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Variations in the 3.3 $μ$m Polycyclic Aromatic Hydrocarbon Feature Across Nearby Galaxies Driven by Metallicity and Radiation Field Spectrum
Authors:
Hannah B. Koziol,
Karin Sandstrom,
Dalya Baron,
Adam K. Leroy,
Jérémy Chastenet,
Ryan Chown,
Daniel A. Dale,
Oleg V. Egorov,
Lindsey Hands,
Mansi Padave,
Debosmita Pathak,
Erik Rosolowsky,
Jessica Sutter,
Tony D. Weinbeck,
Thomas G. Williams,
Alberto Bolatto,
Médéric Boquien,
Yixian Cao,
Enrico Congiu,
Simon C. O. Glover,
Hwihyun Kim,
Ralf S. Klessen,
Kirsten L. Larson,
Justus Neumann,
Elias K. Oakes
, et al. (7 additional authors not shown)
Abstract:
We use JWST NIRCam imaging to investigate the 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) feature in nearby galaxies. NIRCam observations of the 3.3 $μ$m feature are emerging as a powerful tool for studying the structure of the interstellar medium (ISM) and the conditions of the dust at ~0".1 resolution. These maps require accurate subtraction of the underlying continuum emission. We present an…
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We use JWST NIRCam imaging to investigate the 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) feature in nearby galaxies. NIRCam observations of the 3.3 $μ$m feature are emerging as a powerful tool for studying the structure of the interstellar medium (ISM) and the conditions of the dust at ~0".1 resolution. These maps require accurate subtraction of the underlying continuum emission. We present an empirical method to isolate the PAH-correlated emission in the F335M filter using the F300M and F360M filters for continuum subtraction. We find that the slope of the F335M/F300M versus F360M/F300M colors for PAH-correlated emission shows a dependence on local ISM properties, with the strongest dependence on specific star formation rate. Weaker emission features captured by these bands appear suppressed relative to the main 3.3 $μ$m feature in regions of active star formation. We find trends in the 3.3/7.7 and 3.3/11.3 $μ$m ratios that suggest changes in PAH size, charge, and heating by a varying radiation field spectrum. We find decreases in both band ratios with increasing metallicity, which we attribute to a shift to smaller PAH populations at low metallicity. Comparison to optical ionized gas line ratios and dust models show that variations in the interstellar radiation field spectrum influence the PAH feature ratios. This analysis supports inhibited growth formation scenarios for the observed PAH band ratio trends with metallicity and emphasizes the importance of considering the local radiation field characteristics and gas-phase metallicity when using these band ratios as PAH property diagnostics.
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Submitted 5 August, 2026;
originally announced August 2026.
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Mid-Infrared Colors Vary with Galactic Environment: Contrasting Star-Forming Disks, Young Centers, and Quiescent Star-Formation Deserts
Authors:
Debosmita Pathak,
Karin M. Sandstrom,
Adam K. Leroy,
Ryan Chown,
Daniel A. Dale,
Hannah B. Koziol,
Mansi Padave,
Jessica Sutter,
Thomas G. Williams,
Dalya Baron,
Alberto D. Bolatto,
Mederic Boquien,
Oleg V. Egorov,
Eric Emsellem,
Simon C. O. Glover,
Ralf S. Klessen,
Eva Schinnerer,
Todd A. Thompson,
Francesco Belfiore,
Frank Bigiel,
Enrico Congiu,
Ivan Gerasimov,
Jay Gonzalez Lobos,
Annie Hughes,
Laura A. Lopez
, et al. (5 additional authors not shown)
Abstract:
We present $50{-}100\,$pc-resolution JWST/MIRI and NIRCam measurements of mid-infrared (mid-IR) color variations in the diffuse interstellar medium (ISM) of 71 nearby star-forming galaxies from the PHANGS-JWST survey. Mid-IR emission traces the dust column density, intensity ($U$) and hardness of the interstellar radiation field, and the physical state (charge, size) and abundance of polycyclic ar…
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We present $50{-}100\,$pc-resolution JWST/MIRI and NIRCam measurements of mid-infrared (mid-IR) color variations in the diffuse interstellar medium (ISM) of 71 nearby star-forming galaxies from the PHANGS-JWST survey. Mid-IR emission traces the dust column density, intensity ($U$) and hardness of the interstellar radiation field, and the physical state (charge, size) and abundance of polycyclic aromatic hydrocarbons (PAHs). Mid-IR colors that trace PAH band-ratios remain fairly constant in the diffuse ISM of star-forming disks. However, they show stark variations in extreme environments: highly star-forming central molecular zones (CMZs) and star-formation deserts/quiescent bulges. In CMZs, PAH-to-continuum ($3.3/21$, $7.7/21$, and $11.3/21\,μ$m) and the $10/21\,μ$m continuum colors are $0.2{-}0.4$ dex lower than in normal disks. We attribute this to higher $U$ based on the far-IR dust colors and the high $21\,μ{\rm m}/Σ_{\rm Mol}$, which we suggest to be a good tracer of $U$ outside star-forming regions. Meanwhile, star-formation deserts show low $7.7\,μ$m PAH emission, resulting in low $7.7/21\,μ$m and $7.7/11.3\,μ$m, while all other mid-IR colors remain typical. This suggests the presence of more neutral PAHs in star-formation deserts, where low $7.7\,μ$m likely reflects ISM conditions similar to early-type and elliptical galaxies. All environments form part of a continuous trend in $7.7/11.3\,μ$m vs.\ specific star-formation rate.
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Submitted 24 August, 2026; v1 submitted 21 July, 2026;
originally announced July 2026.
