Probing the Dependence of the Intergalactic Medium on Large Scale Environment Using the Low Redshift Lyman Alpha Forest

TL;DR

This study investigates how the large-scale environment influences the properties of the low-redshift Lyman Alpha forest using high-resolution cosmological simulations, revealing environmental dependence in absorber statistics and IGM physical conditions.

Contribution

It provides the first detailed analysis of the environmental dependence of the low-redshift Ly-alpha forest using adaptive mesh refinement simulations, linking absorber properties to large-scale density and temperature structures.

Findings

The HI column density distribution slope varies with environment.

Filaments in overdense regions are hotter with lower HI fractions.

Environmental effects influence the physical conditions of the intergalactic medium.

Abstract

We examine the statistics of the low-redshift Ly-alpha forest in an adaptive mesh refinement hydrodynamic cosmological simulation of sufficient volume to include distinct large-scale environments. We compare our HI 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 in Kollmeier et al. (2014) 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 HI column density distribution in the 10$^{12.5}$ $\le$ N$_{HI}$/cm$^{-2}$ $\le$ 10$^{14.5}$ range depends on environment such that the slope becomes steeper for higher environmental density, and this difference reflects distinct physical conditions of the intergalactic medium 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 HI fractions than those in underdense environments. Our results highlight that in order to understand the physics driving the HI CDD, we need not only improved accounting of the sources of ionizing UV photons, but also of the physical conditions of the IGM and how this may vary as a function of large-scale environment.