The Effect Of AGN Heating On The Low-Redshift Ly{\alpha} Forest
  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. 
Alex Gurvich, Blakesley Burkhart, Simeon Bird
astro-ph.CO astro-ph.GA