Chemistry and radiative shielding in star forming galactic disks

TL;DR

This study uses detailed simulations to analyze how various factors influence molecular gas formation and chemical states in star-forming galactic disks, highlighting the importance of self-shielding and the effectiveness of local shielding models.

Contribution

It introduces a comprehensive approach combining ray-tracing radiative transfer with chemical networks to study molecular gas formation in galactic disks, and evaluates local shielding prescriptions against detailed models.

Findings

Self-shielding enables ~10-15% molecular hydrogen in dense filaments.

Significant CO forms only in dense regions with n>10^3 cm^-3.

Local shielding models, especially the Jeans Length approach, approximate detailed calculations well.

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-disk-scale simulations with ray-tracing based 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 ISRF strength, vertical scale height of stellar sources, 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 disk midplane, accounting for approximately ~10-15% of the total gas mass. Significant CO fractions only form in the densest, n>~10^3 cm^-3, gas where a combination of dust, H2, and self-shielding attenuate the FUV background. We additionally compare these ray-tracing based solutions to photochemistry with complementary models where photo-shielding is accounted for with 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 regards to a number of different quantitative measures based on the H2 and CO abundances.