Probing the Dependence of the Intergalactic Medium on Large-scale Environment Using the Low-redshift Lyα Forest

We examine the statistics of the low-redshift Ly alpha forest in a cosmological hydrodynamic simulation using adaptive mesh refinement that has sufficient volume to include distinct large-scale environments. We compare our H I column density distribution of absorbers both with recent work and between two highly refined regions of our simulation: a large-scale overdensity and a large-scale underdensity (on scales of approximately 20 Mpc). We recover the average results presented by Kollmeier et al. using different simulation methods. We further break down these results as a function of environment to examine the detailed dependence of absorber statistics on large-scale density. We find that the slope of the H I column density distribution (CDD) in the range 10(12.5) <= N-H I/cm(-2) <= 10(14.5) depends on environment such that the slope becomes steeper for higher environmental density, and this difference reflects distinct physical conditions of the intergalactic medium (IGM) on these scales. We track this difference to the different temperature structures of filaments in varying environments. Specifically, filaments in the overdensity are hotter, and correspondingly are composed of gas with lower H I fractions than those in underdense environments. Our results highlight that in order to understand the physics driving the H I CDD, we need not only improved accounting of the sources of ionizing UV photons, but also a clearer picture of the physical conditions of the IGM and how it may vary as a function of large-scale environment.