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A steep mass transition for bar-driven ISM structuring revealed by PHANGS-JWST
Authors:
Eric Emsellem,
Jessica Sutter,
Ryan Chown,
Adam Leroy,
Debosmita Pathak,
Eva Schinnerer,
Thomas Williams,
Karin Sandstrom,
Oscar Agertz,
Francesco Belfiore,
Amelia Fraser-McKelvie,
Kirsten Larson,
Janice Lee,
Jay Gonzàlez Lobos,
Sharon Meidt,
Miguel Querejeta,
Sophia Stuber,
Pierrick Verwilghen,
Oleg Egorov,
Damian Gleis,
Jonathan Henshaw,
Justus Neumann,
Yixian Cao,
Daniel Dale,
Daizhong Liu
, et al. (4 additional authors not shown)
Abstract:
Galactic bars play a critical role in the secular evolution of their hosts by reorganising the ISM. We use a sample of 57 star-forming disc galaxies observed with JWST at 3 and 7.7 $μ$m to probe how the spatial distribution of PAH emission, as a structural marker of the cold ISM, depends on stellar mass and bar presence. We find evidence for a "watershed" at a stellar mass of $10^{10}$ Msun, marki…
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Galactic bars play a critical role in the secular evolution of their hosts by reorganising the ISM. We use a sample of 57 star-forming disc galaxies observed with JWST at 3 and 7.7 $μ$m to probe how the spatial distribution of PAH emission, as a structural marker of the cold ISM, depends on stellar mass and bar presence. We find evidence for a "watershed" at a stellar mass of $10^{10}$ Msun, marking a fundamental transition in the bar-driven distribution of PAH emission. This confirms trends previously predicted by numerical simulations and observed via ionised gas or UV light. While lower-mass galaxies exhibit a disordered and clumpy distribution of PAH emission regardless of bar presence, higher-mass barred hosts display well-structured dynamical features traced by PAH emission with significant gas reservoirs (e.g., discs and rings) within the central 15% of the bar radius (Rb). Furthermore, we observe a systematic depletion of PAH emission within the [0.2-0.8] Rb range in barred systems with stellar masses above $10^{10}$ Msun. Such central discs, rings, and associated radial dips ("bar deserts") appear to be a mass-dependent phenomenon: ubiquitous in massive galaxies but mostly absent in lower-mass counterparts. In contrast to the structured features in massive hosts, the disorganised ISM in lower-mass galaxies masks commonly observed bar-driven signatures. This suggests that tracer selection and dust obscuration may significantly bias observed bar fractions. Our study underlines two regimes of secular evolution, with different impacts and observability of bar-driven processes: it reaffirms bars as primary drivers of rapid secular evolution in galaxies above $10^{10}$ Msun$, while their impact is significantly reduced or delayed below this threshold. It further underscores the need to account for these processes when modelling galaxy evolution in cosmological simulations.
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Submitted 3 July, 2026;
originally announced July 2026.
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Multi-scale Gas Structure and Dynamics in an Extragalactic Central Molecular Zone
Authors:
Liam M. Wang,
Jiayi Sun,
Yu-Hsuan Teng,
Eric W. Koch,
Sanghyuk Moon,
Elias Oakes,
Erik Rosolowsky
Abstract:
The structures and dynamics of the interstellar medium are governed by a combination of self-gravity, external gravity, and various sources of ordered and random motions on different spatial scales. This paper uses ALMA CO (3-2) observations at 0.1" $\approx$ 5 pc resolution to examine the scale dependence of molecular gas structure and dynamics in the central molecular zone (CMZ) of a nearby gala…
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The structures and dynamics of the interstellar medium are governed by a combination of self-gravity, external gravity, and various sources of ordered and random motions on different spatial scales. This paper uses ALMA CO (3-2) observations at 0.1" $\approx$ 5 pc resolution to examine the scale dependence of molecular gas structure and dynamics in the central molecular zone (CMZ) of a nearby galaxy, NGC 3351. We use the dendrogram technique to characterize hierarchical molecular gas structures spanning two decades in spatial scales and measure their size, gas mass, and velocity dispersion. Their size-linewidth relation shows a power-law slope of 0.58, comparable to measurements for CMZs in other galaxies and suggestive of significant contribution from ordered motion on large scales. We further decompose the observed velocity dispersion in each gas structure into ordered versus random motions. The former appears stronger in gas structures at $\gtrsim$ 30 pc while the latter becomes more dominant at $\lesssim$ 30 pc. Modulo uncertainties with the CO-to-H$_2$ conversion factor, the estimated gravitational free-fall time is comparable to the crossing time of ordered motions for structures on all spatial scales, and both becomes longer than the crossing time of random motions at small, $\lesssim$ 10 pc scales. Our results highlight the varying sources and drivers of gas motions on different spatial scales in the CMZ of a Milky Way-like galaxy.
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Submitted 24 April, 2026; v1 submitted 7 April, 2026;
originally announced April 2026.
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PAH Marks the Spot: Digging for Buried Clusters in Nearby Star-forming Galaxies
Authors:
Gabrielle B. Graham,
Daniel A. Dale,
Chase L. Smith,
Elisabeth Brann,
Kaycee D. Conder,
Samuel Crowe,
Sumitra Dhileepkumar,
Nicole A. Imming,
Emilio Mendez,
Zachary Pleska,
Kelsey Sako,
Amirnezam Amiri,
Ashley T. Barnes,
Médéric Boquien,
Rupali Chandar,
Ryan Chown,
Oleg Y. Gnedin,
Kathryn Grasha,
Stephen Hannon,
Hamid Hassani,
Rémy Indebetouw,
Hwihyun Kim,
Jaeyeon Kim,
Hannah Koziol,
Kirsten L. Larson
, et al. (17 additional authors not shown)
Abstract:
The joint capabilities of the Hubble Space Telescope (HST) and JWST allow for an unparalleled look at the early lives of star clusters at near- and mid-infrared wavelengths. We present here a multiband analysis of embedded young stellar clusters in 11 nearby, star-forming galaxies, using the PHANGS-JWST and PHANGS-HST datasets. We use the Zooniverse citizen science platform to conduct an initial b…
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The joint capabilities of the Hubble Space Telescope (HST) and JWST allow for an unparalleled look at the early lives of star clusters at near- and mid-infrared wavelengths. We present here a multiband analysis of embedded young stellar clusters in 11 nearby, star-forming galaxies, using the PHANGS-JWST and PHANGS-HST datasets. We use the Zooniverse citizen science platform to conduct an initial by-eye search for embedded clusters in near-UV/optical/near-infrared images that trace stellar continuum emission, the Paschen$α$ and H$α$ recombination lines, and the 3.3 $μ$m polycyclic aromatic hydrocarbon feature and its underlying continuum. With this approach, we identify 292 embedded cluster candidates for which we characterize their ages, masses, and levels of line-of-sight extinction by comparing the photometric data to predictions from stellar population models. The embedded cluster candidates have a median age of 4.5 Myr and an average line-of-sight extinction $\left< A_V \right> = 6.0$ mag. We determine lower limits on source stellar masses, resulting in a median stellar mass of $10^3$ $M_{\odot}$. We use this sample of embedded cluster candidates to train multiple convolutional neural network models to carry out deep transfer learning-based searches for embedded clusters. With the aim of optimizing models for future catalog production, we compare results for four variations of training data using two neural networks. Confusion matrices for all eight model configurations, as well as inter-model identification trends, are presented. With refinement of the training sample, we determine that optimized models could serve as a pathway for future embedded cluster identification beyond our 11 galaxy sample.
