outskirts from a more massive galaxy. Ultimately, the presented data does not allow us to put a firm constraint on the progenitor mass. We show that at least two globular cluster candidates possibly associated with the shell are quite young, with ages below 1.5~Gyr. We also note the presence of a young (~1Gyr) stellar population in the center of NGC 474. The two may have resulted from the same event.
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Submitted 5 November, 2020; v1 submitted 7 July, 2020;
originally announced July 2020.
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Hunting ghosts: the iconic stellar stream(s) around NG5907 under scrutiny
Authors:
Oliver Müller,
Ana Vudragović,
Michal Bílek
Abstract:
Stellar streams are regarded as crucial objects to test galaxy formation models, with their morphology tracing the underlying potentials and their occurrence tracking the assembly history of the galaxies. The existence of one of the most iconic stellar streams, the double loop around NGC5907, has recently been questioned by new observations with the Dragonfly telescope. This new work only finds pa…
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Stellar streams are regarded as crucial objects to test galaxy formation models, with their morphology tracing the underlying potentials and their occurrence tracking the assembly history of the galaxies. The existence of one of the most iconic stellar streams, the double loop around NGC5907, has recently been questioned by new observations with the Dragonfly telescope. This new work only finds parts of the stream, even though they reach a 1 sigma surface brightness limit of 30.3 mag per sq. arcsec in the g-band. Using 7.2 hours of Luminance L-band imaging with the Milanković 1.4 meter telescope, we have re-observed the putative double loop part to confirm or reject this assessment. We do not find signs of the double loop, but see only a single, knee-shaped stellar stream. Comparing our to the data by the Dragonfly team, we find the same features. Our observations reach a 1 sigma surface brightness limit of 29.7 mag per sq. arcsec in the g-band. These findings emphasize the need for independent confirmation of detections of very low-surface brightness features.
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Submitted 28 November, 2019;
originally announced November 2019.
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Newly discovered dwarf galaxies in the MATLAS low density fields
Authors:
Rebecca Habas,
Francine R. Marleau,
Pierre-Alain Duc,
Patrick R. Durrell,
Sanjaya Paudel,
Mélina Poulain,
Rubén Sánchez-Janssen,
Sreevarsha Sreejith,
Joanna Ramasawmy,
Bryson Stemock,
Christopher Leach,
Jean-Charles Cuillandre,
Stephen Gwyn,
Adriano Agnello,
Michal Bílek,
Jérémy Fensch,
Oliver Müller,
Eric W. Peng,
Remco F. J. van der Burg
Abstract:
We present the photometric properties of 2210 newly identified dwarf galaxy candidates in the MATLAS fields. The Mass Assembly of early Type gaLAxies with their fine Structures (MATLAS) deep imaging survey mapped $\sim$142 deg$^2$ of the sky around nearby isolated early type galaxies using MegaCam on the Canada-France-Hawaii Telescope, reaching surface brightnesses of $\sim$ 28.5 - 29 in the g-ban…
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We present the photometric properties of 2210 newly identified dwarf galaxy candidates in the MATLAS fields. The Mass Assembly of early Type gaLAxies with their fine Structures (MATLAS) deep imaging survey mapped $\sim$142 deg$^2$ of the sky around nearby isolated early type galaxies using MegaCam on the Canada-France-Hawaii Telescope, reaching surface brightnesses of $\sim$ 28.5 - 29 in the g-band. The dwarf candidates were identified through a direct visual inspection of the images and by visually cleaning a sample selected using a partially automated approach, and were morphologically classified at the time of identification. Approximately 75% of our candidates are dEs, indicating that a large number of early type dwarfs also populate low density environments, and 23.2% are nucleated. Distances were determined for 13.5% of our sample using pre-existing $z_{spec}$ measurements and HI detections. We confirm the dwarf nature for 99% of this sub-sample based on a magnitude cut $M_g$ = -18. Additionally, most of these ($\sim$90%) have relative velocities suggesting that they form a satellite population around nearby massive galaxies rather than an isolated field sample. Assuming that the candidates over the whole survey are satellites of the nearby galaxies, we demonstrate that the MATLAS dwarfs follow the same scaling relations as dwarfs in the Local Group as well as the Virgo and Fornax clusters. We also find that the nucleated fraction increases with $M_g$, and find evidence of a morphology-density relation for dwarfs around isolated massive galaxies.
