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

The most distant known quasar recently discovered by Bañados et al. (2018) is at z=7.5 (690 Myr after the Big Bang), at the dawn of galaxy formation. We explore the host galaxy of the brightest quasar in the large volume cosmological hydrodynamic simulation BlueTides, which in Phase II has reached these redshifts. The brightest quasar in BlueTides has a luminosity of a ~ few 10 13 L ? and a black hole mass of 6.4×10 8 M ? at z~7.5 , comparable to the observed quasar (the only one in this large volume). The quasar resides in a rare halo of mass M H ~10 12 M ? and has a host galaxy of stellar mass of 4×10 10 M ? with an ongoing (intrinsic) star formation rate of ~80M ? yr -1. The corresponding intrinsic UV magnitude of the galaxy is -23.1 , which is roughly 2.7 magnitudes fainter than the quasar's magnitude of -25.9. We find that the galaxy is highly metal enriched with a mean metallicity equal to the solar value. We derive quasar and galaxy spectral energy distribution (SED) in the mid and near infrared JWST bands. We predict a significant amount of dust attenuation in the rest-frame UV corresponding to A 1500 ~1.7 giving an UV based SFR of ~14M ? yr -1. We present mock JWST images of the galaxy with and without central point source, in different MIRI and NIRCam filters. The host galaxy is detectable in NIRCam filters, but it is extremely compact (R E =0.35 kpc). It will require JWST's exquisite sensitivity and resolution to separate the galaxy from the central point source. Finally within the FOV of the quasar in BlueTides there are two more sources that would be detectable by JWST.