The clustering of $z > 7$ galaxies: Predictions from the BLUETIDES simulation

Abstract

We study the clustering of the highest-z galaxies (from ~ $0.1$ to a few tens Mpc scales) using the BLUETIDES simulation and compare it to current observational constraints from Hubble legacy and Hyper Suprime Cam (HSC) fields (at $z=6-7.2$). With a box length of $400$ $Mpc/h$ on each side and $0.7$ trillion particles, BLUETIDES is the largest high resolution cosmological hydrodynamic simulation to date ideally suited for studies of high-z galaxies. We find that galaxies with magnitude $m_{UV}<27.7$ have a bias ($b_g$) of $8.1\pm 1.2$ at $z=8$, and typical halo masses $M_H \gtrsim 6\times10^{10} M_{\odot}$. Given the redshift evolution between $z=8$ to $z=10$ ($b_g\propto(1+z)^{1.6}$), our inferred values of the bias and halo masses are consistent with measured angular clustering at $z \sim 6.8$ from these brighter samples. The bias of fainter galaxies (in the Hubble legacy field at $H_{160} \lesssim29.5$) is $5.9\pm0.9$ at $z=8$ corresponding to halo masses $M_H \gtrsim 10^{10} M_{\odot}$. We investigate directly the 1-halo term inthe clustering and show that it dominates on scales $r \lesssim 0.1$ Mpc/$h$ ($\Theta \lesssim 3"$) with non-linear effect at transition scales between the 1-halo and 2-halo term affecting scales 0.1 $\lesssim r \lesssim $ 20 Mpc/$h$ ($3"\lesssim \Theta \lesssim 90"$). Current clustering measurements probe down to the scales in the transition between 1-halo to 2-halo regime where non-linear effects are important. The amplitude of the 1-halo term implies that occupation numbers for satellites in \texttt{BLUETIDES} are somewhat higher than standard HODs adopted in these analyses (which predict amplitudes in the 1-halo regime suppressed by a factor 2-3). That possibly implies a higher number of galaxies detected by JWST (at small scales and even fainter magnitudes) observing these fields.