Chemistry and radiative shielding in star forming galactic disks
  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.
