-
Identification of low redshift groups and clusters of galaxies in the X-CLASS survey and the X-ray luminosity-temperature relation
Authors:
Q. Moysan,
F. Sarron,
N. Clerc,
G. Soucail,
C. Adami,
B. Altieri,
R. Cabanac,
M. Chira,
J. Comparat,
D. Coia,
E. Drigga,
E. Gaynullina,
A. Khalikova,
E. Koulouridis,
K. Migkas,
M. Molham,
L. Paquereau,
T. Sadibekova,
I. Valtchanov
Abstract:
Properties of the hot intracluster and intragroup medium are mostly set by the underlying gravitational potential well, although complex astrophysical processes at play during their buildup may leave a significant imprint. Observational constraints on the degree and scales of such non-gravitational processes require well-selected samples of objects and deep observations of their gas content. We ai…
▽ More
Properties of the hot intracluster and intragroup medium are mostly set by the underlying gravitational potential well, although complex astrophysical processes at play during their buildup may leave a significant imprint. Observational constraints on the degree and scales of such non-gravitational processes require well-selected samples of objects and deep observations of their gas content. We aim to study the scaling relation between two global properties of the hot gas, namely its soft-band X-ray luminosity ($L_X$) and its temperature ($T$), by studying a sample of low-mass systems associated with precise redshifts, simultaneously accounting for sample selection biases and associated measurement uncertainties. This work takes as input a large catalogue of X-ray-selected galaxy clusters (X-CLASS). We perform a thorough revision of the redshifts of sources using deep photometric data from the Legacy Surveys and our own tailored spectroscopic follow-up of 52 low-redshift systems. We devise a spectroscopically complete sample of 155 low-redshift ($0.07<z<0.2$) systems, and we measure properties of their X-ray emitting gas, with median $\overline{T}=1.7$ keV and median $\overline{L_X}=10^{43}$ erg s$^{-1}$. We infer the relation between $L_X$ and $T$ in a Bayesian framework. Our sample of groups and clusters with median total mass $\sim 6 \times 10^{13}M_\odot$ reveals a relation $L_X-T$ steeper than predicted by the self-similar model, with a slope $B=3.2 \pm 0.1$. This result fits well within recent studies that together indicate a trend of increasing slope with decreasing median halo mass. This work supports a scenario of a stronger decrease in luminosity with decreasing mass in the group regime than for massive galaxy clusters. This effect is possibly due to strong and sustained feedback expelling gas efficiently from their relatively shallower potential wells.
△ Less
Submitted 24 March, 2026;
originally announced March 2026.
-
CIV wind properties of the SDSS-V X-ray selected quasars: strong optical-to-UV emission is key regardless of X-ray strength
Authors:
Amy L. Rankine,
David Homan,
James Aird,
Pranavi Hiremath,
Scott F. Anderson,
Roberto J. Assef,
Franz E. Bauer,
W. N. Brandt,
Marcella Brusa,
Johannes Buchner,
Maria Chira,
Yaherlyn Díaz,
Patrick B. Hall,
Anton M. Koekemoer,
Mirko Krumpe,
Georg Lamer,
Teng Liu,
Sean Morrison,
Blessing Musiimenta,
C. A. Negrete,
Qingling Ni,
Paola Rodríguez Hidalgo,
Mara Salvato,
Donald P. Schneider,
Yue Shen
, et al. (3 additional authors not shown)
Abstract:
We present an investigation of the rest-frame optical/UV and X-ray properties for a sample of 3027 X-ray selected quasars between $1.5 \leq z \leq 3.5$ detected in the deepest Spectrum Roentgen Gamma/eROSITA data available and observed by the fifth iteration of the Sloan Digital Sky Survey (SDSS-V). We parametrize the CIV$\lambda1549$ emission line to infer the strength of accretion disc winds and…
▽ More
