268 research outputs found
Off the Beaten Path: A New Approach to Realistically Model The Orbital Decay of Supermassive Black Holes in Galaxy Formation Simulations
We introduce a force correction term to better model the dynamical friction
(DF) experienced by a supermassive black hole (SMBH) as it orbits within its
host galaxy. This new approach accurately follows the orbital decay of a SMBH
and drastically improves over commonly used advection methods. The force
correction introduced here naturally scales with the force resolution of the
simulation and converges as resolution is increased. In controlled experiments
we show how the orbital decay of the SMBH closely follows analytical
predictions when particle masses are significantly smaller than that of the
SMBH. In a cosmological simulation of the assembly of a small galaxy, we show
how our method allows for realistic black hole orbits. This approach overcomes
the limitations of the advection scheme, where black holes are rapidly and
artificially pushed toward the halo center and then forced to merge, regardless
of their orbits. We find that SMBHs from merging dwarf galaxies can spend
significant time away from the center of the remnant galaxy. Improving the
modeling of SMBH orbital decay will help in making robust predictions of the
growth, detectability, and merger rates of SMBHs, especially at low galaxy
masses or at high redshift.Comment: 8 pages, 4 figure, Accepted by MNRA
Dancing to ChaNGa: A Self-Consistent Prediction For Close SMBH Pair Formation Timescales Following Galaxy Mergers
We present the first self-consistent prediction for the distribution of
formation timescales for close Supermassive Black Hole (SMBH) pairs following
galaxy mergers. Using ROMULUS25, the first large-scale cosmological simulation
to accurately track the orbital evolution of SMBHs within their host galaxies
down to sub-kpc scales, we predict an average formation rate density of close
SMBH pairs of 0.013 cMpc^-3 Gyr^-1. We find that it is relatively rare for
galaxy mergers to result in the formation of close SMBH pairs with sub-kpc
separation and those that do form are often the result of Gyrs of orbital
evolution following the galaxy merger. The likelihood and timescale to form a
close SMBH pair depends strongly on the mass ratio of the merging galaxies, as
well as the presence of dense stellar cores. Low stellar mass ratio mergers
with galaxies that lack a dense stellar core are more likely to become tidally
disrupted and deposit their SMBH at large radii without any stellar core to aid
in their orbital decay, resulting in a population of long-lived 'wandering'
SMBHs. Conversely, SMBHs in galaxies that remain embedded within a stellar core
form close pairs in much shorter timescales on average. This timescale is a
crucial, though often ignored or very simplified, ingredient to models
predicting SMBH mergers rates and the connection between SMBH and star
formation activity.Comment: 11 pages, 7 figures, accepted for publication in MNRA
Shining Light on the Hosts of the Nano-Hertz Gravitational Wave Sources: A Theoretical Perspective
The formation of supermassive black holes (SMBHs) in the Universe and its
role in the properties of the galaxies is one of the open questions in
astrophysics and cosmology. Though, traditionally, electromagnetic waves have
been instrumental in direct measurements of SMBHs, significantly influencing
our comprehension of galaxy formation, gravitational waves (GW) bring an
independent avenue to detect numerous binary SMBHs in the observable Universe
in the nano-Hertz range using the pulsar timing array observation. This brings
a new way to understand the connection between the formation of binary SMBHs
and galaxy formation if we can connect theoretical models with multi-messenger
observations namely GW data and galaxy surveys. Along these lines, we present
here the first paper on this series based on {\sc Romulus} cosmological
simulation on the properties of the host galaxies of SMBHs and propose on how
this can be used to connect with observations of nano-Hertz GW signal and
galaxy surveys. We show that the most dominant contribution to the background
will arise from sources with high chirp masses which are likely to reside in
low redshift early-type galaxies with high stellar mass, largely old stellar
population, and low star formation rate, and that reside at centers of galaxy
groups and manifest evidence of recent mergers. The masses of the sources show
a correlation with the halo mass and stellar mass of the host galaxies. This
theoretical study will help in understanding the host properties of the GW
sources and can help in establishing a connection with observations.Comment: 16 pages, 12 figures. Submitted to MNRA
Wandering Supermassive Black Holes in Milky Way Mass Halos
We present a self-consistent prediction from a large-scale cosmological
simulation for the population of `wandering' supermassive black holes (SMBHs)
of mass greater than M on long-lived, kpc-scale orbits within
Milky Way (MW)-mass galaxies. We extract a sample of MW-mass halos from the
Romulus25 cosmological simulation (Tremmel et al. 2017), which is uniquely able
to capture the orbital evolution of SMBHs during and following galaxy mergers.
