Abstract
We investigate the low-redshift Lyman-alpha forest column density
distribution in the Illustris simulation. We show that Illustris reproduces
observations extremely well in the column density range 10^12.5-10^14.5 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-Giguere (2009)
ultra-violet background, which is more ionizing at z=0.1 than the Haardt &
Madau (2012) 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. We further consider
the effect of supernova feedback, Voigt profile fitting and finite resolution,
all of which we show to be small. 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 in
cosmological simulations should be further refined. However the "photon
underproduction crisis," can be resolved by including AGN feedback and standard
ionizing background models.
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