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

Madeline A. Marshall, Yueying Ni, Tiziana Di Matteo, J. Stuart B. Wyithe, Stephen Wilkins, Rupert A. C. Croft

Published 2019-12-07Version 1

We examine the properties of the host galaxies of $z=7$ quasars using the large volume, cosmological hydrodynamical simulation BlueTides. We find that the most massive black holes and quasars are hosted by massive galaxies with stellar masses $\log(M_\ast/M_\odot)=10.8\pm0.2$, and $10.2\pm0.4$, which have a wide range of star formation rates, of $50\substack{+119 \\ -35}M_\odot/\rm{yr}$ and $19\substack{+28 \\ -12}M_\odot/\rm{yr}$, respectively. The hosts of the most massive black holes and quasars in BlueTides are generally bulge-dominated, with bulge-to-total mass ratio $B/T\simeq0.85\pm0.1$, 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 significantly more compact, with half-mass radii $R_{0.5}=0.29\substack{+0.15 \\ -0.10}$ kpc and $0.28\substack{+0.08 \\ -0.06}$ kpc respectively, relative to galaxies with similar masses, which have $R_{0.5}=0.5\substack{+0.3 \\ -0.2}$ 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 currently observable quasar samples are biased tracers of the intrinsic black hole--stellar mass relations, following a relation that is 0.27 dex higher than that of the full galaxy sample. Finally, we find that black hole hosts are generally quite isolated. However, the most massive black holes are more likely to be found in denser environments than the typical $M_{\textrm{BH}}>10^{6.5}M_\odot$ black hole, indicating that minor mergers at least play some role in growing black holes in the early Universe.