CO Isotopologue-derived Molecular Gas Conditions and CO-to-H<sub>2</sub> Conversion Factors in M51

Abstract Over the past decade, several millimeter interferometer programs have mapped the nearby star-forming galaxy M51 at a spatial resolution of ≤170 pc. This study combines observations from three major programs: the PdBI Arcsecond Whirlpool Survey, the SMA M51 large program, and the Surveying the Whirlpool at Arcseconds with NOEMA. The data set includes the (1–0) and (2–1) rotational transitions of 12 CO, 13 CO, and C 18 O isotopologues. The observations cover the r &lt; 3 kpc region, including the center and part of the disk, thereby ensuring strong detections of the weaker 13 CO and C 18 O lines. All observations are convolved in this analysis to an angular resolution of 4″, corresponding to a physical scale of 170 pc. We investigate empirical line ratio relations and quantitatively evaluate molecular gas conditions such as temperature, density, and the CO-to-H 2 conversion factor ( α CO ). We employ two approaches to study the molecular gas conditions: (i) assuming local thermodynamic equilibrium (LTE) to analytically determine the CO column density and α CO , and (ii) using non-LTE modeling with RADEX to fit physical conditions to observed CO isotopologue intensities. We find that the α CO values in the center and along the inner spiral arm are ∼0.5 dex (LTE) and 0.1 dex (non-LTE) below the Milky Way inner disk value. The average non-LTE α CO is 2.4 ± 0.5 M ⊙ pc −2 (K km s −1 ) −1 . While both methods show dispersion due to underlying assumptions, the scatter is larger for LTE-derived values. This study underscores the necessity for robust CO line modeling to accurately constrain the molecular interstellar medium’s physical and chemical conditions in nearby galaxies.

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Publication Details

Journal
The Astronomical Journal
Published
2024-12-10
DOI
https://doi.org/10.3847/1538-3881/ad888a
Citations
6
Primary Topic
Astrophysics and Star Formation Studies
Type
article
Field-Weighted Citation Impact
1.99

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article

CO Isotopologue-derived Molecular Gas Conditions and CO-to-H2 Conversion Factors in M51

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6 citations
The Astronomical Journal
Astrophysics and Star Formation Studies
1.99
article

CO Isotopologue-derived Molecular Gas Conditions and CO-to-H2 Conversion Factors in M51

Karin M. Sandstrom, Yu-Hsuan Teng, Valencia Zhang, J. Pety, David J. Wilner, Mélanie Chevance, Erik Rosolowsky, Sophia K. Stuber, Miguel Querejeta, Jakob S. den Brok, Frank Bigiel, Thomas G. Williams, Simon C. O. Glover, Eva Schinnerer, P. K. Humire, Cosima Eibensteiner, Adam K. Leroy, María J. Jiménez-Donaire, A. Usero, Eric W. Koch, G. Petitpas, Hsi-An Pan, Daizhong Liu, Daniel A. Dale, Qizhou Zhang, Rowan J. Smith, Ina Galić
article en
6 citations

Abstract

Abstract Over the past decade, several millimeter interferometer programs have mapped the nearby star-forming galaxy M51 at a spatial resolution of ≤170 pc. This study combines observations from three major programs: the PdBI Arcsecond Whirlpool Survey, the SMA M51 large program, and the Surveying the Whirlpool at Arcseconds with NOEMA. The data set includes the (1–0) and (2–1) rotational transitions of 12 CO, 13 CO, and C 18 O isotopologues. The observations cover the r &lt; 3 kpc region, including the center and part of the disk, thereby ensuring strong detections of the weaker 13 CO and C 18 O lines. All observations are convolved in this analysis to an angular resolution of 4″, corresponding to a physical scale of 170 pc. We investigate empirical line ratio relations and quantitatively evaluate molecular gas conditions such as temperature, density, and the CO-to-H 2 conversion factor ( α CO ). We employ two approaches to study the molecular gas conditions: (i) assuming local thermodynamic equilibrium (LTE) to analytically determine the CO column density and α CO , and (ii) using non-LTE modeling with RADEX to fit physical conditions to observed CO isotopologue intensities. We find that the α CO values in the center and along the inner spiral arm are ∼0.5 dex (LTE) and 0.1 dex (non-LTE) below the Milky Way inner disk value. The average non-LTE α CO is 2.4 ± 0.5 M ⊙ pc −2 (K km s −1 ) −1 . While both methods show dispersion due to underlying assumptions, the scatter is larger for LTE-derived values. This study underscores the necessity for robust CO line modeling to accurately constrain the molecular interstellar medium’s physical and chemical conditions in nearby galaxies.

The Astronomical JournalVol. 169(1)
National Science Foundation, Deutsche Forschungsgemeinschaft, Fundação de Amparo à Pesquisa do Estado de São Paulo, Natural Sciences and Engineering Research Council of Canada, Science and Technology Facilities Council, HORIZON EUROPE European Research Council
Openalex Percentile: Top 14%
Astrophysics and Star Formation Studies
1.99
DOIOpen Access PDFOpenAlex
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