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Submitted 17 December, 2019; v1 submitted 29 October, 2019;
originally announced October 2019.
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Utilizing Shell Galaxies
Authors:
Ivana Ebrová,
Michal Bílek,
Bruno Jungwiert
Abstract:
Stellar shells are low surface brightness features in the form of open, concentric arcs, formed in close-to-radial collisions of galaxies. They occur in at least 10% of early-type galaxies and a small portion of spirals and their unique kinematics carry valuable information about the host galaxies. We discuss a method using measurements of the number and distribution of shells to estimate the mass…
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Stellar shells are low surface brightness features in the form of open, concentric arcs, formed in close-to-radial collisions of galaxies. They occur in at least 10% of early-type galaxies and a small portion of spirals and their unique kinematics carry valuable information about the host galaxies. We discuss a method using measurements of the number and distribution of shells to estimate the mass distribution of the galaxies and the time since the merger. The method is applied on the shells of NGC 4993 - a galaxy hosting the electromagnetic counterpart of the gravitational wave event GW170817, to estimate the probable time since the galactic merger. We used analytical calculations and particle simulations to show that, in special cases, when kinematic data are available, further constraints on mass distribution and merger time can be derived. Applying the methods to the rapidly growing sample of known shell galaxies will constrain the dark-matter content in the galaxies and reveal detailed information on the recent merger history of the Universe.
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Submitted 16 September, 2019;
originally announced September 2019.
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Origin Of Tidal Structures In Modified Gravity
Authors:
Michal Bílek,
Ingo Thies,
Pavel Kroupa,
Benoit Famaey
Abstract:
The missing mass problem has not been solved decisively yet. Observations show that if gravity is to be modified, then the MOND theory is its excellent approximation on galactic scales. MOND suggests an adjustments of the laws of physics in the limit of low accelerations. Comparative simulations of interacting galaxies in MOND and Newtonian gravity with dark matter revealed two principal differenc…
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The missing mass problem has not been solved decisively yet. Observations show that if gravity is to be modified, then the MOND theory is its excellent approximation on galactic scales. MOND suggests an adjustments of the laws of physics in the limit of low accelerations. Comparative simulations of interacting galaxies in MOND and Newtonian gravity with dark matter revealed two principal differences: 1) galaxies can have close flybys without ending in mergers in MOND because of weaker dynamical friction, and 2) tidal dwarf galaxies form very easily in MOND. When this is combined with the fact that many interacting galaxies are observed at high redshift, we obtain a new perspective on tidal features: they are often formed by non-merging encounters and tidal disruptions of tidal dwarf galaxies. Here we present the results from our self-consistent MOND $N$-body simulation of a close flyby of two galaxies similar to the Milky Way. It turns out that most types of the structures that are traditionally assigned to galaxy mergers can be formed by non-merging encounters, including tidal arms, bridges, streams, shells, disk warps, thick disks, and most probably also disks of satellites. The success of MOND in explaining the dynamics of galaxies hints us that this way of formation of tidal structures should be considered seriously.
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Submitted 20 August, 2019;
originally announced August 2019.
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Imprint of the galactic acceleration scale on globular cluster systems
Authors:
Michal Bílek,
Srdjan Samurović,
Florent Renaud
Abstract:
We report that the density profiles of globular cluster (GC) systems in a sample of 17 early-type galaxies (ETGs) show breaks at the radii where the gravitational acceleration exerted by the stars equals the galactic acceleration scale $a_0$ known from the radial acceleration relation or MOND. The match with the other characteristic radii in the galaxy is not that close. We propose possible explan…
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We report that the density profiles of globular cluster (GC) systems in a sample of 17 early-type galaxies (ETGs) show breaks at the radii where the gravitational acceleration exerted by the stars equals the galactic acceleration scale $a_0$ known from the radial acceleration relation or MOND. The match with the other characteristic radii in the galaxy is not that close. We propose possible explanations in the frameworks of the $Λ$CDM model and MOND. We find tentative evidence that in the $Λ$CDM context, GCs reveal not only the masses of the dark halos through the richness of the GC systems but also the concentrations through the break radii of the GC systems.