We present an investigation of the rest-frame optical/UV and X-ray properties for a sample of 3027 X-ray selected quasars between $1.5 \leq z \leq 3.5$ detected in the deepest Spectrum Roentgen Gamma/eROSITA data available and observed by the fifth iteration of the Sloan Digital Sky Survey (SDSS-V). We parametrize the CIV$\lambda1549$ emission line to infer the strength of accretion disc winds and perform X-ray spectral fitting. The X-ray spectral properties -- namely, the 2keV monochromatic luminosity (L$_\text{2keV}$) and spectral slope -- are not strongly correlated with wind strength. Despite this result, the X-ray selected sample is shifted towards lower CIV blueshifts and higher equivalent widths than the optically selected sample observed in previous SDSS surveys, and matching in optical luminosity, redshift, and Eddington ratio does not reduce these differences. We estimate the far-UV luminosity using the HeII$\lambda1640$ line luminosity and define the slopes between this and the 2500A monochromatic luminosity ($L_{2500}$) and L$_\text{2keV}$ ($α_\text{ouv}$ and $α_\text{uvx}$, respectively) in a similar manner to the familiar $α_\text{ox}$ parameter, which tracks the spectral slope between $L_{2500}$ and L$_\text{2keV}$. The quantity $α_\text{ouv}$ is more strongly correlated with wind strength in our sample than $α_\text{ox}$. We show that the correlation between $α_\text{ox}$ and wind strength is driven by the relationship between the optical luminosity and wind strength. Our results are consistent with a radiation line-driven wind, whereby the ionising far-UV photons must not over-ionise the gas. The hard X-ray photons are few enough in number to have a negligible effect on the ionisation state of the material.
△ Less
Submitted 16 March, 2026;
originally announced March 2026.
-
Revisiting the X-ray-to-UV relation of Quasars in the era of all-sky surveys
Authors:
Maria Chira,
Antonis Georgakakis,
Angel Ruiz,
Shi-Jiang Chen,
Johannes Buchner,
Amy L. Rankine,
Elias Kammoun,
Catarina Aydar,
Mara Salvato,
Andrea Merloni,
Mirko Krumpe
Abstract:
The X-ray--to--UV relation of active galactic nuclei (AGNs), commonly parametrized via the monochromatic luminosities at $2500\,\mathring{A}$ and $2\,keV$, reflects the energetic interplay between the accretion disc and the X-ray-emitting corona, and is key for understanding accretion physics. Previous studies suggest that disc-dominated emission becomes more prominent with increasing optical lumi…
▽ More
The X-ray--to--UV relation of active galactic nuclei (AGNs), commonly parametrized via the monochromatic luminosities at $2500\,\mathring{A}$ and $2\,keV$, reflects the energetic interplay between the accretion disc and the X-ray-emitting corona, and is key for understanding accretion physics. Previous studies suggest that disc-dominated emission becomes more prominent with increasing optical luminosity. However, the redshift evolution of this relation remains debated, and a dependence on Eddington ratio, predicted by accretion flow models, is still observationally unconstrained. We revisit this relation using a large, nearly all-sky sample by combining the SDSS DR16Q QSO catalogue with X-ray data from XMM-Newton and the SRG/eROSITA All-Sky Survey DR1, yielding 136,745 QSOs at redshifts $0.5 \leq z < 3.0$. We introduce a hierarchical Bayesian framework that treats X-ray detections and upper limits uniformly, enabling robust inference from both parametric and non-parametric models. We confirm a tight, sublinear $\log L_X({\rm 2\,keV})$-$\log L_ν({\rm 2500\,\mathring{A}})$ correlation, but with a normalization at the lower end of previous estimates. Contrary to most literature results, we detect a mild but systematic redshift evolution: the relation flattens and its intrinsic scatter decreases at higher redshift. This trend is consistent with disc emission increasingly dominated by scattering and enhanced energy transfer to the X-ray corona, potentially indicating redshift evolution in the X-ray bolometric correction. We find no significant dependence on Eddington ratio, in tension with recent accretion flow models.
△ Less
Submitted 10 December, 2025;
originally announced December 2025.
