Decoding Molecular Gas Conditions: Insights from Comprehensive CO Isotopologue Surveys across Nearby Galaxies
Jakob den Brok
Thursday, Dec. 5th, 14:40CET
The PHANGS-ALMA Large Program has reshaped our understanding of the intricate relationship between molecular gas and star formation across various environments within nearby galaxies. The 12CO(2-1) line has proven to be a robust tracer, probing fundamental properties of molecular gas in giant molecular clouds. However, as an optically thick emission line, we face inherent challenges when interpreting its emission. In contrast, the fainter 13CO and C18O lines generally remain optically thin, offering critical insights and enabling a more nuanced exploration of the diverse molecular gas conditions prevalent in extragalactic environments. My talk aims to present and establish new benchmarks in our comprehension of molecular gas properties, such as temperature, density, opacity, and isotopologue species abundances, by incorporating additional observations from CO isotopologues across entire galaxies. Drawing from the findings of den Brok+(2022, 2023, 2024) and leveraging data from the IRAM 30m, SMA, and ALMA large program, I will showcase the ability to characterize the molecular gas in distinct environments across star-forming galaxies. The current sample includes the nearby galaxies M101 and M51, which, in combination, allow us to probe a large dynamical range of properties, including AGN influence (M51) and large metallicity gradients (1 dex in M101), which we expect to affect the molecular gas significantly. The combination of various CO isotopologue line ratios proves instrumental in discerning the impact of selective nucleosynthesis on C and O gas abundances and the influence of gas turbulence on line opacities. These expensive interferometric observing programs allow us to probe the ISM at high angular resolution, and I will show how particularly the observed contrast between center and disk reflects changes in the molecular gas dynamics and conditions at GMC-scale. Including CO isotopologue lines opens a new window for studying molecular gas in nearby galaxies with significant implications for modeling the physical and chemical processes regulating star formation. These comprehensive surveys offer critical insights for the high redshift community, which relies on calibrating higher-J CO lines to probe the molecular gas conditions. This work marks a pivotal step towards characterizing CO-bright, star-forming molecular gas, and enhancing our grasp of galactic evolution's intricate processes.