The origin of the most massive black holes at high-z: BlueTides and the next quasar frontier

Abstract The growth of the most massive black holes in the early Universe, consistent with the detection of highly luminous quasars at z > 6 implies sustained, critical accretion of material to grow and power them. Given a black hole (BH) seed scenario, it is still uncertain which conditions in the early Universe allow the fastest BH growth. Large-scale hydrodynamical cosmological simulations of structure formation allow us to explore the conditions conducive to the growth of the earliest supermassive BHs. We use the cosmological hydrodynamic simulation BlueTides, which incorporates a variety of baryon physics in a (400 h−1Mpc)3 volume with 0.7 trillion particles to follow the earliest phases of BH critical growth. At z = 8 the most massive BHs (a handful) approach masses of $10^{8} \rm \thinspace M_{{\odot }}$ with the most massive (with $M_{\rm BH} = 4 \times 10^8\,\rm \thinspace M_{{\odot }}$) being found in an extremely compact (compared to present day) spheroid-dominated host galaxy. Examining the large-scale environment of hosts, we find that the initial tidal field is more important than overdensity in setting the conditions for early BH growth. In regions of low tidal fields gas accretes ‘cold’ on to the BH and falls along thin, radial filaments straight into the centre forming the most compact galaxies and most massive BHs at the earliest times. Regions of high tidal fields instead induce larger coherent angular momenta and influence the formation of the first population of massive compact discs. The extreme early growth depends on the early interplay of high gas densities and the tidal field that shapes the mode of accretion. Mergers may play a minor role in the formation of the first generation, rare massive BHs.