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Submitted 14 November, 2025;
originally announced November 2025.
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Resolved HII regions in NGC 253: Ionized gas structure and suggestions of a universal density-surface brightness relation
Authors:
Rebecca L. McClain,
Adam K. Leroy,
Enrico Congiu,
Ashley. T. Barnes,
Francesco Belfiore,
Oleg Egorov,
Eric Emsellem,
Erik Rosolowsky,
Amirnezam Amiri,
Mederic Boquien,
Jeremy Chastenet,
Ryan Chown,
Daniel A. Dale,
Sanskriti Das,
Simon C. O. Glover,
Kathryn Grasha,
Remy Indebetouw,
Eric W. Koch,
Smita Mathur,
J. Eduardo Mendez-Delgado,
Elias K. Oakes,
Hsi-An Pan,
Karin Sandstrom,
Sumit K. Sarbadhicary,
Bradley C. Whitmore
, et al. (1 additional authors not shown)
Abstract:
We use the full-disk VLT-MUSE mosaic of NGC 253 to identify 2492 HII regions and study their resolved structure. With an average physical resolution of 17 pc, this is one of the largest samples of highly resolved spectrally mapped extragalactic HII regions. Regions of all luminosities exhibit a characteristic emission profile described by a double Gaussian with a marginally resolved or unresolved…
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We use the full-disk VLT-MUSE mosaic of NGC 253 to identify 2492 HII regions and study their resolved structure. With an average physical resolution of 17 pc, this is one of the largest samples of highly resolved spectrally mapped extragalactic HII regions. Regions of all luminosities exhibit a characteristic emission profile described by a double Gaussian with a marginally resolved or unresolved core with radius <10 pc surrounded by a more extended halo of emission with radius 20-30 pc. Approximately 80% of the emission of a region originates from the halo component. As a result of this compact structure, the luminosity-radius relations for core and effective radii of HII regions depend sensitively on the adopted methodology. Only the isophotal radius yields a robust relationship in NGC 253, but this measurement has an ambiguous physical meaning. We invert the measured emission profiles to infer density profiles and find central densities of n_e = 10-100 cm-3. In the brightest regions, these agree well with densities inferred from the [SII]6716,30 doublet. The central density of HII regions correlates well with the surface brightness within the effective radius. We show that this same scaling relation applies to the recent MUSE+HST catalog for 19 nearby galaxies. We also discuss potential limitations, including completeness, impacts of background subtraction and spatial resolution, and the generality of our results when applied to other galaxies.
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Submitted 29 October, 2025;
originally announced October 2025.
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The Hidden Life of Stars: Embedded Beginnings to AGB Endings in the PHANGS-JWST Sample. I. Catalog of Mid-IR Sources
Authors:
Hamid Hassani,
Erik Rosolowsky,
Adam K. Leroy,
Karin Sandstrom,
Médéric Boquien,
David A. Thilker,
Bradley C. Whitmore,
Gagandeep S. Anand,
Ashley T. Barnes,
Yixian Cao,
Ryan Chown,
Enrico Congiu,
Daniel A. Dale,
Oleg V. Egorov,
Ivan Gerasimov,
Kathryn Grasha,
Remy Indebetouw,
Janice C. Lee,
Fu-Heng Liang,
Daniel Maschmann,
Sharon E. Meidt,
Elias K. Oakes,
Ismael Pessa,
Jérôme Pety,
Miguel Querejeta
, et al. (6 additional authors not shown)
Abstract:
We present a multiwavelength catalog of mid-infrared-selected compact sources in 19 nearby galaxies, combining JWST NIRCam/MIRI, HST UV-optical broadband, H$α$ narrow-band, and ALMA CO observations. We detect 24,945 compact sources at 21 $μ$m and 55,581 at 10 $μ$m. Artificial star tests show 50% completeness limits of $\sim$5 $μ$Jy for the 10 $μ$m catalog, and $\sim$24 $μ$Jy for the 21 $μ$m catalo…
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We present a multiwavelength catalog of mid-infrared-selected compact sources in 19 nearby galaxies, combining JWST NIRCam/MIRI, HST UV-optical broadband, H$α$ narrow-band, and ALMA CO observations. We detect 24,945 compact sources at 21 $μ$m and 55,581 at 10 $μ$m. Artificial star tests show 50% completeness limits of $\sim$5 $μ$Jy for the 10 $μ$m catalog, and $\sim$24 $μ$Jy for the 21 $μ$m catalog. We find that 21 $μ$m compact sources contribute $\sim$20% of the total galaxy emission in that band, but only contribute $5%$ at 10 $μ$m. We classify sources using stellar evolution and population synthesis models combined with empirical classifications derived from the literature. Our classifications include H$α$-bright and dust-embedded optically faint clusters, red supergiants (RSGs), oxygen-rich and carbon-rich AGB stars, and a range of rarer stellar types. In sampling a broad range of star forming environments with a uniform, well-characterized selection, this catalog enables enables analyses of infrared-bright stellar populations. We find that H$α$-faint sources account for only 10% of dusty (likely young) clusters, implying that the infrared-bright, optically-faint phase of cluster evolution is short compared to the H$α$-bright stage. The luminosity functions of 10 and 21 $μ$m sources follow power-law distributions, with the 21 $μ$m slope ($-1.7 \pm 0.1$) similar to that of giant molecular cloud mass functions and ultraviolet bright star-forming complexes, while the 10 $μ$m slope ($-2.0 \pm 0.1$) is closer to that of young stellar clusters.