We predict that such halos, regardless of recent merger history or morphology,
host an average of SMBHs, including their central black hole,
within 10 kpc from the galactic center and an average of SMBHs
total within their virial radius, not counting those in satellite halos.
Wandering SMBHs exist within their host galaxies for several Gyrs, often
accreted by their host halo in the early Universe. We find, with
significance, that wandering SMBHs are preferentially found outside of galactic
disks.Comment: 7 pages, 5 figures, accepted for publication in ApJ Letter
An Enhanced Massive Black Hole Occupation Fraction Predicted in Cluster Dwarf Galaxies
The occupation fraction of massive black holes (MBHs) in low mass galaxies
offers interesting insights into initial black hole seeding mechanisms and
their mass assembly history, though disentangling these two effects remains
challenging. Using the Romulus cosmological simulations we examine the impact
of environment on the occupation fraction of MBHs in low mass galaxies. Unlike
most modern cosmological simulations, Romulus seeds MBHs based on local gas
properties, selecting very dense, pristine, and rapidly collapsing regions in
the early Universe as sites to host MBHs without assuming anything about MBH
occupation as a function of galaxy stellar mass, or halo mass, a priori. The
simulations predict that dwarf galaxies with M M in
cluster environments are approximately two times more likely to host a MBH
compared to those in the field. The predicted occupation fractions are
remarkably consistent with those of nuclear star clusters. Across cluster and
field environments, dwarf galaxies with earlier formation times are more likely
to host a MBH. Thus, while the MBH occupation function is similar between
cluster and field environments at high redshift (), a difference arises as
late-forming dwarfs -- which do not exist in the cluster environment -- begin
to dominate in the field and pull the MBH occupation fraction down for low mass
galaxies. Additionally, prior to in-fall some cluster dwarfs are similar to
progenitors of massive, isolated galaxies, indicating that they might have
grown to higher masses had they not been impeded by the cluster environment.
While the population of MBHs in dwarf galaxies is already widely understood to
be important for understanding MBH formation, this work demonstrates that
environmental dependence is important to consider as future observations search
for low mass black holes in dwarf galaxies.Comment: 16 pages, 7 figures, to be submitted to the Open Journal of
Astrophysic
Modeling the Redshift Evolution of the Normal Galaxy X-ray Luminosity Function
Emission from X-ray binaries (XRBs) is a major component of the total X-ray
luminosity of normal galaxies, so X-ray studies of high redshift galaxies allow
us to probe the formation and evolution of X-ray binaries on very long
timescales. In this paper, we present results from large-scale population
synthesis models of binary populations in galaxies from z = 0 to 20. We use as
input into our modeling the Millennium II Cosmological Simulation and the
updated semi-analytic galaxy catalog by Guo et al. (2011) to self-consistently
account for the star formation history (SFH) and metallicity evolution of each
galaxy. We run a grid of 192 models, varying all the parameters known from
previous studies to affect the evolution of XRBs. We use our models and
observationally derived prescriptions for hot gas emission to create
theoretical galaxy X-ray luminosity functions (XLFs) for several redshift bins.
Models with low CE efficiencies, a 50% twins mass ratio distribution, a steeper
IMF exponent, and high stellar wind mass loss rates best match observational
results from Tzanavaris & Georgantopoulos (2008), though they significantly
underproduce bright early-type and very bright (Lx > 10d41) late-type galaxies.
These discrepancies are likely caused by uncertainties in hot gas emission and
SFHs, AGN contamination, and a lack of dynamically formed Low-mass XRBs. In our
highest likelihood models, we find that hot gas emission dominates the emission
for most bright galaxies. We also find that the evolution of the normal galaxy
X-ray luminosity density out to z = 4 is driven largely by XRBs in galaxies
with X-ray luminosities between 10d40 and 10d41 erg/s.Comment: Accepted into ApJ, 17 pages, 3 tables, 7 figures. Text updated to
address referee's comment
- …