Localized Deviations from the CO–Polycyclic Aromatic Hydrocarbon Relation in PHANGS-JWST Galaxies: Faint Polycyclic Aromatic Hydrocarbon Emission or Elevated CO Emissivity?

Abstract Polycyclic aromatic hydrocarbon (PAH) emission is widely used to trace the distribution of molecular gas in the interstellar medium, exhibiting a tight correlation with CO(2–1) emission across nearby galaxies. Using PHANGS-JWST and PHANGS-Atacama Large Millimeter/submillimeter Array (ALMA) data, we identify localized regions where this correlation fails, with CO flux exceeding that predicted from 7.7 μ m PAH emission by more than an order of magnitude. These outlier regions are found in 20 out of 70 galaxies and are located in galaxy centers and bars, without signs of massive star formation. We explore two scenarios to explain the elevated CO-to-PAH ratios, which can either be due to suppressed PAH emission or enhanced CO emissivity. We examine PAH emission in other bands (3.3 and 11.3 μ m) and the dust-continuum-dominated bands (10 and 21 μ m), finding consistently high CO-to-PAH (or CO-to-dust continuum) emission ratios, suggesting that 7.7 μ m PAH emission is not particularly suppressed. In some outlier regions, PAH sizes and spectral energy distribution of the radiation differ slightly from nearby control regions with normal CO-to-PAH ratios, though without a consistent trend. We find that the outlier regions show higher CO velocity dispersions (Δ v CO ). This increase in Δ v CO lowers CO optical depth and raises its emissivity for a given gas mass. Our results favor a scenario where shear along the bar lanes and shocks at the bar ends elevate CO emissivity, leading to the breakdown of the CO–PAH correlation. Future JWST spectroscopy and deep ALMA observations of CO isotopologues will provide critical tests of this scenario.

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Journal
The Astrophysical Journal
Published
2026-04-06
DOI
https://doi.org/10.3847/1538-4357/ae4599
Primary Topic
Astrophysics and Star Formation Studies
Type
article
Field-Weighted Citation Impact
0.00

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article

Localized Deviations from the CO–Polycyclic Aromatic Hydrocarbon Relation in PHANGS-JWST Galaxies: Faint Polycyclic Aromatic Hydrocarbon Emission or Elevated CO Emissivity?

Karin M. Sandstrom, Susan E. Clark, H. L. Wendy Pan, Ryan Chown et al.
The Astrophysical Journal
Astrophysics and Star Formation Studies
article

Localized Deviations from the CO–Polycyclic Aromatic Hydrocarbon Relation in PHANGS-JWST Galaxies: Faint Polycyclic Aromatic Hydrocarbon Emission or Elevated CO Emissivity?

Karin M. Sandstrom, Susan E. Clark, H. L. Wendy Pan, Ryan Chown, Lukas Neumann, Tony D. Weinbeck, Simon C. O. Glover, Dalya Baron, Jessica Sutter, I-Da Chiang, Rupali Chandar, Eric W. Koch, Kathryn Kreckel, Hannah Koziol, S. Meidt, Jakob den Brok, Miguel Querejeta, Sophia K. Stuber, Jaeyeon Kim, Yu-Hsuan Teng, Adam K. Leroy, Daniel A. Dale, Ralf S. Klessen, Kathryn Grasha, Oleg V. Egorov, Thomas G. Williams, Eva Schinnerer, Ashley T. Barnes
article en

Abstract

Abstract Polycyclic aromatic hydrocarbon (PAH) emission is widely used to trace the distribution of molecular gas in the interstellar medium, exhibiting a tight correlation with CO(2–1) emission across nearby galaxies. Using PHANGS-JWST and PHANGS-Atacama Large Millimeter/submillimeter Array (ALMA) data, we identify localized regions where this correlation fails, with CO flux exceeding that predicted from 7.7 μ m PAH emission by more than an order of magnitude. These outlier regions are found in 20 out of 70 galaxies and are located in galaxy centers and bars, without signs of massive star formation. We explore two scenarios to explain the elevated CO-to-PAH ratios, which can either be due to suppressed PAH emission or enhanced CO emissivity. We examine PAH emission in other bands (3.3 and 11.3 μ m) and the dust-continuum-dominated bands (10 and 21 μ m), finding consistently high CO-to-PAH (or CO-to-dust continuum) emission ratios, suggesting that 7.7 μ m PAH emission is not particularly suppressed. In some outlier regions, PAH sizes and spectral energy distribution of the radiation differ slightly from nearby control regions with normal CO-to-PAH ratios, though without a consistent trend. We find that the outlier regions show higher CO velocity dispersions (Δ v CO ). This increase in Δ v CO lowers CO optical depth and raises its emissivity for a given gas mass. Our results favor a scenario where shear along the bar lanes and shocks at the bar ends elevate CO emissivity, leading to the breakdown of the CO–PAH correlation. Future JWST spectroscopy and deep ALMA observations of CO isotopologues will provide critical tests of this scenario.

The Astrophysical JournalVol. 1001(1)
Tamkang University (TW), Australian National University (AU), University of Wyoming (US), Institute of Astronomy and Astrophysics, Academia Sinica (TW), European Southern Observatory (DE), Whitman College (US), Heidelberg University (DE), National Radio Astronomy Observatory (US), University of Manchester (GB), Ghent University (BE), Kyung Hee University (KR), University of California San Diego (US), Kavli Institute for Particle Astrophysics and Cosmology (US), Max Planck Institute for Astronomy (DE), Spanish National Observatory (ES), Max Planck Institute for Radio Astronomy (DE), National Astronomical Observatory of Japan (JP), Columbus Community Clinical Oncology Program (US), The Ohio State University (US), University of Maryland, College Park (US), Algoma University (CA), University of Toledo (US), Stanford University (US)
Deutsche Forschungsgemeinschaft
Openalex Percentile: Top 26%
Astrophysics and Star Formation Studies
DOIOpen Access PDFOpenAlex
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