Author:
den Brok Jakob,Bigiel Frank,
Abstract
Robust knowledge of the distribution, amount, and physical/chemical state of the cold molecular (H2) gas is key to understanding galaxy evolution. With the help of multi-CO line observations, it is possible to study the molecular gas distribution and disentangle numerous physical and chemical processes that shape and govern the molecular interstellar medium (ISM). For the first time, we obtain full-galaxy mapping data of faint CO isotopologues (13CO, C18O, C17O) at 1mm and 3mm wavelengths across the disk of the nearby spiral galaxy M51. With the help of these CO isotopologues, it is possible to constrain the bulk physical and chemical conditions in the molecular gas. We study potential explanations for why CO isotopologue emission varies. Likely drivers include CO abundance variations due to selective nucleosynthesis and changes in the optical depth. Our analysis concludes that a combination of variation in opacity and relative abundances is the dominant driver for the observed CO isotopologue ratio trends on large (kpc) scales. In contrast, abundance variation due to selective photodissociation and chemical fractionation seem to only play a minor or negligible role on galaxy-wide scales.