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PHANGS-JWST: the largest extragalactic molecular cloud catalog traced by polycyclic aromatic hydrocarbon emission

Z. Bazzi, D. Colombo, F. Bigiel, A. K. Leroy, E. Rosolowsky, K. Sandstrom, A. Duarte-Cabral, H. Faustino Vieira, M. I. N. Kobayashi, H. He, S. E. Meidt, A. T. Barnes, R. S. Klessen, S. C. O. Glover, M. D. Thorp, H. -A. Pan, R. Chown, R. J. Smith, D. A. Dale, T. G. Williams, A. Amiri, S. Dlamini, J. Chastenet, S. K. Sarbadhicary, A. Hughes, J. C. Lee, L. Hands, the PHANGS collaboration

TL;DR

This work leverages high-resolution JWST PAH maps from the PHANGS-JWST program to identify and characterize extragalactic PAH-traced clouds across 66 nearby galaxies, pushing to sub-GMC scales with a 30 pc common resolution. By converting PAH intensities to CO(2-1) luminosities through a calibrated relation, the authors derive molecular properties for a large cloud population, enabling environment-resolved analyses using morphological masks. The study finds that spiral arms host the densest and most massive PAH-identified clouds, while interarm and bar regions host less massive structures; the cloud mass spectrum is well described by a lognormal form, with the width and scale correlating with star formation and galaxy properties. The catalogs, including a 30 pc homogeneous and a native-resolution version, provide a valuable resource for comparing PAH-traced and CO-traced molecular clouds and for testing cloud formation and evolution in diverse galactic environments.

Abstract

High-resolution JWST images of nearby spiral galaxies reveal polycyclic aromatic hydrocarbon (PAH) emission structures that trace molecular gas, including CO-dark regions. We identify ISM cloud structures in PHANGS-JWST 7.7 $μ$m PAH maps for 66 galaxies, smoothed to 30 pc and at native resolution, extracting 108,466 and 146,040 clouds, respectively. Molecular properties were inferred using a linear conversion from PAH to CO. Given the tendency for clouds in galaxy centers to overlap in velocity space, we opted to flag these and omit them from the analysis in this work. The remaining clouds correspond to giant molecular clouds, such as those detected in CO(2-1) emission by ALMA, or lower surface density clouds that either fall below the ALMA detection limits of existing maps or genuinely have no molecular counterpart. Cross-matching with ALMA CO maps at 90 pc in 27 galaxies shows that 41 % of PAH clouds have CO associations. The converted molecular properties vary little across environments, but the most massive clouds are preferentially found in spiral arms. Fitting lognormal mass distributions down to $2\times10^{3} M_{\odot}$ shows that spiral arms host the highest-mass clouds, consistent with enhanced formation in arm gravitational potentials. Cloud molecular surface densities decline by a factor of $\sim 1.5-2$ toward $2 - 3 R_{e}$. However, the trend largely varies in individual galaxies, with flat, decreasing, and even no trend as a function of galactocentric radius. Factors like large-scale processes and morphologies might influence the observed trends. We publish two catalogs online, one at the common resolution of 30 pc and another at the native resolution. We expect them to have broad utility for future PAH clouds, molecular clouds, and star formation studies.

PHANGS-JWST: the largest extragalactic molecular cloud catalog traced by polycyclic aromatic hydrocarbon emission

TL;DR

This work leverages high-resolution JWST PAH maps from the PHANGS-JWST program to identify and characterize extragalactic PAH-traced clouds across 66 nearby galaxies, pushing to sub-GMC scales with a 30 pc common resolution. By converting PAH intensities to CO(2-1) luminosities through a calibrated relation, the authors derive molecular properties for a large cloud population, enabling environment-resolved analyses using morphological masks. The study finds that spiral arms host the densest and most massive PAH-identified clouds, while interarm and bar regions host less massive structures; the cloud mass spectrum is well described by a lognormal form, with the width and scale correlating with star formation and galaxy properties. The catalogs, including a 30 pc homogeneous and a native-resolution version, provide a valuable resource for comparing PAH-traced and CO-traced molecular clouds and for testing cloud formation and evolution in diverse galactic environments.

