Publication Date:
2016-12-04
Description:
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-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 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 (H 2 ) abundances in dense filaments around the disc mid-plane, accounting for approximately ~10–15 per cent of the total gas mass. Significant CO fractions only form in the densest, $n_{\mathrm{{\rm H}}}\gtrsim 10^3\,\mathrm{cm}^{-3}$ , gas where a combination of dust, H 2 , and self-shielding attenuates the far-ultraviolet background. We additionally compare these ray-tracing-based solutions to photochemistry with complementary models where photoshielding 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 = 40 K temperature cap performs the best in regard to a number of different quantitative measures based on the H 2 and CO abundances.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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