A tiny host galaxy for the first giant black hole: Z = 7.5 quasar in BlueTides

Abstract

The most distant known quasar recently discovered by Banados et al. is at z = 7.5. We explore the host galaxy of the brightest quasar in the large-volume cosmological hydrodynamic simulation BlueTides. The brightest quasar in BlueTides has a luminosity of ∼ a few 1013 L☉ and black hole mass of 6.4 x 108 M☉ at z ∼ 7.5, comparable to the observed quasar. The quasar resides in a rare halo of mass MH ∼ 1012 M☉ and has a host galaxy of stellar mass 4 x 1010 M☉ with an ongoing (intrinsic) star formation rate of ∼80 M☉ yr−1. The corresponding intrinsic ultraviolet (UV) magnitude of the galaxy is roughly 2.7 mag fainter than the quasar's magnitude and the galaxy is highly metal enriched with mean metallicity equal to the solar value. We derive the quasar and galaxy spectral energy distribution in the mid- and near-infrared JWST bands. Finally, we predict a significant amount of dust attenuation in the rest-frame UV corresponding to A1500 ∼ 1.7, giving a UV-based star formation rate of ∼14 M☉ yr−1. We present mock JWST images of the galaxy in different MIRI and NIRCam filters. The host galaxy is detectable in NIRCam filters, but is extremely compact (RE = 0.35 kpc). It will require JWST's exquisite sensitivity and resolution to separate the galaxy from the central point source.