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

¹ Argelander-Institut für Astronomie, University of Bonn Auf dem Hügel 71 53121 Bonn, Germany
² Department of Astronomy, Ohio State University 180 W. 18th Ave Columbus OH 43210, USA
³ Center for Cosmology and Astroparticle Physics 191 West Woodruff Avenue Columbus OH 43210, USA
⁴ Dept. of Physics, 4-183 CCIS, University of Alberta Edmonton AB T6G 2E1, Canada
⁵ Department of Astronomy & Astrophysics, University of California, San Diego 9500 Gilman Dr. La Jolla CA 92093, USA
⁶ Cardiff Hub for Astrophysics Research and Technology (CHART), School of Physics & Astronomy, Cardiff University The Parade CF24 3AA Cardiff, UK
⁷ Department of Astronomy, Oskar Klein Center, Stockholm University, AlbaNova University Center SE-106 91 Stockholm, Sweden
⁸ I. Physikalisches Institut, Universität zu Köln Zülpicher Str 77 D-50937 Köln, Germany
⁹ Sterrenkundig Observatorium, Universiteit Gent Krijgslaan 281 S9 B-9000 Gent, Belgium
¹⁰ European Southern Observatory (ESO) Karl-Schwarzschild-Straße 2 85748 Garching, Germany
¹¹ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik Albert-Ueberle-Str. 2 69120 Heidelberg, Germany
¹² Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen Im Neuenheimer Feld 205 D-69120 Heidelberg, Germany
¹³ Harvard-Smithsonian Center for Astrophysics 60 Garden Street Cambridge MA 02138, USA
¹⁴ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University 10 Garden Street Cambridge MA 02138, USA
¹⁵ Department of Physics, Tamkang University, No.151, Yingzhuan Road Tamsui District New Taipei City 251301, Taiwan
¹⁶ School of Physics and Astronomy, University of, N Haugh St Andrews KY16 9SS, United Kingdom
¹⁷ Department of Physics & Astronomy, University of Wyoming Laramie WY 82071, USA
¹⁸ Department of Physics, University of Arkansas, 226 Physics Building 825 West Dickson Street Fayetteville AR 72701, USA
¹⁹ Department of Astronomy, University of Cape Town Rondebosch 7701, South Africa
²⁰ Sub-department of Astrophysics, Department of Physics, University of Oxford Keble Road Oxford OX1 3RH, UK
²¹ UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester Oxford Road Manchester M13 9PL, UK
²² Department of Physics and Astronomy, The Johns Hopkins University Baltimore MD 21218, USA
²³ IRAP/OMP/Université de Toulouse, 9 Av. du Colonel Roche BP 44346 F-31028 Toulouse cedex 4, France
²⁴ Space Telescope Science Institute Baltimore MD 21218, USA

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Received: 12 June 2025
Accepted: 7 November 2025

Abstract

High-resolution JWST images of nearby spiral galaxies reveal polycyclic aromatic hydrocarbon (PAH) structures that potentially trace molecular clouds, even CO-dark regions. For this paper, we identified ISM cloud structures in PHANGS-JWST 7.7 μm PAH emission maps for 66 galaxies, smoothed to a common physical resolution of 30 pc and at native resolution. We extracted 108 466 cloud structures in the 30 pc sample and 146 040 clouds in the native resolution sample. We then calculated their molecular properties following 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 clouds 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. We specifically used the homogenized sample for our analysis. Upon cross-matching the PAH clouds to the ALMA CO clouds at a homogenized resolution of 90 pc in 27 galaxies, we find that 41% of the PAH clouds are associated with a CO counterpart. We also show that the converted molecular cloud properties of the PAH clouds do not differ much when compared in different galactic environments. However, outside the central environment, the highest molecular mass surface density clouds are preferentially found in spiral arms. We further apply a lognormal fit to the mass spectra to an unprecedented extragalactic completeness limit of 2 × 10³ M_⊙, and find that spiral arms contain the most massive clouds compared to other galactic environments. Our findings support the idea that spiral arm gravitational potentials foster the formation of high surface density clouds, and that lower surface density clouds form in the interarm regions. The cloud Σ_mol values show a decline of a factor of ∼1.5 − 2 toward the outer 2 − 3 R_e. However, the trend largely varies in individual galaxies, with flat, decreasing, and even no trend as a function of R_gal. Factors such as large-scale processes, galaxy types, and morphologies might influence the observed trends. We note that combining homogenized molecular properties of individual galaxies leads to the loss of information about the physical processes that are driving deviations in trends of those properties across different galactic environments. We published two catalogs at the CDS, one at the common resolution of 30 pc and another at the native resolution. We expect them to have broad utility for future studies of PAH clouds, molecular clouds, and star formation.