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Submitted 19 March, 2026; v1 submitted 19 September, 2025;
originally announced September 2025.
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The Hierarchical Dynamical State of Molecular Gas from 3 to 300 pc in NGC 253
Authors:
Elias K. Oakes,
Christopher M. Faesi,
Erik Rosolowsky,
Adam K. Leroy,
Simon C. O. Glover,
Annie Hughes,
Sharon E. Meidt,
Eva Schinnerer,
Jiayi Sun,
Amirnezam Amiri,
Ashley T. Barnes,
Zein Bazzi,
Ivana Bešlić,
Guillermo A. Blanc,
Charlie Burton,
Ryan Chown,
Enrico Congiu,
Daniel A. Dale,
Simthembile Dlamini,
Hao He,
Eric W. Koch,
Fu-Heng Liang,
Jérôme Pety,
Miguel Querejeta,
Sumit K. Sarbadhicary
, et al. (3 additional authors not shown)
Abstract:
Understanding how the dynamical state of the interstellar medium (ISM) changes across spatial scales can provide important insights into how the gas is organized and ultimately collapses to form stars. To this end, we present ALMA $^{12}\mathrm{CO}(2-1)$ observations at $7$ pc ($0''.4$) spatial resolution across a $1.4~\mathrm{kpc}\times5.6~\mathrm{kpc}$ ($1'.3\times1'.3$) region located in the di…
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Understanding how the dynamical state of the interstellar medium (ISM) changes across spatial scales can provide important insights into how the gas is organized and ultimately collapses to form stars. To this end, we present ALMA $^{12}\mathrm{CO}(2-1)$ observations at $7$ pc ($0''.4$) spatial resolution across a $1.4~\mathrm{kpc}\times5.6~\mathrm{kpc}$ ($1'.3\times1'.3$) region located in the disk of the nearby ($D = 3.5$ Mpc), massive, star-forming galaxy NGC 253. We decompose this emission with a hierarchical, multiscale dendrogram algorithm to identify 2463 structures with deconvolved sizes ranging from $\sim3$ to $300$ pc, complete to a limiting mass of $10^4~M_\odot$. By comparing the virial parameter of these structures against physical properties including size, mass, surface density, velocity dispersion, and hierarchical position, we carry out a comprehensive search for a preferred scale at which gravitationally bound structures emerge. Ultimately, we do not identify evidence of an emergent scale for bound objects in our data, nor do we find a significant correlation between the virial parameter and structure sizes. These findings suggest that simple observational estimates of gravitational binding cannot be used to define molecular clouds and emphasize the need for multiscale approaches to characterize the ISM.
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Submitted 4 November, 2025; v1 submitted 4 July, 2025;
originally announced July 2025.
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The MUSE view of the Sculptor galaxy: survey overview and the planetary nebulae luminosity function
Authors:
E. Congiu,
F. Scheuermann,
K. Kreckel,
A. Leroy,
E. Emsellem,
F. Belfiore,
J. Hartke,
G. Anand,
O. V. Egorov,
B. Groves,
T. Kravtsov,
D. Thilker,
C. Tovo,
F. Bigiel,
G. A. Blanc,
A. D. Bolatto,
S. A. Cronin,
D. A. Dale,
R. McClain,
J. E. Méndez-Delgado,
E. K. Oakes,
R. S. Klessen,
E. Schinnerer,
T. G. Williams
Abstract:
NGC 253, the Sculptor galaxy, is the southern, massive, star-forming disk galaxy closest to the Milky Way. In this work, we present a new 103-pointing MUSE mosaic of this galaxy covering the majority of its star-forming disk up to 0.75xR25. With an area of ~20x5 arcmin2 (~20x5 kpc2, projected) and a physical resolution of ~15 pc, this mosaic constitutes one of the largest, highest physical resolut…
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NGC 253, the Sculptor galaxy, is the southern, massive, star-forming disk galaxy closest to the Milky Way. In this work, we present a new 103-pointing MUSE mosaic of this galaxy covering the majority of its star-forming disk up to 0.75xR25. With an area of ~20x5 arcmin2 (~20x5 kpc2, projected) and a physical resolution of ~15 pc, this mosaic constitutes one of the largest, highest physical resolution integral field spectroscopy surveys of any star-forming galaxy to date. Here, we exploit the mosaic to identify a sample of ~500 planetary nebulae (~20 times larger than in previous studies) to build the planetary nebula luminosity function (PNLF) and obtain a new estimate of the distance to NGC 253. The value obtained is 17% higher than estimates returned by other reliable measurements, mainly obtained via the top of the red giant branch method (TRGB). The PNLF also varies between the centre (r < 4 kpc) and the disk of the galaxy. The distance derived from the PNLF of the outer disk is comparable to that of the full sample, while the PNLF of the centre returns a distance ~0.9 Mpc larger. Our analysis suggests that extinction related to the dust-rich interstellar medium and edge-on view of the galaxy (the average E(B-V) across the disk is ~0.35 mag) plays a major role in explaining both the larger distance recovered from the full PNLF and the difference between the PNLFs in the centre and in the disk.
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Submitted 17 June, 2025;
originally announced June 2025.
