32 research outputs found
Supermassive black hole pairs in clumpy galaxies at high redshift: delayed binary formation and concurrent mass growth
Massive gas-rich galaxy discs at host massive star-forming
clumps with typical baryonic masses in the range ~M which
can affect the orbital decay and concurrent growth of supermassive black hole
(BH) pairs. Using a set of high-resolution simulations of isolated clumpy
galaxies hosting a pair of unequal-mass BHs, we study the interaction between
massive clumps and a BH pair at kpc scales, during the early phase of the
orbital decay. We find that both the interaction with massive clumps and the
heating of the cold gas layer of the disc by BH feedback tend to delay
significantly the orbital decay of the secondary, which in many cases is
ejected and then hovers for a whole Gyr around a separation of 1--2 kpc. In the
envelope, dynamical friction is weak and there is no contribution of disc
torques: these lead to the fastest decay once the orbit of the secondary BH has
circularised in the disc midplane. In runs with larger eccentricities the delay
is stronger, although there are some exceptions. We also show that, even in
discs with very sporadic transient clump formation, a strong spiral pattern
affects the decay time-scale for BHs on eccentric orbits. We conclude that,
contrary to previous belief, a gas-rich background is not necessarily conducive
to a fast BH decay and binary formation, which prompts more extensive
investigations aimed at calibrating event-rate forecasts for ongoing and future
gravitational-wave searches, such as with Pulsar Timing Arrays and the future
evolved Laser Interferometer Space Antenna.Comment: Accepted by MNRA
Growth and activity of black holes in galaxy mergers with varying mass ratios
We study supermassive black holes (BHs) in merging galaxies, using a suite of
hydrodynamical simulations with very high spatial (~10 pc) and temporal (~1
Myr) resolution, where we vary the initial mass ratio, the orbital
configuration, and the gas fraction. (i) We address the question of when and
why, during a merger, increased BH accretion occurs, quantifying gas inflows
and BH accretion rates. (ii) We also quantify the relative effectiveness in
inducing AGN activity of merger-related versus secular-related causes, by
studying different stages of the encounter: the stochastic (or early) stage,
the (proper) merger stage, and the remnant (or late) stage. (iii) We assess
which galaxy mergers preferentially enhance BH accretion, finding that the
initial mass ratio is the most important factor. (iv) We study the evolution of
the BH masses, finding that the BH mass contrast tends to decrease in minor
mergers and to increase in major mergers. This effect hints at the existence of
a preferential range of mass ratios for BHs in the final pairing stages. (v) In
both merging and dynamically quiescent galaxies, the gas accreted by the BH is
not necessarily the gas with angular momentum, but the gas that
angular momentum.Comment: Accepted for publication in MNRAS, 23 pages, 22 figures, 3 table
The birth of a supermassive black hole binary
We study the dynamical evolution of supermassive black holes, in the late
stage of galaxy mergers, from kpc to pc scales. In particular, we capture the
formation of the binary, a necessary step before the final coalescence, and
trace back the main processes causing the decay of the orbit. We use
hydrodynamical simulations of galaxy mergers with different resolutions, from
down to , in order to study the effects of the
resolution on our results, remove numerical effects, and assess that resolving
the influence radius of the orbiting black hole is a minimum condition to fully
capture the formation of the binary. Our simulations include the relevant
physical processes, namely star formation, supernova feedback, accretion onto
the black holes and the ensuing feedback. We find that, in these mergers,
dynamical friction from the smooth stellar component of the nucleus is the main
process that drives black holes from kpc to pc scales. Gas does not play a
crucial role and even clumps do not induce scattering or perturb the orbits. We
compare the time needed for the formation of the binary to analytical
predictions and suggest how to apply such analytical formalism to obtain
estimates of binary formation times in lower resolution simulations.Comment: 12 pages, 12 Figures, submitted to MNRA
Merging Black Holes in Dwarf Galaxies: Calculating Binary Black Hole Coalescence Timescales from Simulations for LISA Detection
Supermassive black holes (SMBHs) merging in dwarf galaxies will be detectable
by the Laser Interferometer Space Antenna (LISA) in the mid-2030s. Previous
cosmological hydrodynamic simulations have shown the prediction of massive
black holes merging in dwarf galaxies, but these simulations are limited by
their resolution and cannot follow black hole pairs all the way to coalescence.