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Submitted 13 August, 2019;
originally announced August 2019.
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Discussing the first velocity dispersion profile of an ultra-diffuse galaxy in MOND
Authors:
Michal Bílek,
Oliver Müller,
Benoit Famaey
Abstract:
Using Jeans modelling, we calculate the velocity dispersion profile of the ultra-diffuse galaxy (UDG) Dragonfly 44 in MOND. For the nominal mass-to-light ratio from the literature and an isotropic profile, the agreement with the data is excellent near the center of the galaxy. However, in modified gravity, close to the cluster core, the gravitational environment should bring the galaxy back toward…
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Using Jeans modelling, we calculate the velocity dispersion profile of the ultra-diffuse galaxy (UDG) Dragonfly 44 in MOND. For the nominal mass-to-light ratio from the literature and an isotropic profile, the agreement with the data is excellent near the center of the galaxy. However, in modified gravity, close to the cluster core, the gravitational environment should bring the galaxy back towards the Newtonian behavior. This success of the isolated MOND prediction for the central velocity dispersion could then mean that the galaxy is at large distances ($\gg5$ Mpc) from the cluster core, as hinted by the fact that close-by UDGs share similar velocities with a dispersion well below that of the cluster itself. There is however a two-sigma tension in the outer part of the UDG due to an increase of the observed dispersion profile with respect to the flat MOND prediction. This deviation could simply be a measurement error. Other possibilities could be, for a UDG far from the cluster, a higher-than-nominal baryonic mass with a tangentially anisotropic dispersion profile, or even a sign of a dark baryonic halo. If the UDG is closer to the cluster core, the deviation could be a sign that it is in the process of disruption.
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Submitted 4 June, 2019;
originally announced June 2019.
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A tidal tale: detection of multiple stellar streams in the environment of NGC1052
Authors:
Oliver Müller,
R. Michael Rich,
Javier Román,
Mustafa K. Yıldız,
Michal Bílek,
Pierre-Alain Duc,
Jérémy Fensch,
Ignacio Trujillo,
Andreas Koch
Abstract:
The possible existence of two dark matter free galaxies (NGC1052-DF2 and NGC1052-DF4) in the field of the early-type galaxy NGC1052 presents a challenge to theories of dwarf galaxy formation according to the current cosmological paradigm. We carried out a search for signatures of past interactions connected to the putative hosts of NGC1052-DF2 and NGC1052-DF4 using a very deep L-band image obtaine…
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The possible existence of two dark matter free galaxies (NGC1052-DF2 and NGC1052-DF4) in the field of the early-type galaxy NGC1052 presents a challenge to theories of dwarf galaxy formation according to the current cosmological paradigm. We carried out a search for signatures of past interactions connected to the putative hosts of NGC1052-DF2 and NGC1052-DF4 using a very deep L-band image obtained with the 0.7m Jeanne Rich telescope that reach a surface brightness limit of 28.5 mag arcsec-2 in the r band. We found several low-surface brightness features, possibly consistent with an ongoing merger history in this group. We find a tidal interaction between NGC1052 and NGC1047, confirming a physical association. Furthermore we find a stellar loop around NGC1052 in the direction of NGC1042 and a stellar stream pointing in the direction of NGC1052-DF2 - but both are not directly connected. We find no evidence for a recent tidal interaction for NGC1052-DF2 and NGC1052-DF4. No low surface brightness features have been uncovered around the spiral galaxy NGC1042, which leaves the association (physical or projected) between NGC1052 and NGC1042 ambiguous, though they have similar radial velocities. Their association will only be established once accurate distances to both objects have been measured.
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Submitted 26 March, 2019; v1 submitted 18 March, 2019;
originally announced March 2019.
