Abstract
The dark matter halo mass function is an extremely important quantity that
will be measured to high precision by ongoing and next generation surveys. It
is therefore crucial to determine which are the theoretical uncertainties that
need to be taken into account when comparing the observed halo mass function to
the predictions of theoretical cosmological models and numerical simulations.
In this paper we study the effect of baryonic processes on the halo mass
function in the galaxy cluster mass range using a catalogue of 153 high
resolution cosmological hydrodynamical simulations performed with the AMR code
ramses. We use the results of our simulations within a simple analytical model
to gauge the effects of baryon physics on the halo mass function. We find that
neglect of AGN feedback leads to a significant boost in the cluster mass
function similar to that reported by other authors. However, including AGN
feedback not only gives rise to systems that are similar to observed galaxy
clusters, but they also reverse the global baryonic effects on the clusters.
The resulting mass function is closer to the unmodified dark matter halo mass
function but still contains a mass dependent bias at the 5-10% level. We then
explore how these effects bias measurements of the cosmological parameters,
such as $\sigma_8$ and $Ømega_m$. For current cluster surveys baryonic effects
are within the noise for current survey volumes, but forthcoming and planned
large surveys will be highly biased by these processes.
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