-
Characterising the X-ray variability of QSOs to the highest Eddington ratios and black hole masses with eROSITA light curves
Authors:
Antonis Georgakakis,
Angel Ruiz,
Johannes Buchner,
Iossif Papadakis,
Maria Chira,
Kirpal Nandra,
Shi-Jiang Chen,
Maurizio Paolillo,
Qingling Ni,
Mara Salvato,
Thomas Boller,
Andrea Merloni
Abstract:
An important diagnostic of the inner structure of accretion flows onto supermassive black holes are the stochastic flux variations at X-ray wavelengths. Despite its significance, a systematic characterisation of the statistical properties of the X-ray variability to the highest Eddington ratios and most massive black holes is still lacking. In this paper we address this issue using SRG/eROSITA 5-e…
▽ More
An important diagnostic of the inner structure of accretion flows onto supermassive black holes are the stochastic flux variations at X-ray wavelengths. Despite its significance, a systematic characterisation of the statistical properties of the X-ray variability to the highest Eddington ratios and most massive black holes is still lacking. In this paper we address this issue using SRG/eROSITA 5-epoch light curves to characterise the mean X-ray variability of optically selected SDSS QSOs extending to black holes masses of $10^{10}$ solar and accretion rates close to the Eddington limit. The adopted variability statistic is the ensemble normalised excess variance, which is measured using a novel hierarchical Bayesian model (eBExVar) tailored to the Poisson nature of the X-ray light curves. We find a clear anti-correlation of the ensemble variability with black hole mass, extending previous results to time scales of months. This can be interpreted as evidence for an X-ray corona size and/or physical conditions that scale with black holes mass. We also find an unexpected increase of the ensemble normalised excess variance close to the Eddington limit, which is contrary to the predictions of empirical variability models. This result suggests an additional variability component for fast growing black holes that may be related to systematic variations of the hot corona size with Eddington ratio or shielding of the hot corona by an inner puffed-up disk and/or outflows.
△ Less
Submitted 9 December, 2025;
originally announced December 2025.
-
The X-CLASS survey: A catalogue of 1646 X-ray-selected galaxy clusters up to z$\sim$1.5
Authors:
E. Koulouridis,
N. Clerc,
T. Sadibekova,
M. Chira,
E. Drigga,
L. Faccioli,
J. P. Le Fèvre,
C. Garrel,
E. Gaynullina,
A. Gkini,
M. Kosiba,
F. Pacaud,
M. Pierre,
J. Ridl,
K. Tazhenova,
C. Adami,
B. Altieri,
J. -C. Baguley,
R. Cabanac,
E. Cucchetti,
A. Khalikova,
M. Lieu,
J. -B. Melin,
M. Molham,
M. E. Ramos-Ceja
, et al. (3 additional authors not shown)
Abstract:
Cosmological probes based on galaxy clusters rely on cluster number counts and large-scale structure information. X-ray cluster surveys are well suited for this purpose, since they are far less affected than optical surveys by projection effects, and cluster properties can be predicted with good accuracy. The XMM Cluster Archive Super Survey, X-CLASS, is a serendipitous search of X-ray-detected ga…
▽ More
Cosmological probes based on galaxy clusters rely on cluster number counts and large-scale structure information. X-ray cluster surveys are well suited for this purpose, since they are far less affected than optical surveys by projection effects, and cluster properties can be predicted with good accuracy. The XMM Cluster Archive Super Survey, X-CLASS, is a serendipitous search of X-ray-detected galaxy clusters in 4176 XMM-Newton archival observations until August 2015. All observations are clipped to exposure times of 10 and 20 ks to obtain uniformity and they span ~269 deg$^2$ across the high-Galactic latitude sky ($|b|> 20^o$). The main goal of the survey is the compilation of a well-selected cluster sample suitable for cosmological analyses. We describe the detection algorithm, the visual inspection, the verification process and the redshift validation of the cluster sample, as well as the cluster selection function computed by simulations. We also present the various metadata that are released with the catalogue, along with the redshifts of 124 clusters obtained with a dedicated multi-object spectroscopic follow-up programme. With this publication we release the new X-CLASS catalogue of 1646 well-selected X-ray-detected clusters over a wide sky area, along with their selection function. The sample spans a wide redshift range, from the local Universe up to z~1.5, with 982 spectroscopically confirmed clusters, and over 70 clusters above z=0.8. Because of its homogeneous selection and thorough verification, the cluster sample can be used for cosmological analyses, but also as a test-bed for the upcoming eROSITA observations and other current and future large-area cluster surveys. It is the first time that such a catalogue is made available to the community via an interactive database which gives access to a wealth of supplementary information, images, and data.
