The impact of different physical processes on the statistics of Lyman-limit and damped Lyman α absorbers

We compute the z = 3 neutral hydrogen column density distribution function f(N-Hi) for 19 simulations drawn from the Overwhelmingly Large Simulations project using a post-processing correction for self-shielding calculated with full radiative transfer of the ionizing background radiation. We investigate how different physical processes and parameters affect the abundance of Lyman-limit systems (LLSs) and damped Lyman alpha absorbers including: (i) metal-line cooling; (ii) the efficiency of feedback from supernovae and active galactic nuclei; (iii) the effective equation of state for the interstellar medium; (iv) cosmological parameters; (v) the assumed star formation law and (vi) the timing of hydrogen reionization. We find that the normalization and slope, D = d log(10) f/d log(10) N-H1, of f(N-Hi) in the LLS regime are robust to changes in these physical processes. Among physically plausible models, f(N-Hi) varies by less than 0.2 dex and D varies by less than 0.18 for LLSs. This is primarily due to the fact that these uncertain physical processes mostly affect star-forming gas which contributes less than 10 per cent to f(N-Hi) in the LLS column density range. At higher column densities, variations in f(N-Hi) become larger (approximately 0.5 dex at f(N-Hi) = 10(22) cm(-2) and 1.0 dex at f(N-Hi) = 10(22) cm(-2)) and molecular hydrogen formation also becomes important. Many of these changes can be explained in the context of self-regulated star formation in which the amount of star-forming gas in a galaxy will adjust such that outflows driven by feedback balance inflows due to accretion. Tools to reproduce all figures in this work can be found at the following url: https://bitbucket.org/galtay/hi-cddf-owls-1.