HOD modelling of high redshift galaxies using the BLUETIDES simulation

Abstract

We construct halo occupation distribution (HOD) models of high redshift ($z \gtrsim 7.5$) galaxies with $M_{*}>10^8~M_{\odot}/h$ using the BlueTides hydrodynamic simulation suite, with a particular emphasis on modelling the small scale / 1-halo clustering ($0.01\lesssim r \lesssim 1~ h^{-1}\rm{Mpc}$). Similar to low redshift studies, we find that the central and satellite mean HODs ($\left<N_{\mathrm{cen}}\right>$ and $\left<N_{\mathrm{sat}}\right>$) can be modeled by a smoothed step function and a power law respectively. The number density of satellite galaxies is however significantly suppressed compared to low redshift (satellite fractions drop from $\sim 50 \%$ at $z=0$ to $\lesssim 10 \%$ at $z=7.5$). The mean number of satellites, $\left<N_{\mathrm{sat}}\right> < 1$ for halo masses below $3 \times 10^{11} M_{\odot}/h$ (a rare halo at these redshifts). For the radial number density profiles, satellites with $10^8 \lesssim M^* \lesssim 10^{9} M_{\odot}/h$ in halos with $M_H \gtrsim 3 \times10^{11} M_{\odot}/h$ are consistent with NFW (with concentrations $c_{\mathrm{sat}} \sim 10-40$). Within halos of mass $M_H\lesssim 3 \times 10^{11} M_{\odot}/h$ satellites exhibit a power law profile with slope -3. Because these halos dominate the small scale clustering, the resulting 1-halo term is steeper than predicted using standard NFW profiles. Using this power-law profile for satellites, we can successfully reproduce the small-scale clustering exhibited by BlueTides galaxies using HOD modelling. We predict the highest probability of detecting satellites at $z>7.5$ is around centrals of $M^*\sim 3 \times 10^{10} M_{\odot}/h$ (with $M^{*}\gtrsim$ a few $10^{7} M_{\odot}/h$ ). This should be achievable with the James Webb Space Telescope (JWST).