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Constraining resolved extragalactic $R_{21}$ variation with well calibrated ALMA observations
Authors:
Jakob den Brok,
Elias K. Oakes,
Adam K. Leroy,
Eric W. Koch,
Antonio Usero,
Erik W. Rosolowsky,
Frank Bigiel,
Jiayi Sun,
Hao He,
Ashley T. Barnes,
Yixian Cao,
Fu-Heng Liang,
Hsi-An Pan,
Toshiki Saito,
Sumit K. Sarbadhicary,
Thomas G. Williams
Abstract:
CO(1-0) and CO(2-1) are commonly used as bulk molecular gas tracers. The CO line ratios (especially CO(2-1)/CO(1-0) - $R_{21}$) vary within and among galaxies, yet previous studies on $R_{21}$ and alike often rely on measurements constructed by combining data from facilities with substantial relative calibration uncertainties that have the same order as physical line ratio variations. Hence robust…
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CO(1-0) and CO(2-1) are commonly used as bulk molecular gas tracers. The CO line ratios (especially CO(2-1)/CO(1-0) - $R_{21}$) vary within and among galaxies, yet previous studies on $R_{21}$ and alike often rely on measurements constructed by combining data from facilities with substantial relative calibration uncertainties that have the same order as physical line ratio variations. Hence robustly determining systematic $R_{21}$ variations is challenging. Here, we compare CO(1-0) and CO(2-1) mapping data from ALMA for 14 nearby galaxies, at a common physical resolution of 1.7 kpc. Our dataset includes new ALMA (7m+TP) CO(1-0) maps of 12 galaxies. We investigate $R_{21}$ variation to understand its dependence on global galaxy properties, kpc-scale environmental factors, and its correlation with star formation rate (SFR) surface density and metallicity. We find that the galaxy-to-galaxy scatter is 0.05 dex. This is lower than previous studies which reported over 0.1 dex variation, likely reflecting significant flux calibration uncertainties in single-dish surveys. Within individual galaxies, $R_{21}$ has a typical mean value of ~0.64 and 0.1 dex variation, with an increase to ~0.75 towards galactic centers. We find strong correlations between $R_{21}$ and various galactic parameters, particularly SFR surface density, which shows a power-law slope of 0.10-0.11 depending on the adopted binning/fitting methods. Our findings suggest that, for studies covering main sequence galaxy samples, assuming a fixed $R_{21}$=0.64 does not significantly bias kpc-scale molecular gas mass estimates from CO(2-1). Instead, systematic uncertainties from flux calibration and the CO-to-H$_2$ conversion factor account for more systematic scatter of CO-derived molecular gas properties.
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Submitted 10 June, 2025;
originally announced June 2025.
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Cloud-scale gas properties, depletion times, and star formation efficiency per free-fall time in PHANGS--ALMA
Authors:
Adam K. Leroy,
Jiayi Sun,
Sharon Meidt,
Oscar Agertz,
I-Da Chiang,
Jindra Gensior,
Simon C. O. Glover,
Oleg Y. Gnedin,
Annie Hughes,
Eva Schinnerer,
Ashley T. Barnes,
Frank Bigiel,
Alberto D. Bolatto,
Dario Colombo,
Jakob den Brok,
Melanie Chevance,
Ryan Chown,
Cosima Eibensteiner,
Damian R. Gleis,
Kathryn Grasha,
Jonathan D. Henshaw,
Ralf S. Klessen,
Eric W. Koch,
Elias K. Oakes,
Hsi-An Pan
, et al. (9 additional authors not shown)
Abstract:
We compare measurements of star formation efficiency to cloud-scale gas properties across PHANGS-ALMA. Dividing 67 galaxies into 1.5 kpc scale regions, we calculate the molecular gas depletion time, tau_dep= Sigma_mol/Sigma_SFR, and the star formation efficiency per free-fall time, eff=tau_ff/tau_dep, for each region. Then we test how tau_dep and eff vary as functions of the regional mass-weighted…
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We compare measurements of star formation efficiency to cloud-scale gas properties across PHANGS-ALMA. Dividing 67 galaxies into 1.5 kpc scale regions, we calculate the molecular gas depletion time, tau_dep= Sigma_mol/Sigma_SFR, and the star formation efficiency per free-fall time, eff=tau_ff/tau_dep, for each region. Then we test how tau_dep and eff vary as functions of the regional mass-weighted mean molecular gas properties on cloud scales (60-150pc): gas surface density, <Sigma_mol^cloud>, velocity dispersion, <sigma_mol^cloud>, virial parameter, <alpha_vir^cloud>, and gravitational free-fall time, <tau_ff^cloud>. <tau_ff^cloud> and tau_dep correlate positively, consistent with the expectation that gas density plays a key role in setting the rate of star formation. Our fiducial measurements suggest tau_dep \propto <tau_ff^cloud>^0.5 and eff \approx 0.39%, though the exact numbers depend on the adopted fitting methods. We also observe anti-correlations between tau_dep and <Sigma_mol^cloud> and between tau_dep^mol and <sigma_mol^cloud> . All three correlations may reflect the same underlying link between density and star formation efficiency combined with systematic variations in the degree to which self-gravity binds molecular gas in galaxies. We highlight the tau_dep-<sigma_mol^cloud> relation because of the lower degree of correlation between the axes. Contrary to theoretical expectations, we observe an anti-correlation between tau_dep^mol and <alpha_vir^cloud> and no significant correlation between eff and <alpha_vir^cloud>. Our results depend sensitively on the adopted CO-to-H2 conversion factor, with corrections for excitation and emissivity effects in inner galaxies playing an important role. We emphasize that our simple methodology and clean selection allow easy comparison to numerical simulations and highlight this as a logical next direction.
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Submitted 1 March, 2026; v1 submitted 6 February, 2025;
originally announced February 2025.
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Polycyclic Aromatic Hydrocarbon and CO(2-1) Emission at 50-150 pc Scales in 70 Nearby Galaxies
Authors:
Ryan Chown,
Adam K. Leroy,
Karin Sandstrom,
Jeremy Chastenet,
Jessica Sutter,
Eric W. Koch,
Hannah B. Koziol,
Lukas Neumann,
Jiayi Sun,
Thomas G. Williams,
Dalya Baron,
Gagandeep S. Anand,
Ashley T. Barnes,
Zein Bazzi,
Francesco Belfiore,
Alberto Bolatto,
Mederic Boquien,
Frank Bigiel,
Yixian Cao,
Melanie Chevance,
Dario Colombo,
Daniel A. Dale,
Jakob den Brok,
Oleg V. Egorov,
Cosima Eibensteiner
, et al. (22 additional authors not shown)
Abstract:
Combining Atacama Large Millimeter/sub-millimeter Array CO(2-1) mapping and JWST near- and mid-infrared imaging, we characterize the relationship between CO(2-1) and polycyclic aromatic hydrocarbon (PAH) emission at ~100 pc resolution in 70 nearby star-forming galaxies. Leveraging a new Cycle 2 JWST treasury program targeting nearby galaxies, we expand the sample size by more than an order of magn…
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Combining Atacama Large Millimeter/sub-millimeter Array CO(2-1) mapping and JWST near- and mid-infrared imaging, we characterize the relationship between CO(2-1) and polycyclic aromatic hydrocarbon (PAH) emission at ~100 pc resolution in 70 nearby star-forming galaxies. Leveraging a new Cycle 2 JWST treasury program targeting nearby galaxies, we expand the sample size by more than an order of magnitude compared to previous ~100 pc resolution CO-PAH comparisons. Focusing on regions of galaxies where most of the gas is likely to be molecular, we find strong correlations between CO(2-1) and 3.3 um, 7.7 um, and 11.3 um PAH emission, estimated from JWST's F335M, F770W, and F1130W filters. We derive power law relations between CO(2-1) and PAH emission, which have indices in the range 0.8-1.3, implying relatively weak variations in the observed CO-to-PAH ratios across the regions that we study. We find that CO-to-PAH ratios and scaling relationships near HII regions are similar to those in diffuse sight lines. The main difference between the two types of regions is that sight lines near HII regions show higher intensities in all tracers. Galaxy centers, on the other hand, show higher overall intensities and enhanced CO-to-PAH ratios compared to galaxy disks. Individual galaxies show 0.19 dex scatter in the normalization of CO at fixed I_PAH, and this normalization anti-correlates with specific star formation rate (sSFR) and correlates with stellar mass. We provide a prescription that accounts for these galaxy-to-galaxy variations and represents our best current empirical predictor to estimate CO(2-1) intensity from PAH emission, which allows one to take advantage of JWST's excellent sensitivity and resolution to trace cold gas.