△ Less
Submitted 27 May, 2021; v1 submitted 14 April, 2021;
originally announced April 2021.
-
Host galaxy and orientation differences between AGN different types
Authors:
Anamaria Gkini,
Manolis Plionis,
Maria Chira,
Elias Koulouridis
Abstract:
Aims.The main purpose of this study is to investigate aspects regarding the validity of the active galactic nucleus (AGN) unification paradigm (UP). In particular, we focus on the AGN host galaxies, which according to the UP should show no systematic differences depending on the AGN classification. Methods.For the purpose of this study, we used (a) the spectroscopic Sloan Digital Sky Survey (SDSS)…
▽ More
Aims.The main purpose of this study is to investigate aspects regarding the validity of the active galactic nucleus (AGN) unification paradigm (UP). In particular, we focus on the AGN host galaxies, which according to the UP should show no systematic differences depending on the AGN classification. Methods.For the purpose of this study, we used (a) the spectroscopic Sloan Digital Sky Survey (SDSS) Data Release (DR) 14 catalogue, in order to select and classify AGNs using emission line diagnostics, up to a redshift of z=0.2, and (b) the Galaxy Zoo Project catalogue, which classifies SDSS galaxies in two broad Hubble types: spirals and ellipticals. Results.We find that the fraction of type 1 Seyfert nuclei (Sy1) hosted in elliptical galaxies is significantly larger than the corresponding fraction of any other AGN type, while there is a gradient of increasing spiral-hosts from Sy1 to LINER, type 2 Seyferts (Sy2) and composite nuclei. These findings cannot be interpreted within the simple unified model, but possibly by a co-evolution scheme for supermassive black holes (SMBH) and galactic bulges. Furthermore, for the case of spiral host galaxies we find the Sy1 population to be strongly skewed towards face-on configurations, while the corresponding Sy2 population range in all host galaxy orientation configurations has a similar, but not identical, orientation distribution to star-forming (SF) galaxies. These results also cannot be interpreted by the standard unification paradigm, but point towards a significant contribution of the galactic disc to the obscuration of the nuclear region. This is also consistent with the observed preference of Sy1 nuclei to be hosted by ellipticals, that is, the dusty disc of spiral hosts contributes to the obscuration of the broad-line region (BLR), and thus relatively more ellipticals are expected to appear hosting Sy1 nuclei.
△ Less
Submitted 2 September, 2021; v1 submitted 5 January, 2021;
originally announced January 2021.
-
Dependence of the dynamical properties of light-cone simulation dark matter halos on their environment
Authors:
Maria Chira,
Manolis Plionis,
Shankar Agarwal
Abstract:
Aims: We study the dependence of the dynamical properties of dark matter halos on their environment in a whole-sky $Λ$CDM light-cone simulation extending to $z\sim 0.65$. The properties of interest are halo shape (parametrized by its principal axes), spin and virialisation status, the alignment of halo spin and shape, as well as the shape-shape and spin-spin alignments among halo neighbours. Metho…
▽ More
Aims: We study the dependence of the dynamical properties of dark matter halos on their environment in a whole-sky $Λ$CDM light-cone simulation extending to $z\sim 0.65$. The properties of interest are halo shape (parametrized by its principal axes), spin and virialisation status, the alignment of halo spin and shape, as well as the shape-shape and spin-spin alignments among halo neighbours. Methods: We define the halo environment using the notion of halo isolation status determined by the distance to its nearest neighbor. This defines a maximum spherical region around each halo devoid of other halos, above the catalog threshold mass. We consider as 'close halo pairs', the pairs that are separated by a distance lower than a specific threshold. In order to decontaminate our results from the known dependence of halo dynamical properties on mass, we use a random sampling procedure in order to compare properties of similar halo abundance distributions. Results: (a) We find a strong dependence of halo properties on their environment, confirming that isolated halos are more aspherical and more prolate with lower spin values. (b) Correlations between halo properties exist and are mostly independent of halo environment. (c) Halo spins are aligned with the minor axis, regardless of halo shape. (d) Close halo neighbors have their major axes statistically aligned, while they show a slight but statistically significant preference for anti-parallel spin directions. The latter result is enhanced for the case of close halo pairs in low-density environments. Furthermore, we find a preference of the spin vectors to be oriented perpendicular to the line connecting such close halo pairs.
