35 research outputs found
Driving the Growth of the Earliest Supermassive Black Holes with Major Mergers of Host Galaxies
The formation mechanism of supermassive black holes (SMBHs) in general, and
of SMBHs observed as luminous quasars at redshifts
in particular, remains an open fundamental question. The presence of
such massive BHs at such early times, when the Universe was less than a billion
years old, implies that they grew via either super-Eddington accretion, or
nearly uninterrupted gas accretion near the Eddington limit; the latter, at
first glance, is at odds with empirical trends at lower redshifts, where quasar
episodes associated with rapid BH growth are rare and brief. In this work, I
examine whether and to what extent the growth of the quasar SMBHs can be
explained within the standard quasar paradigm, in which major mergers of host
galaxies trigger episodes of rapid gas accretion below or near the Eddington
limit. Using a suite of Monte Carlo merger tree simulations of the assembly
histories of the likely hosts of the quasars, I investigate (i) their
growth and major merger rates out to , and (ii) how long the feeding
episodes induced by host mergers must last in order to explain the observed quasar population without super-Eddington accretion. The halo major merger
rate scales roughly as , with quasar hosts typically
experiencing major mergers between (), compared to for typical massive galaxies at
(). An example of a viable sub-Eddington SMBH growth
model is one where a host merger triggers feeding for a duration comparable to
the halo dynamical time. These findings suggest that the growth mechanisms of
the earliest quasar SMBHs need not have been drastically different from their
counterparts at lower redshifts.Comment: 24 pages, 5 figures, invited submission to a focus issue on galactic
centers in Classical and Quantum Gravit
Recurring flares from supermassive black hole binaries: implications for tidal disruption candidates and OJ 287
I discuss the possibility that accreting supermassive black hole (SMBH)
binaries with sub-parsec separations produce periodically recurring luminous
outbursts that interrupt periods of relative quiescence. This hypothesis is
motivated by two characteristics found generically in simulations of binaries
embedded in prograde accretion discs: (i) the formation of a central,
low-density cavity around the binary, and (ii) the leakage of gas into this
cavity, occurring once per orbit via discrete streams on nearly radial
trajectories. The first feature would reduce the emergent optical/UV flux of
the system relative to active galactic nuclei powered by single SMBHs, while
the second can trigger quasiperiodic fluctuations in luminosity. I argue that
the quasiperiodic accretion signature may be much more dramatic than previously
thought, because the infalling gas streams can strongly shock-heat via
self-collision and tidal compression, thereby enhancing viscous accretion. Any
optically thick gas that is circularized about either SMBH can accrete before
the next pair of streams is deposited, fueling transient, luminous flares that
recur every orbit. Due to the diminished flux in between accretion episodes,
such cavity-accretion flares could plausibly be mistaken for the tidal
disruptions of stars in quiescent nuclei. The flares could be distinguished
from tidal disruption events if their quasiperiodic recurrence is observed, or
if they are produced by very massive SMBHs that cannot disrupt solar-type
stars. They may be discovered serendipitously in surveys such as LSST or
eROSITA. I present a heuristic toy model as a proof of concept for the
production of cavity-accretion flares, and generate mock light curves and
specta. I also apply the model to the active galaxy OJ 287, whose production of
quasiperiodic pairs of optical flares has long fueled speculation that it hosts
a SMBH binary.Comment: 15 pages, 5 figures. Accepted for publication in MNRA
Electromagnetic signatures of supermassive black hole binaries resolved by PTAs
Pulsar timing arrays (PTAs) may eventually be able to detect not only the
stochastic gravitational-wave (GW) background of SMBH binaries, but also
individual, particularly massive binaries whose signals stick out above the
background. In this contribution, we discuss the possibility of identifying and
studying such `resolved' binaries through their electromagnetic emission. The
host galaxies of such binaries are themselves expected to be also very massive
and rare, so that out to redshifts z~2 a unique massive galaxy may be
identified as the host. At higher redshifts, the PTA error boxes are larger and
may contain as many as several hundred massive-galaxy interlopers. In this
case, the true counterpart may be identified, if it is accreting gas
efficiently, as an active galactic nucleus (AGN) with a peculiar spectrum and
variable emission features. Specifically, the binary's tidal torques expel the
gas from the inner part of the accretion disk, making it unusually dim in X-ray
and UV bands and in broad optical emission lines. The tails of the broad wings
of any FeK-alpha emission line may also be `clipped' and missing. The binary's
orbital motion, as well as the gas motions it induces, may trigger
quasiperiodic variations. These include coherent flux variability, such as
luminous, multi-wavelength flares, as well as Doppler shifts of broad emission
lines and `see-saw' oscillations in the FeK-alpha line. Additional features,
such as evidence for a recent major merger or dual collimated jets, could also
corroborate the counterpart. These properties would make resolved PTA sources
stand out among AGN with similar overall luminosities and allow their
identification.Comment: 18 pages, 5 figures, accepted for publication in a focus issue on
PTAs in Classical and Quantum Gravit
Population III X-Ray Binaries
Understanding of the role of X-rays for driving the thermal evolution of the
intergalactic medium (IGM) at high redshifts is one of important questions in
astrophysics. High-mass X-ray binaries (HMXBs) in early stellar populations are
prime X-ray source; however, their formation efficiency is not well understood.
