Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA
Abstract
We measure the low-J CO line ratio R21=CO(2-1)/CO(1-0), R32=CO(3-2)/CO(2-1), and R31 = CO(3-2)/CO(1-0) using whole-disk CO maps of nearby galaxies. We draw CO(2-1) from PHANGS--ALMA, HERACLES, and follow-up IRAM surveys; CO(1-0) from COMING and the Nobeyama CO Atlas of Nearby Spiral Galaxies; and CO(3-2) from the JCMT NGLS and APEX LASMA mapping. Altogether this yields 76, 47, and 29 maps of R21, R32, and R31 at 20" \sim 1.3 kpc resolution, covering 43, 34, and 20 galaxies. Disk galaxies with high stellar mass, log10 M_* [Msun]=10.25-11 and star formation rate, SFR=1-5 Msun/yr, dominate the sample. We find galaxy-integrated mean values and 16%-84% range of R21 = 0.65 (0.50-0.83), R32=0.50 (0.23-0.59), and R31=0.31 (0.20-0.42). We identify weak trends relating galaxy-integrated line ratios to properties expected to correlate with excitation, including SFR/M_* and SFR/L_CO. Within galaxies, we measure central enhancements with respect to the galaxy-averaged value of \sim 0.18^{+0.09}_{-0.14} dex for R21, 0.27^{+0.13}_{-0.15} dex for R31, and 0.08^{+0.11}_{-0.09} dex for R32. All three line ratios anti-correlate with galactocentric radius and positively correlate with the local star formation rate surface density and specific star formation rate, and we provide approximate fits to these relations. The observed ratios can be reasonably reproduced by models with low temperature, moderate opacity, and moderate densities, in good agreement with expectations for the cold ISM. Because the line ratios are expected to anti-correlate with the CO(1-0)-to-H_2 conversion factor, alphaCO^(1-0), these results have general implications for the interpretation of CO emission from galaxies.
Keywords
Cite
@article{arxiv.2109.11583,
title = {Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA},
author = {Adam K. Leroy and Erik Rosolowsky and Antonio Usero and Karin Sandstrom and Eva Schinnerer and Andreas Schruba and Alberto D. Bolatto and Jiayi Sun and Ashley. T. Barnes and Francesco Belfiore and Frank Bigiel and Jakob S. den Brok and Yixian Cao and I-Da Chiang and Mélanie Chevance and Daniel A. Dale and Cosima Eibensteiner and Christopher M. Faesi and Simon C. O. Glover and Annie Hughes and Maria J. Jiménez Donaire and Ralf S. Klessen and Eric W. Koch and J. M. Diederik Kruijssen and Daizhong Liu and Sharon E. Meidt and Hsi-An Pan and Jérôme Pety and Johannes Puschnig and Miguel Querejeta and Toshiki Saito and Amy Sardone and Elizabeth J. Watkins and Axel Weiss and Thomas G. Williams},
journal= {arXiv preprint arXiv:2109.11583},
year = {2022}
}
Comments
34 pages; 12 figures; accepted for publication in the Astrophysical Journal; data tables available http://www.astronomy.ohio-state.edu/~leroy.42/int_corat_tab.ecsv and http://www.astronomy.ohio-state.edu/~leroy.42/co_model.ecsv before publication