△ Less
Submitted 25 July, 2021; v1 submitted 10 September, 2020;
originally announced September 2020.
-
A Simulated Annealing algorithm to quantify patterns in astronomical data
Authors:
Maria Chira,
Manolis Plionis
Abstract:
We develop an optimization algorithm, using simulated annealing for the quantification of patterns in astronomical data based on techniques developed for robotic vision applications. The methodology falls in the category of cost minimization algorithms and it is based on user-determined interaction - among the pattern elements - criteria that define the properties of the sought structures. We appl…
▽ More
We develop an optimization algorithm, using simulated annealing for the quantification of patterns in astronomical data based on techniques developed for robotic vision applications. The methodology falls in the category of cost minimization algorithms and it is based on user-determined interaction - among the pattern elements - criteria that define the properties of the sought structures. We applied the algorithm on a large variety of mock images and we constrained the free parameters; α and k, which express the amount of noise in the image and how strictly the algorithm seeks for cocircular structures, respectively. We find that the two parameters are interrelated and also that, independently of the pattern properties, an appropriate selection for most of the images would be log(k) = -2 and 0 < α \lesssim 0.04. The width of the effective α-range, for different values of k, is reduced when more interaction coefficients are taken into account for the definition of the patterns of interest. Finally, we applied the algorithm on N-body simulation dark-matter halo data and on the HST image of the lensing Abell 2218 cluster to conclude that this versatile technique could be applied for the quantification of structure and for identifying coherence in astronomical patterns.
△ Less
Submitted 8 September, 2020; v1 submitted 10 October, 2019;
originally announced October 2019.
-
Dependence on the environment of the abundance function of light-cone simulation dark matter haloes
Authors:
Maria Chira,
Manolis Plionis,
Pier-Stefano Corasaniti
Abstract:
Aims.We study the dependence of the halo abundance function (AF) on different environments in a whole-sky $Λ$CDM light-cone halo catalogue extending to z < 0.65, using a simple and well-defined halo isolation criterion. Methods. The isolation status of each individual dark matter halo is determined by the distance to its nearest neighbour, which defines the maximum spherical region devoid of halos…
▽ More
Aims.We study the dependence of the halo abundance function (AF) on different environments in a whole-sky $Λ$CDM light-cone halo catalogue extending to z < 0.65, using a simple and well-defined halo isolation criterion. Methods. The isolation status of each individual dark matter halo is determined by the distance to its nearest neighbour, which defines the maximum spherical region devoid of halos above a threshold mass around it (although the true size of such region may be much larger since it is not necessarily spherical). A versatile double power-law Schechter function is used to fit the dark matter halo AF, and its derived parameters are studied as a function of halo isolation status. Results. (a) Our function fits the halo abundances for all halo isolation statuses extremely well, while the well-established theoretical mass functions, integrated over the volume of the light-cone, provide an adequate but poorer fit than our phenomenological model. (b) As expected, and in agreement with other studies based on snap-shot simulations, we find significant differences of the halo abundance function as a function of halo isolation, indicating different rates of halo formation. The slope of the power law and the characteristic mass of the Schechter-like fitting function decrease with isolation, a result consistent with the formation of less massive haloes in lower density regions. (c) We find an unexpected upturn of the characteristic mass of the most isolated haloes of our sample. This upturn originates and characterises only the higher redshift regime (z > 0.45), which probably implies a significant and recent evolution of the isolation status of the most isolated and most massive haloes.
△ Less
Submitted 30 May, 2018; v1 submitted 16 May, 2018;
originally announced May 2018.