The Effect Of AGN Heating On The Low-Redshift Ly{\alpha} Forest

TL;DR

This study uses the Illustris simulation to show that AGN feedback and a more ionizing ultraviolet background can explain the observed properties of the low-redshift Lyman-alpha forest, addressing the photon underproduction crisis.

Contribution

It demonstrates that AGN feedback and updated UV background models in simulations can reproduce observed low-redshift Lyman-alpha forest data, resolving previous discrepancies.

Findings

Illustris reproduces observations at $z=0.1$ for certain column densities.

AGN feedback influences gas distribution, mimicking extra photon effects.

Discrepancy at higher column densities suggests need for refining AGN feedback models.

Abstract

We investigate the effects of AGN heating and the ultraviolet background on the low-redshift Lyman-$\alpha$ forest column density distribution (CDD) using the Illustris simulation. We show that Illustris reproduces observations at $z =0.1$ in the column density range $10^{12.5} - 10^{13.5}$cm$^{-2}$, relevant for the "photon underproduction crisis." We attribute this to the inclusion of AGN feedback, which changes the gas distribution so as to mimic the effect of extra photons, as well as the use of the Faucher-Gigu\`ere ultraviolet background, which is more ionizing at $z=0.1$ than the Haardt & Madau background previously considered. We show that the difference between simulations run with smoothed particle hydrodynamics and simulations using a moving mesh is small in this column density range but can be more significant at larger column densities. We further consider the effect of supernova feedback, Voigt profile fitting and finite resolution, all of which we show to have little influence on the CDD. Finally, we identify a discrepancy between our simulations and observations at column densities $10^{14} - 10^{16}$cm$^{-2}$, where Illustris produces too few absorbers, which suggests the AGN feedback model should be further refined. Since the "photon underproduction crisis" primarily affects lower column density systems, we conclude that AGN feedback and standard ionizing background models can resolve the crisis.