Using -body simulations, we estimate the HMXB formation rate via mutual
gravitational interactions of nascent, small groups of the Population~III
stars. We find that HMXBs form at a rate of one per
in newly born stars, and that they emit with a power of in the keV band per star formation rate (SFR). This
value is a factor larger than what is observed in star forming
galaxies at lower redshifts; the X-ray production from early HMXBs would have
been even more copious, if they also formed \textit{in situ} or via migration
in protostellar disks. Combining our results with earlier studies suggests that
early HMXBs were highly effective at heating the IGM and leaving a strong 21 cm
signature. We discuss broader implications of our results, such as the rate of
long gamma-ray bursts from Population~III stars and the direct collapse channel
for massive black hole formation.Comment: 19 pages, 8 figures, conference title : Frontier Research in
Astrophysics - II (https://pos.sissa.it/269/
The effect of baryonic streaming motions on the formation of the first supermassive black holes
Observations of quasars at redshifts z > 6 reveal that 10^9 Msol supermassive
black holes (SMBHs) had already formed when the Universe was < 0.9 Gyr old. One
hypothesis for the origins of these SMBHs is that they grew from the remnants
of the first generation of massive stars, which formed in low-mass (~ 10^5 to
10^6 Msol) dark matter minihaloes at z > 20. This is the regime where baryonic
streaming motions--the relative velocities of baryons with respect to dark
matter in the early Universe--most strongly inhibit star formation by
suppressing gas infall and cooling. We investigate the impact of this effect on
the growth of the first SMBHs using a suite of high-fidelity,
ellipsoidal-collapse Monte Carlo merger-tree simulations. We find that the
suppression of seed BH formation by the streaming motions significantly reduces
the number density of the most massive BHs at z > 15, but the residual effect
at lower redshifts is essentially negligible. The streaming motions can reduce
by a factor of few the number density of the most luminous quasars at z ~
10-11, where such objects could be detected by the James Webb Space Telescope.
We conclude, with minor theoretical caveats, that baryonic streaming motions
are unlikely to pose a significant additional obstacle to the formation of the
observed high-redshift quasar SMBHs. Nor do they appreciably affect the heating
and reionization histories of the Universe or the merger rates of nuclear BHs
in the mass and redshift ranges of interest for proposed gravitational-wave
detectors.Comment: 9 pages, 5 figures, accepted for publication in MNRA
The suppression of direct collapse black hole formation by soft X-ray irradiation
The origin of supermassive black holes (SMBHs) in galactic nuclei is one of
the major unsolved problems in astrophysics. One hypothesis is that they grew
from >10^5 M_sun black holes that formed in the `direct collapse' of massive
gas clouds that have low concentrations of both metals and molecular hydrogen
(H_2). Such clouds could form in the early (z>10) Universe if pre-galactic gas
is irradiated by H_2-photodissociating, far-ultraviolet (FUV) light from a
nearby star-forming galaxy. In this work, we re-examine the critical FUV flux
J_crit that is required to keep H_2 photodissociated and lead to direct
collapse. We submit that the same galaxies that putatively supply the
extraordinary FUV fluxes required for direct collapse should also produce
copious amounts of soft X-rays, which work to offset H_2 photodissociation by
increasing the ionization fraction and promoting H_2 formation. Accounting for
this effect increases the value of J_crit by a factor of at least 3-10,
depending on the brightness temperature of FUV radiation. This enhancement of
J_crit suppresses the abundance of potential direct collapse sites at z>10 by
several orders of magnitude. Recent studies---without accounting for the soft
X-rays from the FUV source galaxies---had already arrived at large values of
J_crit that implied that direct collapse may occur too rarely to account for
the observed abundance of high-redshift quasars. Our results suggest that
J_crit should be even higher than previously estimated, and pose an additional
challenge for the direct collapse scenario via strong FUV radiation to explain
the high-redshift quasar population.Comment: 16 pages, 8 figures, 5 tables, accepted for publication in MNRA
A Rich Cluster of Galaxies Near the Quasar B2 1335+28 at z=1.1: Color Distribution and Star-Formation Properties
We previously reported a significant clustering of red galaxies (R-K=3.5--6)
around the radio-loud quasar B2 1335+28 at z=1.086. In this paper, we establish
the existence of a rich cluster at the quasar redshift, and study the
properties of the cluster galaxies through further detailed analysis of the
photometric data. The color distribution of the galaxies in the cluster is
quite broad and the fraction of blue galaxies (\sim 70%) is much larger than in
intermediate-redshift clusters. Using evolutionary synthesis models, we show
that this color distribution can be explained by galaxies with various amounts
of star-formation activity mixed with the old stellar populations. Notably,
there are about a dozen galaxies which show very red optical-NIR colors but
also show significant UV excess with respect to passive-evolution models. They
can be interpreted as old early-type galaxies with a small amount of star
formation. The fact that the UV-excess red galaxies are more abundant than the
quiescent red ones suggests that a large fraction of old galaxies in this
cluster are still forming stars to some extent. However, a sequence of
quiescent red galaxies is clearly identified on the R-K versus K
color-magnitude (C-M) diagram. The slope and zero point of their C-M relation
appear to be consistent with those expected for the precursors of the C-M
relation of present-day cluster ellipticals when observed at z=1.1. We estimate
the Abell richness class of the cluster to be R \sim 1. New X-ray data
presented here place an upper limit of L_x < 2 10^{44} erg s^{-1} for the
cluster luminosity. Inspections of the wider optical images reveal some lumpy
structure, suggesting that the whole system is still dynamically young.Comment: 54 pages including 13 Postscript figures, 1 jpg figure, and 1 table,
uses aasms4.sty and epsf.sty. Accepted for publication in ApJ: Replaced as
the older verison was missed to include the figure 2c, 2d, and figure