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Submitted 19 March, 2025; v1 submitted 7 October, 2024;
originally announced October 2024.
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The Fraction of Dust Mass in the Form of PAHs on 10-50 pc Scales in Nearby Galaxies
Authors:
Jessica Sutter,
Karin Sandstrom,
Jérémy Chastenet,
Adam K. Leroy,
Eric W. Koch,
Thomas G. Williams,
Ryan Chown,
Francesco Belfiore,
Frank Bigiel,
Médéric Boquien,
Yixian Cao,
Mélanie Chevance,
Daniel A. Dale,
Oleg V. Egorov,
Simon C. O. Glover,
Brent Groves,
Ralf S. Klessen,
Kathryn Kreckel,
Kirsten L. Larson,
Elias K. Oakes,
Debosmita Pathak,
Lise Ramambason,
Erik Rosolowsky,
Elizabeth J. Watkins
Abstract:
Polycyclic aromatic hydrocarbons (PAHs) are a ubiquitous component of the interstellar medium (ISM) in z~0 massive, star-forming galaxies and play key roles in ISM energy balance, chemistry, and shielding. Wide field of view, high resolution mid-infrared (MIR) images from JWST provides the ability to map the fraction of dust in the form of PAHs and the properties of these key dust grains at 10-50…
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Polycyclic aromatic hydrocarbons (PAHs) are a ubiquitous component of the interstellar medium (ISM) in z~0 massive, star-forming galaxies and play key roles in ISM energy balance, chemistry, and shielding. Wide field of view, high resolution mid-infrared (MIR) images from JWST provides the ability to map the fraction of dust in the form of PAHs and the properties of these key dust grains at 10-50 pc resolution in galaxies outside the Local Group. We use MIR JWST photometric observations of a sample of 19 nearby galaxies from the "Physics at High Angular Resolution in Nearby GalaxieS" (PHANGS) survey to investigate the variations of the PAH fraction. By comparison to lower resolution far-IR mapping, we show that a combination of the MIRI filters (R$_{\rm{PAH}}$ = [F770W+F1130W]/F2100W) traces the fraction of dust by mass in the form of PAHs (i.e., the PAH fraction, or q$_{\rm{PAH}}$). Mapping R$_{\rm{PAH}}$ across the 19 PHANGS galaxies, we find that the PAH fraction steeply decreases in HII regions, revealing the destruction of these small grains in regions of ionized gas. Outside HII regions, we find R$_{\rm{PAH}}$ is constant across the PHANGS sample with an average value of 3.43$\pm$0.98, which, for an illuminating radiation field of intensity 2-5 times that of the radiation field in the solar neighborhood, corresponds to q$_{\rm{PAH}}$ values of 3-6%.
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Submitted 23 May, 2024;
originally announced May 2024.
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PHANGS-JWST: Data Processing Pipeline and First Full Public Data Release
Authors:
Thomas G. Williams,
Janice C. Lee,
Kirsten L. Larson,
Adam K. Leroy,
Karin Sandstrom,
Eva Schinnerer,
David A. Thilker,
Francesco Belfiore,
Oleg V. Egorov,
Erik Rosolowsky,
Jessica Sutter,
Joseph DePasquale,
Alyssa Pagan,
Travis A. Berger,
Gagandeep S. Anand,
Ashley T. Barnes,
Frank Bigiel,
Médéric Boquien,
Yixian Cao,
Jérémy Chastenet,
Mélanie Chevance,
Ryan Chown,
Daniel A. Dale,
Sinan Deger,
Cosima Eibensteiner
, et al. (33 additional authors not shown)
Abstract:
The exquisite angular resolution and sensitivity of JWST is opening a new window for our understanding of the Universe. In nearby galaxies, JWST observations are revolutionizing our understanding of the first phases of star formation and the dusty interstellar medium. Nineteen local galaxies spanning a range of properties and morphologies across the star-forming main sequence have been observed as…
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The exquisite angular resolution and sensitivity of JWST is opening a new window for our understanding of the Universe. In nearby galaxies, JWST observations are revolutionizing our understanding of the first phases of star formation and the dusty interstellar medium. Nineteen local galaxies spanning a range of properties and morphologies across the star-forming main sequence have been observed as part of the PHANGS-JWST Cycle 1 Treasury program at spatial scales of $\sim$5-50pc. Here, we describe pjpipe, an image processing pipeline developed for the PHANGS-JWST program that wraps around and extends the official JWST pipeline. We release this pipeline to the community as it contains a number of tools generally useful for JWST NIRCam and MIRI observations. Particularly for extended sources, pjpipe products provide significant improvements over mosaics from the MAST archive in terms of removing instrumental noise in NIRCam data, background flux matching, and calibration of relative and absolute astrometry. We show that slightly smoothing F2100W MIRI data to 0.9" (degrading the resolution by about 30 percent) reduces the noise by a factor of $\approx$3. We also present the first public release (DR1.1.0) of the pjpipe processed eight-band 2-21 $μ$m imaging for all nineteen galaxies in the PHANGS-JWST Cycle 1 Treasury program. An additional 55 galaxies will soon follow from a new PHANGS-JWST Cycle 2 Treasury program.
