Abstract
Observations of the most distant bright quasars imply that billion solar mass supermassive black holes (SMBH) have to be assembled within the first eight hundred million years. Under our standard galaxy formation scenario such fast growth implies large gas densities providing sustained accretion at critical or supercritical rates onto an initial black hole seed. It has been a long standing question whether and how such high black hole accretion rates can be achieved and sustained at the centers of early galaxies. Here we use our new cosmological hydrodynamic simulation (MassiveBlack) covering a volume (0.75 \Gpc)^3 appropriate for studying the rare first quasars to show that steady high density cold gas flows responsible for assembling the first galaxies produce the high gas densities that lead to sustained critical accretion rates and hence rapid growth commensurate with the existence of ~10^9 solar mass black holes as early as z~7. We find that under these conditions quasar feedback is not effective at stopping the cold gas from penetrating the central regions and hence cannot quench the accretion until the host galaxy reaches M_halo > 10^{12} solar masses. This cold-flow driven scenario for the formation of quasars implies that they should be ubiquitous in galaxies in the early universe and that major (proto)galaxy mergers are not a requirement for efficient fuel supply and growth, particularly for the earliest SMBHs.
Highlights
It is well established that the properties of supermassive black holes (SMBH) found at the centers of galaxies today are tightly coupled to those of their hosts implying a strong link between black hole and galaxy formation
The cosmological gas density distribution in the full volume of the MassiveBlack is shown in the large scale image of Figure 1
With our new large cosmological simulation MassiveBlack we show that the short timescale associated with infall via cold flows and the short cooling timescales in cold radial streams that penetrate the halo render the flow into the central regions unstoppable by feedback allowing it to sustain BH growth at the Eddington rates to build up the required BH masses by z = 6 − 7
Summary
It is well established that the properties of supermassive black holes (SMBH) found at the centers of galaxies today are tightly coupled to those of their hosts implying a strong link between black hole and galaxy formation. Rare (the comoving space density of z ∼ 6 quasars is roughly n ∼ a few Gpc−3) the inferred hole masses of these quasars are in excess of 109 M⊙ comparable to the masses of the most massive black holes in the Universe today. The origin of these massive black hole seed and the physical conditions that allow early growth to supermassive black holes remain a challenging problem. Two distinct populations of seed masses, in the range of 100 − 105 M⊙ have been proposed: the small mass seeds are usually thought to be the remnants of the first generation of PopIII stars formed of metalfree gas at z ∼ 20 − 30 (Bromm et al 1999; Abel et al 2000; Nakamura & Umemura 2001; Yoshida et al 2003; Gao et al 2006, e.g.), while the large seeds form in direct dynamical collapse in metal-free galaxies
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