Chemistry and radiative shielding in star-forming galactic discs

Abstract

To understand the conditions under which dense, molecular gas is able to form within a galaxy, we post-process a series of three-dimensional galactic-disc-scale simulations with ray-tracingbased radiative transfer and chemical network integration to compute the equilibrium chemical and thermal state of the gas. In performing these simulations, we vary a number of parameters, such as the interstellar radiation field strength, vertical scaleheight of stellar sources, and cosmic ray flux, to gauge the sensitivity of our results to these variations. Self-shielding permits significant molecular hydrogen (H2) abundances in dense filaments around the disc mid-plane, accounting for approximately~10-15 per cent of the total gasmass. SignificantCO fractions only form in the densest, nH ≳ 103 cm-3, gas where a combination of dust, H2, and self-shielding attenuates the far-ultraviolet background. We additionally compare these raytracing- based solutions to photochemistry with complementary models where photoshielding is accounted forwith locally computed prescriptions.With some exceptions, these local models for the radiative shielding length perform reasonably well at reproducing the distribution and amount of molecular gas as compared with a detailed, global ray-tracing calculation. Specifically, an approach based on the Jeans length with a T = 40K temperature cap performs the best in regard to a number of different quantitative measures based on the H2 and CO abundances