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Submitted 9 May, 2024; v1 submitted 26 January, 2024;
originally announced January 2024.
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The impact of HII regions on Giant Molecular Cloud properties in nearby galaxies sampled by PHANGS ALMA and MUSE
Authors:
Antoine Zakardjian,
Jérôme Pety,
Cinthya N. Herrera,
Annie Hughes,
Elias Oakes,
Kathryn Kreckel,
Chris Faesi,
Simon C. O. Glover,
Brent Groves,
Ralf S. Klessen,
Sharon Meidt,
Ashley Barnes,
Francesco Belfiore,
Ivana Bešlić,
Frank Bigiel,
Guillermo A. Blanc,
Mélanie Chevance,
Daniel A. Dale,
Jakob den Brok,
Cosima Eibensteiner,
Eric Emsellem,
Axel García-Rodríguez,
Kathryn Grasha,
Eric W. Koch,
Adam K. Leroy
, et al. (14 additional authors not shown)
Abstract:
We identify giant molecular clouds (GMCs) associated with HII regions for a sample of 19 nearby galaxies using catalogs of GMCs and H regions released by the PHANGS-ALMA and PHANGS-MUSE surveys, using the overlap of the CO and Hα emission as the key criterion for physical association. We compare the distributions of GMC and HII region properties for paired and non-paired objects. We investigate co…
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We identify giant molecular clouds (GMCs) associated with HII regions for a sample of 19 nearby galaxies using catalogs of GMCs and H regions released by the PHANGS-ALMA and PHANGS-MUSE surveys, using the overlap of the CO and Hα emission as the key criterion for physical association. We compare the distributions of GMC and HII region properties for paired and non-paired objects. We investigate correlations between GMC and HII region properties among galaxies and across different galactic environments to determine whether GMCs that are associated with HII regions have significantly distinct physical properties to the parent GMC population. We identify trends between the Hα luminosity of an HII region and the CO peak brightness and the molecular mass of GMCs that we tentatively attribute to a direct physical connection between the matched objects, and which arise independently of underlying environmental variations of GMC and HII region properties within galaxies. The study of the full sample nevertheless hides a large variability galaxy by galaxy. Our results suggests that at the ~100 pc scales accessed by the PHANGS-ALMA and PHANGS-MUSE data, pre-supernova feedback mechanisms in HII regions have a subtle but measurable impact on the properties of the surrounding molecular gas, as inferred from CO observations.
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Submitted 5 May, 2023;
originally announced May 2023.
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Distances to Local Group Galaxies via Population II, Stellar Distance Indicators I: The Sculptor Dwarf Spheroidal
Authors:
Quang H. Tran,
Taylor J. Hoyt,
Wendy L. Freedman,
Barry F. Madore,
Elias K. Oakes,
William Cerny,
Dylan Hatt,
Rachael L. Beaton
Abstract:
We determine the distance to the Sculptor Dwarf Spheroidal via three Population II stellar distance indicators: (a) the Tip of the Red Giant Branch (TRGB), (b) RR Lyrae variables (RRLs), and (c) the ridgeline of the blue horizontal branch (HB). High signal-to-noise, wide-field $VI$ imaging that covers an area $48' \times 48'$ and reaches a photometric depth approximately 2 mag fainter than the HB…
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We determine the distance to the Sculptor Dwarf Spheroidal via three Population II stellar distance indicators: (a) the Tip of the Red Giant Branch (TRGB), (b) RR Lyrae variables (RRLs), and (c) the ridgeline of the blue horizontal branch (HB). High signal-to-noise, wide-field $VI$ imaging that covers an area $48' \times 48'$ and reaches a photometric depth approximately 2 mag fainter than the HB was acquired with the Magellan-Baade 6.5m telescope. The true modulus derived from Sculptor's TRGB is found to be $μ^\mathrm{TRGB}_o = 19.59 \pm 0.07_\mathrm{stat} \pm 0.05_\mathrm{sys}$ mag. Along with periods adopted from the literature, newly acquired RRL phase points are fit with template light curves to determine $μ_{W_{I,V-I}}^\mathrm{RRL} = 19.60 \pm 0.01_\mathrm{stat} \pm 0.05_\mathrm{sys}$ mag. Finally, the HB distance is found to be $μ^\mathrm{HB}_o = 19.54 \pm 0.03_\mathrm{stat} \pm 0.09_\mathrm{sys}$ mag. Absolute calibrations of each method are anchored by independent geometric zero-points, utilizes a different class of stars, and are determined from the same photometric calibration.
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Submitted 17 May, 2022;
originally announced May 2022.
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Distances to Local Group Galaxies via Population II, Stellar Distance Indicators. II. The Fornax Dwarf Spheroidal
Authors:
Elias K. Oakes,
Taylor J. Hoyt,
Wendy L. Freedman,
Barry F. Madore,
Quang H. Tran,
William Cerny,
Rachael L. Beaton,
Mark Seibert
Abstract:
We determine three independent Population II distance moduli to the Fornax dwarf spheroidal (dSph) galaxy, using wide-field, ground-based $VI$ imaging acquired with the Magellan-Baade telescope at Las Campanas Observatory. After subtracting foreground stars using Gaia EDR3 proper motions, we measure an $I$-band tip of the red giant branch (TRGB) magnitude of…
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We determine three independent Population II distance moduli to the Fornax dwarf spheroidal (dSph) galaxy, using wide-field, ground-based $VI$ imaging acquired with the Magellan-Baade telescope at Las Campanas Observatory. After subtracting foreground stars using Gaia EDR3 proper motions, we measure an $I$-band tip of the red giant branch (TRGB) magnitude of $I_0^\mathrm{TRGB} = 16.753 \pm 0.03_\mathrm{stat} \pm 0.037_\mathrm{sys}$ mag, with a calibration based in the LMC giving a distance modulus of $μ_0^\mathrm{TRGB} = 20.80 \pm 0.037_\mathrm{stat} \pm 0.057_\mathrm{sys}$ mag. We determine an RR Lyrae (RRL) distance from template mean magnitudes, with periods adopted from the literature. Adopting a Gaia DR2 calibration of first overtone RRL period-luminosity and period-Wesenheit relations, we find $μ_0^\mathrm{PLZ} = 20.74 \pm 0.01_\mathrm{stat} \pm 0.12_\mathrm{sys}$ mag and $μ_0^\mathrm{PWZ} = 20.68 \pm 0.02_\mathrm{stat} \pm 0.07_\mathrm{sys}$ mag. Finally, we determine a distance from Fornax's horizontal branch (HB) and two galactic globular cluster calibrators, giving $μ_0^\mathrm{HB} = 20.83 \pm 0.03_\mathrm{stat} \pm 0.09_\mathrm{sys}$ mag. These distances are each derived from homogeneous IMACS photometry, are anchored to independent geometric zero-points, and utilize different classes of stars. We therefore average over independent uncertainties and report the combined distance modulus $\langle μ_0\rangle = 20.770 \pm 0.042_\mathrm{stat} \pm 0.024_\mathrm{sys}$ mag (corresponding to a distance of $143\pm3$ kpc).