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Study of gravitational fields and globular cluster systems of early-type galaxies
Authors:
Michal Bílek,
Srdjan Samurović,
Florent Renaud
Abstract:
(abridged)
Context. Gravitational fields at the outskirts of early-type galaxies (ETGs) are difficult to constrain observationally. It thus remains poorly explored how well the LCDM and MOND hypotheses agree with ETGs.
Aims. This led us to gather a large sample of ETGs and examine homogeneously which dark matter halos they occupy, whether the halos follow the theoretically predicted stellar-to…
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(abridged)
Context. Gravitational fields at the outskirts of early-type galaxies (ETGs) are difficult to constrain observationally. It thus remains poorly explored how well the LCDM and MOND hypotheses agree with ETGs.
Aims. This led us to gather a large sample of ETGs and examine homogeneously which dark matter halos they occupy, whether the halos follow the theoretically predicted stellar-to-halo mass relation (SHMR) and the halo mass-concentration relation (HMCR), whether ETGs obey MOND and the radial acceleration relation (RAR) observed for late-type galaxies (LTGs), and finally whether LCDM or MOND perform better in ETGs.
Methods. We employed Jeans analysis of radial velocities of globular clusters (GCs). We analysed nearly all ETGs having more than about 100 archival GC radial velocity measurements. The GC systems of our 17 ETGs extend mostly over ten effective radii. A LCDM simulation of GC formation helped us to interpret the results.
Results. Successful LCDM fits are found for all galaxies, but compared to the theoretical HMCR and SHMR, the best-fit halos usually have concentrations that are too low and stellar masses that are too high for their masses. This might be because of tidal stripping of the halos or because ETGs and LTGs occupy different halos. Most galaxies can be fitted by the MOND models successfully as well, but for some of the galaxies, especially those in centers of galaxy clusters, the observed GC velocity dispersions are too high. This might be a manifestation of the additional dark matter that MOND requires in galaxy clusters. Additionally, we find many signs that the GC systems were perturbed by galaxy interactions. Formal statistical criteria prefer the best-fit LCDM models over the MOND models, but this might be due to the higher flexibility of the LCDM models. The MOND approach can predict the GC velocity dispersion profiles better.
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Submitted 13 March, 2019;
originally announced March 2019.
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NGC 4993, the shell galaxy host of GW170817: constraints on the recent galactic merger
Authors:
Ivana Ebrová,
Michal Bílek,
Mustafa K. Yıldız,
Jiří Eliášek
Abstract:
NGC 4993 is the shell galaxy host of the GRB170817A short gamma-ray burst and the GW170817 gravitational-wave event produced during a binary-neutron-star coalescence. The galaxy shows signs, including the stellar shells, that it has recently accreted a smaller, late-type galaxy. The accreted galaxy might be the original host of the binary neutron star. We measured the positions of the stellar shel…
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NGC 4993 is the shell galaxy host of the GRB170817A short gamma-ray burst and the GW170817 gravitational-wave event produced during a binary-neutron-star coalescence. The galaxy shows signs, including the stellar shells, that it has recently accreted a smaller, late-type galaxy. The accreted galaxy might be the original host of the binary neutron star. We measured the positions of the stellar shells of NGC 4993 in an HST/ACS archival image and use the shell positions to constrain the time of the galactic merger. According to the analytical model of the evolution of the shell structure in the expected gravitational potential of NGC 4993, the galactic merger happened at least 200 Myr ago, with a probable time roughly around 400 Myr and the estimates higher than 600 Myr being improbable. This constitutes the lower limit on the age of the binary neutron star, because the host galaxy was probably quenched even before the galactic merger, and the merger has likely shut down the star formation in the accreted galaxy. We roughly estimate the probability that the binary neutron star originates in the accreted galaxy to be around 30%.
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Submitted 8 December, 2024; v1 submitted 4 January, 2018;
originally announced January 2018.