We calculate the delay time between black hole pairing and merger based on the
properties of the black holes and their host galaxies, and use these properties
to calculate gravitational wave strains for eleven different binary black holes
that merge inside dwarf galaxies from eight cosmological simulations. This
delay time calculation accounts for dynamical friction due to gas and stars,
loss-cone scattering, and hardening of the binary due to gravitational
radiation. Out of the eleven black hole mergers in the simulations, five black
hole pairs will merge within 0.8 - 8 Gyr of forming a close pair and could be
observed by LISA, and the remaining six are unresolved due to resolution
limitations of the simulation. As all five of the resolved close pairs merge
within a Hubble time, we make the broad estimate that close SMBH pairs in dwarf
galaxies will merge and be detectable by LISA, but this estimate depends on
either the presence of gas during orbital decay or a solution to the dynamical
buoyancy problem in cored potentials.Comment: 8 pages, 3 figure
A comparison of black hole growth in galaxy mergers with Gasoline and Ramses
Supermassive black hole dynamics during galaxy mergers is crucial in
determining the rate of black hole mergers and cosmic black hole growth. As
simulations achieve higher resolution, it becomes important to assess whether
the black hole dynamics is influenced by the treatment of the interstellar
medium in different simulation codes. We here compare simulations of black hole
growth in galaxy mergers with two codes: the Smoothed Particle Hydrodynamics
code Gasoline, and the Adaptive Mesh Refinement code Ramses. We seek to
identify predictions of these models that are robust despite differences in
hydrodynamic methods and implementations of sub-grid physics. We find that the
general behavior is consistent between codes. Black hole accretion is minimal
while the galaxies are well-separated (and even as they "fly-by" within 10 kpc
at first pericenter). At late stages, when the galaxies pass within a few kpc,
tidal torques drive nuclear gas inflow that triggers bursts of black hole
accretion accompanied by star formation. We also note quantitative
discrepancies that are model-dependent: our Ramses simulations show less star
formation and black hole growth, and a smoother gas distribution with larger
clumps and filaments, than our Gasoline simulations. We attribute these
differences primarily to the sub-grid models for black hole fueling and
feedback and gas thermodynamics. The main conclusion is that differences exist
quantitatively between codes, and this should be kept in mind when making
comparisons with observations. However, reassuringly, both codes capture the
same dynamical behaviors in terms of triggering of black hole accretion, star
formation, and black hole dynamics.Comment: 11 pages, 7 figures. Submitted to A&A. Comments welcom
Nuclear coups: dynamics of black holes in galaxy mergers
We study the dynamical evolution of supermassive black holes (BHs) in merging
galaxies on scales of hundreds of kpc to 10 pc, to identify the physical
processes that aid or hinder the orbital decay of BHs. We present
hydrodynamical simulations of galaxy mergers with a resolution of 20 pc,
chosen to accurately track the motion of the nuclei and provide a realistic
environment for the evolution of the BHs. We find that, during the late stages
of the merger, tidal shocks inject energy in the nuclei, causing one or both
nuclei to be disrupted and leaving their BH `naked', without any bound gas or
stars. In many cases, the nucleus that is ultimately disrupted is that of the
larger galaxy (`nuclear coup'), as star formation grows a denser nuclear cusp
in the smaller galaxy. We supplement our simulations with an analytical
estimate of the orbital-decay time required for the BHs to form a binary at
unresolved scales, due to dynamical friction. We find that, when a nuclear coup
occurs, the time-scale is much shorter than when the secondary's nucleus is
disrupted, as the infalling BH is more massive, and it also finds itself in a
denser stellar environment.Comment: Accepted for publication in MNRAS, 16 pages, 13 figures, 2 table
Growth and activity of black holes in galaxy mergers with varying mass ratios