Abstract

High-resolution JWST images of nearby spiral galaxies reveal polycyclic aromatic hydrocarbon (PAH) emission structures that trace molecular gas, including CO-dark regions. We identify ISM cloud structures in PHANGS-JWST 7.7 m PAH maps for 66 galaxies, smoothed to 30 pc and at native resolution, extracting 108,466 and 146,040 clouds, respectively. Molecular properties were inferred using a linear conversion from PAH to CO. Given the tendency for clouds in galaxy centers to overlap in velocity space, we opted to flag these and omit them from the analysis in this work. The remaining clouds correspond to giant molecular clouds, such as those detected in CO(2-1) emission by ALMA, or lower surface density clouds that either fall below the ALMA detection limits of existing maps or genuinely have no molecular counterpart. Cross-matching with ALMA CO maps at 90 pc in 27 galaxies shows that 41 % of PAH clouds have CO associations. The converted molecular properties vary little across environments, but the most massive clouds are preferentially found in spiral arms. Fitting lognormal mass distributions down to shows that spiral arms host the highest-mass clouds, consistent with enhanced formation in arm gravitational potentials. Cloud molecular surface densities decline by a factor of toward . However, the trend largely varies in individual galaxies, with flat, decreasing, and even no trend as a function of galactocentric radius. Factors like large-scale processes and morphologies might influence the observed trends. We publish two catalogs online, one at the common resolution of 30 pc and another at the native resolution. We expect them to have broad utility for future PAH clouds, molecular clouds, and star formation studies.

Paper Structure

This paper contains 32 sections, 14 equations, 18 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Data and galaxy sample
  3. JWST mid-IR data
  4. Converting 7.7 micron to CO
  5. Environmental masks
  6. SCIMES cloud extraction
  7. Dendrogram structures
  8. SCIMES
  9. Molecular cloud properties
  10. Directly measured properties
  11. Derived physical properties
  12. Error estimation
  13. Cloud population
  14. Results and discussion
  15. $I^\mathrm{PAH}_{\rm F770W}$ and CO cloud property comparison
  16. ...and 17 more sections

Figures (18)

  • Figure 1: A zoomed-in view of NGC1566, one of the 66 galaxies. Left: continuum-subtracted intensity image of the galaxy in the F770W MIRI band. The 2 $\sigma$ CO intensity contours from PHANGS-ALMA are represented in black. Right: The PAH cloud structures identified by SCIMES are color-coded by their F770W intensities. The green, blue, and red contours indicate the spiral arm, bar, and central region masks, respectively. The interarm region, in this case, consists of the remaining clouds that are not enclosed by the contours. The number of PAH clouds identified in this galaxy is represented in the bottom right. The color bar on top of the image shows the 7.7$\mu$m intensity range of the identified clouds.
  • Figure 2: The GMCs in NGC0628. CO-identified GMCs using CPROPS are in red ellipses (Hughes et al. in prep), and the 7.7$\mu$m identified PAH clouds using SCIMES are shown in the background with different colors. A zoomed-in view of the central region is shown in the upper right region of the image, focusing on the structures identified by both PAH and CO. This image highlights both the resolution advantage of PAH clouds and better sensitivity compared to CO, allowing for the detection of fainter and smaller clouds where CO is not detected.
  • Figure 3: Box plots with quantiles and outliers comparing the $\Sigma_{\rm mol,R}$ (left), cloud radius (middle), and cloud area ($\mathrm{A_{R}}$; right) distributions of cross-matched PAH at 30 pc physical resolution and CO clouds at 90 pc in a subsample of 27 galaxies. The colored boxes represent the PAH cloud property distributions without overlapping clouds. The black boxes represent the property distributions of the cross-matched CO clouds. The dashed and dotted horizontal lines represent the median property of the full sample of CO clouds and PAH clouds, respectively.
  • Figure 4: Violin plots showing the distribution and medians of $\Sigma_{\rm mol}$ in each galactic environment for the full sample of PAH clouds (colored) and the 77,844 clouds without overlap (transparent) in the 66 galaxies. The dashed line represents the median $\Sigma_{\rm mol}$ for the full PAH cloud sample.
  • Figure 5: Left: Cumulative distributions of the molecular mass surface densities from the full cloud sample. The different colors represent the different environments. The y-axis is the fraction of clouds with a surface density greater than a given value. All distributions are normalized by the total area of their specific environment, $\mathrm{A}$. Middle: The same as the left plot but only considering barred spiral galaxies and excluding disks. Right: The same as the left plot, but only considering barred disc galaxies and excluding spirals. We remove the central region from the PDFs due to overlapping cloud bias.
  • ...and 13 more figures