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MOND simulation suggests the origin of some peculiarities in the Local Group
Authors:
Michal Bílek,
Ingo Thies,
Pavel Kroupa,
Benoit Famaey
Abstract:
(abridged) The Milky Way (MW) and Andromeda (M31) galaxies possess rotating planes of satellites. Their formation has not been explained satisfactorily yet. It was suggested that the MW and M31 satellites are ancient tidal dwarf galaxies, which could explain their configuration. This suggestion gained support by an analytic backward calculation of the relative MW-M31 orbit in the MOND modified dyn…
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(abridged) The Milky Way (MW) and Andromeda (M31) galaxies possess rotating planes of satellites. Their formation has not been explained satisfactorily yet. It was suggested that the MW and M31 satellites are ancient tidal dwarf galaxies, which could explain their configuration. This suggestion gained support by an analytic backward calculation of the relative MW-M31 orbit in the MOND modified dynamics paradigm by Zhao et al. (2013) implying their close flyby 7-11 Gyr ago. Here we explore the Local Group history in MOND in more detail using a simplified first-ever self-consistent simulation. We note the features induced by the encounter in the simulation and identify their possible real counterparts. The simulation was set to approximately reproduce the observed MW and M31 masses, effective radii, separation, relative velocity and disk inclinations. We used the publicly available adaptive-mesh-refinement code Phantom of RAMSES. In the simulation, matter was transferred from the MW to M31 along a tidal tail. The encounter induced formation of several structures resembling the peculiarities of the Local Group. Most notably: 1) A rotating planar structure formed around M31 from the transferred material. It had a size similar to the observed satellite plane and was oriented edge-on to the simulated MW, just as the real one. 2) The same structure also resembled the tidal features observed around M31 by its size and morphology. 3) A warp in the MW developed with an amplitude and orientation similar to that observed. 4) A cloud of particles formed around the simulated MW, with the extent of the actual MW satellite system. The encounter did not end by merging in a Hubble time. The simulation thus demonstrated that MOND can possibly explain many peculiarities of the Local Group and, moreover, that non-merging galaxy encounters in MOND can produce tidal features in galaxies.
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Submitted 6 February, 2018; v1 submitted 13 December, 2017;
originally announced December 2017.
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Investigating the radial acceleration relation in early-type galaxies using the Jeans analysis
Authors:
Michal Bílek,
Srdjan Samurović
Abstract:
Investigating the gravitational field in the early-type galaxies (ETGs, i.e. ellipticals and lenticulars) up to large radii is observationally difficult. It is questionable how the radial acceleration (RAR) in the ETGs looks like, i.e. the relation between the dynamically inferred gravitational acceleration and the acceleration expected from the distribution of the visible matter. This relation is…
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Investigating the gravitational field in the early-type galaxies (ETGs, i.e. ellipticals and lenticulars) up to large radii is observationally difficult. It is questionable how the radial acceleration (RAR) in the ETGs looks like, i.e. the relation between the dynamically inferred gravitational acceleration and the acceleration expected from the distribution of the visible matter. This relation is nearly universal for the spiral galaxies, in agreement with the MOND modified dynamics paradigm. In this contribution, we investigate a sample of 15 ETGs. We extract their full kinematic profiles out to several effective radii from their globular cluster systems and estimate their gravitational field using the Jeans equation. We parametrize the gravitational field by that produced by the stars and a Navarro-Frenk-White DM halo. We find that only 4-5 of our ETGs follow the RAR for the spiral galaxies. All these galaxies are fast rotators, have disky isophotes, appear mostly very elongated and the have bluest colors in our sample. This suggests that they might be spiral galaxies which lost their gas. Our galaxies deviating from the RAR for the spirals either disprove MOND, contain unobserved matter, or indicate a flaw in the method.
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Submitted 23 November, 2017; v1 submitted 16 November, 2017;
originally announced November 2017.