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Submitted 10 May, 2022; v1 submitted 20 April, 2022;
originally announced April 2022.
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Multi-Wavelength, Optical (VI) and Near-Infrared (JHK) Calibration of the Tip of the Red Giant Branch Method based on Milky Way Globular Clusters
Authors:
William Cerny,
Wendy L. Freedman,
Barry F. Madore,
Finian Ashmead,
Taylor Hoyt,
Elias Oakes,
Nhat Quang Hoang Tran,
Blake Moss
Abstract:
Using high precision ground-based photometry for 46 low-reddening Galactic globular clusters, in conjunction with Gaia DR2 proper motions for member star selection, we have calibrated the zero point of the tip of the red giant branch (TRGB) method at two optical ($VI$) and three near-infrared ($JHK$) wavelengths. In doing so, we utilized the sharply-defined zero-age horizontal branch (ZAHB) of the…
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Using high precision ground-based photometry for 46 low-reddening Galactic globular clusters, in conjunction with Gaia DR2 proper motions for member star selection, we have calibrated the zero point of the tip of the red giant branch (TRGB) method at two optical ($VI$) and three near-infrared ($JHK$) wavelengths. In doing so, we utilized the sharply-defined zero-age horizontal branch (ZAHB) of these clusters to relatively calibrate our cluster sample into a composite color-magnitude diagram spanning a wide range of metallicities, before setting the absolute zero point of this composite using the geometric detached eclipsing binary distance to the cluster $ω$ Centauri. The $I-$band zero point we measure [$M_I = -4.056 \pm 0.02 \text{ (stat}) \pm 0.10 \text{ (sys)} $] agrees to within one sigma of the two previously published independent calibrations, using TRGB stars in the LMC [$M_I = $ -4.047 mag; Freedman et al. 2019, 2020] and in the maser galaxy NGC 4258 [$M_{F814W} = $ -4.051 mag; Jang et al. 2020]. We also find close agreement for our $J,H,K$ zero points to several literature studies.
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Submitted 17 December, 2020;
originally announced December 2020.
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A Deep Learning Functional Estimator of Optimal Dynamics for Sampling Large Deviations
Authors:
Tom H. E. Oakes,
Adam Moss,
Juan P. Garrahan
Abstract:
In stochastic systems, numerically sampling the relevant trajectories for the estimation of the large deviation statistics of time-extensive observables requires overcoming their exponential (in space and time) scarcity. The optimal way to access these rare events is by means of an auxiliary dynamics obtained from the original one through the so-called ``generalised Doob transformation''. While th…
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In stochastic systems, numerically sampling the relevant trajectories for the estimation of the large deviation statistics of time-extensive observables requires overcoming their exponential (in space and time) scarcity. The optimal way to access these rare events is by means of an auxiliary dynamics obtained from the original one through the so-called ``generalised Doob transformation''. While this optimal dynamics is guaranteed to exist its use is often impractical, as to define it requires the often impossible task of diagonalising a (tilted) dynamical generator. While approximate schemes have been devised to overcome this issue they are difficult to automate as they tend to require knowledge of the systems under study. Here we address this problem from the perspective of deep learning. We devise an iterative semi-supervised learning scheme which converges to the optimal or Doob dynamics with the clear advantage of requiring no prior knowledge of the system. We test our method in a paradigmatic statistical mechanics model with non-trivial dynamical fluctuations, the fully packed classical dimer model on the square lattice, showing that it compares favourably with more traditional approaches. We discuss broader implications of our results for the study of rare dynamical trajectories.
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Submitted 27 January, 2020;
originally announced January 2020.
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Trajectory phase transitions in non-interacting spin systems
Authors:
Loredana M. Vasiloiu,
Tom H. E. Oakes,
Federico Carollo,
Juan P. Garrahan
Abstract:
We show that a collection of independent Ising spins evolving stochastically can display surprisingly large fluctuations towards ordered behaviour, as quantified by certain types of time-integrated plaquette observables, despite the underlying dynamics being non-interacting. In the large deviation (LD) regime of long times and large system size, this can give rise to a phase transition in trajecto…
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We show that a collection of independent Ising spins evolving stochastically can display surprisingly large fluctuations towards ordered behaviour, as quantified by certain types of time-integrated plaquette observables, despite the underlying dynamics being non-interacting. In the large deviation (LD) regime of long times and large system size, this can give rise to a phase transition in trajectory space. As a non-interacting system we consider a collection of spins undergoing single spin-flip dynamics at infinite-temperature. For the dynamical observables we study, the associated tilted generators have an exact and explicit spin-plaquette duality. Such setup suggests the existence of a transition (in the large size limit) at the self-dual point of the tilted generator. The nature of the LD transition depends on the observable. We consider explicitly two situations: (i) for a pairwise bond observable the LD transition is continuous, and equivalent to that of the transverse field Ising model; (ii) for a higher order plaquette observable, in contrast, the LD transition is first order. Case (i) is easy to prove analytically, while we confirm case (ii) numerically via an efficient trajectory sampling scheme that exploits the non-interacting nature of the original dynamics.
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Submitted 26 November, 2019;
originally announced November 2019.