Unravelling obese black holes in the first galaxies

We predict the existence and observational signatures of a new class of objects that assembled early, during the first billion years of cosmic time: obese black hole galaxies (OBGs). OBGs are objects in which the mass of the central black hole (BH) initially exceeds that of the stellar component of the host galaxy, and the luminosity from BH accretion dominates the starlight. Conventional wisdom dictates that the first galaxies light up with the formation of the first stars; we show here that, in fact, there could exist a population of astrophysical objects in which this is not the case. From a cosmological simulation, we demonstrate that there are sites where star formation is initially inhibited and direct-collapse black holes (DCBHs) form due to the photodissociating effect of Lyman-Werner radiation on molecular hydrogen. We show that the formation of OBGs is inevitable, because the probability of finding the required extragalactic environment and the right physical conditions in a halo conducive to DCBH formation is quite high in the early Universe. We estimate an OBG number density of 0.009 Mpc(-3) at z similar to 8 and 0.03 Mpc(-3) at z similar to 6. Extrapolating from our simulation volume, we infer that the most luminous quasars detected at z >= 6 likely transited through an earlier OBG phase. Following the growth history of DCBHs and their host galaxies in an evolving dark matter halo shows that these primordial galaxies start off with an overmassive BH and acquire their stellar component from subsequent merging as well as in situ star formation. In doing so, they inevitably go through an OBG phase dominated by the accretion luminosity at the Eddington rate or below, released from the growing BH. The OBG phase is characterized by an ultraviolet (UV) spectrum f(lambda) proportional to lambda(beta) with a slope of beta similar to -2.3 and the absence of a Balmer break. OBGs should also be spatially unresolved, and are expected to be brighter than the majority of known high-redshift galaxies. They could also display broad high-excitation emission lines, as expected from type I active galactic nuclei, although the strength of lines such as N v and C iv will obviously depend on the chemical enrichment of the host galaxy. OBGs could potentially be revealed via Hubble Space Telescope follow-up imaging of samples of brighter Lyman-break galaxies provided by wide-area ground-based surveys such as UltraVISTA, and should be easily uncovered and studied with instruments aboard the James Webb Space Telescope. The discovery and characterization of OBGs would provide important insights into the formation of the first BH, and their influence on early galaxy formation.