We study supermassive black holes (BHs) in merging galaxies, using a suite of hydrodynamical simulations with very high spatial (∼10 pc) and temporal (∼1Myr) resolution, where we vary the initial mass ratio, the orbital configuration, and the gas fraction. (i) We address the question of when and why, during a merger, increased BH accretion occurs, quantifying gas inflows and BH accretion rates. (ii) We also quantify the relative effectiveness in inducing active galactic nuclei activity of merger-related versus secular-related causes, by studying different stages of the encounter: the stochastic (or early) stage, the (proper) merger stage, and the remnant (or late) stage. (iii) We assess which galaxy mergers preferentially enhance BH accretion, finding that the initial mass ratio is the most important factor. (iv) We study the evolution of the BH masses, finding that the BH mass contrast tends to decrease in minor mergers and to increase in major mergers. This effect hints at the existence of a preferential range of mass ratios for BHs in the final pairing stages. (v) In both merging and dynamically quiescent galaxies, the gas accreted by the BH is not necessarily the gas with low angular momentum, but the gas that loses angular momentu
Dwarf AGNs from Variability for the Origins of Seeds (DAVOS): Intermediate-mass black hole demographics from optical synoptic surveys
We present a phenomenological forward Monte Carlo model for forecasting the
population of active galactic nuclei (AGNs) in dwarf galaxies observable via
their optical variability. Our model accounts for expected changes in the
spectral energy distribution of AGNs in the intermediate-mass black hole (IMBH)
mass range and uses observational constraints on optical variability as a
function of black hole (BH) mass to generate mock light curves. Adopting
several different models for the BH occupation function, including one for
off-nuclear IMBHs, we quantify differences in the predicted local AGN mass and
luminosity functions in dwarf galaxies. As a result, we are able to model the
variable fraction of AGNs as a function of physical host properties, such as
host galaxy stellar mass, in the presence of complex selection effects. We find
that our adopted occupation fractions for the "heavy" and "light" initial BH
seeding scenarios can be distinguished with variability data at the level for galaxy host stellar masses below with the
Vera C. Rubin Observatory. We demonstrate the prevalence of a selection bias
whereby recovered IMBH masses fall, on average, above the predicted value from
the local host galaxy - BH mass scaling relation with the strength of the bias
dependent on the survey sensitivity. The methodology developed in this work can
be used more broadly to forecast and correct for selection effects for AGN
demographic studies in synoptic surveys. Finally, we show that a targeted
hourly cadence program over a few nights with the Rubin Observatory can
provide strong constraints on IMBH masses given their expected rapid
variability timescales.Comment: 26 pages, 16 figures incl. 5 appendices; re-submitted to MNRAS
following referee repor
The Role of the Radial Orbit Instability in Dark Matter Halo Formation and Structure
For a decade, N-body simulations have revealed a nearly universal dark matter
density profile, which appears to be robust to changes in the overall density
of the universe and the underlying power spectrum. Despite its universality,
the physical origin of this profile has not yet been well understood.
Semi--analytic models by Barnes et al. (2005) have suggested that the density
structure of dark matter halos is determined by the onset of the radial orbit
instability (ROI). We have tested this hypothesis using N-body simulations of
collapsing dark matter halos with a variety of initial conditions. For
dynamically cold initial conditions, the resulting halo structures are triaxial
in shape, due to the mild aspect of the instability. We examine how variations
in initial velocity dispersion affect the onset of the instability, and find
that an isotropic velocity dispersion can suppress the ROI entirely, while a
purely radial dispersion does not. The quantity sigma^2/vc^2 is a criterion for
instability, where regions with sigma^2/vc^2 <~1 become triaxial due to the ROI
or other perturbations. We also find that the radial orbit instability sets a
scale length at which the velocity dispersion changes rapidly from isotropic to
radially anisotropic. This scale length is proportional to the radius at which
the density profile changes shape, as is the case in the semi--analytic models;
however, the coefficient of proportionality is different by a factor of ~2.5.
We conclude that the radial orbit instability is likely to be a key physical
mechanism responsible for the nearly universal profiles of simulated dark
matter halos.Comment: 13 pages, 12 figures, accepted to Ap