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CzeV - The Czech Variable Star Catalogue
Authors:
Marek Skarka,
Martin Mašek,
Luboš Brát,
Pavel Cagaš,
Jakub Juryšek,
Kateřina Hoňková,
Miloslav Zejda,
Ladislav Šmelcer,
Martin Jelínek,
František Lomoz,
Martin Tylšar,
Jaroslav Trnka,
Ondřej Pejcha,
Pavel Pintr,
Martin Lehký,
Jan Janík,
Ladislav Červinka,
Václav Přibík,
David Motl,
Filip Walter,
Petr Zasche,
Karel Koss,
Petr Hájek,
František Bílek,
Jiří Liška
, et al. (40 additional authors not shown)
Abstract:
We present the first release of the Czech Variable star catalogue that currently contains 1228 stars whose variability was discovered by 60 Czech observers. The catalogue contains confirmed variable stars of various types, but also candidates. We give precise coordinates, cross identification with other catalogues, information about constellation, variability type, brightness, light elements, name…
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We present the first release of the Czech Variable star catalogue that currently contains 1228 stars whose variability was discovered by 60 Czech observers. The catalogue contains confirmed variable stars of various types, but also candidates. We give precise coordinates, cross identification with other catalogues, information about constellation, variability type, brightness, light elements, name of the discoverer and year of discovery. In eighty-eight percent of stars the variability type is estimated, for more than 60 % of the stars the light ephemerides are given.
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Submitted 26 September, 2017;
originally announced September 2017.
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Galaxy interactions: dark matter vs. Modified Newtonian dynamics (MOND)
Authors:
Michal Bílek
Abstract:
(doctoral thesis of Michal Bilek, finished on June 19, 2015)
MOND is an observational rule for predicting the acceleration of stars and galaxies from the distribution of the visible matter. It possibly stems from a new law of physics. I list the theoretical aspects of MOND, its achievements and problems. MOND has been tested mainly in disc galaxies so far. Its tests in elliptical galaxies are ra…
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(doctoral thesis of Michal Bilek, finished on June 19, 2015)
MOND is an observational rule for predicting the acceleration of stars and galaxies from the distribution of the visible matter. It possibly stems from a new law of physics. I list the theoretical aspects of MOND, its achievements and problems. MOND has been tested mainly in disc galaxies so far. Its tests in elliptical galaxies are rare because the MOND effects are small for them in the parts observable by the conventional methods. In the thesis, I explain the methods and ideas I developed for testing MOND in the ellipticals using stellar shells. Moreover, the shells enable us to test MOND for stars in radial orbits for the first time. The shells are results of galactic interactions. I discuss the shell formation mechanisms and summarize the findings from shell observations and simulations.
The thesis contains as yet unpublished results mainly in: 1) the introduction of Sect. 3 (the expected differences in the shell morphology in the Newtonian dynamics with dark matter and in MOND), and 2) Sect. 2.4.1 (formulas for modeling the evolution of shell radii).
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Submitted 6 January, 2016;
originally announced January 2016.
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Deep imaging of the shell elliptical galaxy NGC3923 with MegaCam
Authors:
Michal Bílek,
Jean-Charles Cuillandre,
Stephen Gwyn,
Ivana Ebrová,
Kateřina Bartošková,
Bruno Jungwiert,
Lucie Jílková
Abstract:
Context. The elliptical galaxy NGC 3923 is known to be surrounded by a number of stellar shells, probable remnants of an accreted galaxy. Despite its uniqueness, the deepest images of its outskirts come from the 1980s. On the basis of the modified Newtonian dynamics (MOND), it has recently been predicted that a new shell lies in this region.
Aims. We obtain the deepest image ever of the galaxy,…
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Context. The elliptical galaxy NGC 3923 is known to be surrounded by a number of stellar shells, probable remnants of an accreted galaxy. Despite its uniqueness, the deepest images of its outskirts come from the 1980s. On the basis of the modified Newtonian dynamics (MOND), it has recently been predicted that a new shell lies in this region.
Aims. We obtain the deepest image ever of the galaxy, map the tidal features in it, and search for the predicted shell.
Methods. The image of the galaxy was taken by the MegaCam camera at the Canada-France-Hawaii Telescope in the g' band. It reached the surface-brightness limit of 29 mag/arcsec2. In addition, we reanalyzed an archival HST image of the galaxy.
