The host galaxies of z = 7 quasars: predictions from the BlueTides simulation

Abstract

ABSTRACT We examine the properties of the host galaxies of $z=7$ quasars using the large volume, cosmological hydrodynamical simulation BlueTides. We find that the 10 most massive black holes and the 191 quasars in the simulation (with $M_{\textrm{UV,AGN}}\lt M_{\textrm{UV,host}}$) are hosted by massive galaxies with stellar masses $\log (M_\ast /\, {\rm M}_{\odot })=10.8\pm 0.2$, and $10.2\pm 0.4$, which have large star formation rates of $513_{-351}^{+1225}\, {\rm M}_{\odot }/\rm {yr}$ and $191_{-120}^{+288}\, {\rm M}_{\odot }/\rm {yr}$, respectively. The hosts of the most massive black holes and quasars in BlueTides are generally bulge-dominated, with bulge-to-total mass ratio $B/T\simeq 0.85\pm 0.1$; however, their morphologies are not biased relative to the overall $z=7$ galaxy sample. We find that the hosts of the most massive black holes and quasars are compact, with half-mass radii $R_{0.5}=0.41_{-0.14}^{+0.18}$ kpc and $0.40_{-0.09}^{+0.11}$ kpc, respectively; galaxies with similar masses and luminosities have a wider range of sizes with a larger median value, $R_{0.5}=0.71_{-0.25}^{+0.28}$ kpc. We make mock James Webb Space Telescope (JWST) images of these quasars and their host galaxies. We find that distinguishing the host from the quasar emission will be possible but still challenging with JWST, due to the small sizes of quasar hosts. We find that quasar samples are biased tracers of the intrinsic black hole–stellar mass relation, following a relation that is 0.2 dex higher than that of the full galaxy sample. Finally, we find that the most massive black holes and quasars are more likely to be found in denser environments than the typical $M_{\textrm{BH}}\gt 10^{6.5}\, {\rm M}_{\odot }$ black hole, indicating that minor mergers play at least some role in growing black holes in the early Universe.