Abstract
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 ˜ 8 and 0.03 Mpc-3 at z ˜
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 fλ ∝ λβ with a
slope of β ˜ -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.
Original language | English |
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Pages (from-to) | 3438-3444 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 432 |
Publication status | Published - 1 Jul 2013 |
Keywords
- quasars: general
- dark ages
- reionization
- first stars
- early Universe