Results. We detected up to 42 shells in NGC 3923. This is by far the highest number among all shell galaxies. We present the description of the shells and other tidal features in the galaxy. A probable progenitor of some of these features was discovered. The shell system likely originates from two or more progenitors. The predicted shell was not detected, but the new image revealed that the prediction was based on incorrect assumptions and poor data.
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Submitted 6 January, 2016; v1 submitted 26 May, 2015;
originally announced May 2015.
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MOND implications for spectral line profiles of shell galaxies: shell formation history and mass-velocity scaling relations
Authors:
Michal Bílek,
Bruno Jungwiert,
Ivana Ebrová,
Kateřina Bartošková
Abstract:
Context. Many ellipticals are surrounded by round stellar shells probably stemming from minor mergers. A new method for constraining gravitational potential in elliptical galaxies has recently been suggested. It uses the spectral line profiles of these shells to measure the circular velocity at the edge of the shell and the expansion velocity of the shell itself. MOND is an alternative to the dark…
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Context. Many ellipticals are surrounded by round stellar shells probably stemming from minor mergers. A new method for constraining gravitational potential in elliptical galaxies has recently been suggested. It uses the spectral line profiles of these shells to measure the circular velocity at the edge of the shell and the expansion velocity of the shell itself. MOND is an alternative to the dark matter framework aiming to solve the missing mass problem.
Aims. We study how the circular and expansion velocities behave in MOND for large shells.
Methods. The asymptotic behavior for infinitely large shells is derived analytically. The applicability of the asymptotic results for finitely sized shells is studied numerically on a grid of galaxies modeled with Sérsic spheres.
Results. Circular velocity settles asymptotically at a value determined by the baryonic mass of the galaxy forming the baryonic Tully-Fisher relation known for disk galaxies. Shell expansion velocity also becomes asymptotically constant. The expansion velocities of large shells form a multibranched analogy to the baryonic Tully-Fisher relation, together with the galactic baryonic masses. For many - but not all - shell galaxies, the asymptotic values of these two types of velocities are reached under the effective radius. If MOND is assumed to work in ellipticals, then the shell spectra allow many details of the history to be revealed about the formation of the shell system, including its age. The results pertaining to circular velocities apply to all elliptical galaxies, not only those with shells.
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Submitted 14 January, 2015; v1 submitted 19 December, 2014;
originally announced December 2014.
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Shell galaxies as laboratories for testing MOND
Authors:
Michal Bílek,
Ivana Ebrová,
Bruno Jungwiert,
Lucie Jílková,
Kateřina Bartošková
Abstract:
Tests of MOND in ellipticals are relatively rare because these galaxies often lack kinematic tracers in the regions where the MOND effects are significant. Stellar shells observed in many elliptical galaxies offer a promising way to constrain their gravitational field. Shells appear as glowing arcs around their host galaxy. They are observed up to ~100 kpc. The stars in axially symmetric shell sys…
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Tests of MOND in ellipticals are relatively rare because these galaxies often lack kinematic tracers in the regions where the MOND effects are significant. Stellar shells observed in many elliptical galaxies offer a promising way to constrain their gravitational field. Shells appear as glowing arcs around their host galaxy. They are observed up to ~100 kpc. The stars in axially symmetric shell systems move in nearly radial orbits. The radial distributions of shell locations and the spectra of stars in shells can be used to constrain the gravitational potential of their host galaxy. The symmetrical shell systems, being especially suitable for these studies, occur in approximately 3% of all early-type galaxies. Hence the shells substantially increase the number of ellipticals in which MOND can be tested up to large radii. In this paper, we review our work on shell galaxies in MOND. We summarize the paper Bílek et al. (2013), where we demonstrated the consistency of shell radii in an elliptical NGC 3923 with MOND, and the work Bílek et al. (2014), in which we predicted a giant (~200 kpc), as yet undiscovered shell of NGC 3923. We explain the shell identification method, which was used in these two papers. We further describe the expected shape of line profiles in shell spectra in MOND which is very special due to the direct relation of the gravitational field and baryonic matter distribution (Bílek et al., 2014, in preparation).
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Submitted 11 July, 2014;
originally announced July 2014.