118 research outputs found
Growing Massive Black Hole Pairs in Minor Mergers of Disk Galaxies
We perform a suite of high-resolution smoothed particle hydrodynamics
simulations to investigate the orbital decay and mass evolution of massive
black hole (MBH) pairs down to scales of ~30 pc during minor mergers of disk
galaxies. Our simulation set includes star formation and accretion onto the
MBHs, as well as feedback from both processes. We consider 1:10 merger events
starting at z~3, with MBH masses in the sensitivity window of the Laser
Interferometer Space Antenna, and we follow the coupling between the merger
dynamics and the evolution of the MBH mass ratio until the satellite galaxy is
tidally disrupted. While the more massive MBH accretes in most cases as if the
galaxy were in isolation, the satellite MBH may undergo distinct episodes of
enhanced accretion, owing to strong tidal torques acting on its host galaxy and
to orbital circularization inside the disk of the primary galaxy. As a
consequence, the initial 1:10 mass ratio of the MBHs changes by the time the
satellite is disrupted. Depending on the initial fraction of cold gas in the
galactic disks and the geometry of the encounter, the mass ratios of the MBH
pairs at the time of satellite disruption can stay unchanged or become as large
as 1:2. Remarkably, the efficiency of MBH orbital decay correlates with the
final mass ratio of the pair itself: MBH pairs that increase significantly
their mass ratio are also expected to inspiral more promptly down to
nuclear-scale separations. These findings indicate that the mass ratios of MBH
pairs in galactic nuclei do not necessarily trace the mass ratios of their
merging host galaxies, but are determined by the complex interplay between gas
accretion and merger dynamics.Comment: 5 pages, 4 figures, replaced to match accepted version on Ap
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
On the rate of black hole binary mergers in galactic nuclei due to dynamical hardening
We assess the contribution of dynamical hardening by direct three-body
scattering interactions to the rate of stellar-mass black hole binary (BHB)
mergers in galactic nuclei. We derive an analytic model for the single-binary
encounter rate in a nucleus with spherical and disk components hosting a
super-massive black hole (SMBH). We determine the total number of encounters
needed to harden a BHB to the point that inspiral due to
gravitational wave emission occurs before the next three-body scattering event.
This is done independently for both the spherical and disk components. Using a
Monte Carlo approach, we refine our calculations for to include
gravitational wave emission between scattering events. For astrophysically
plausible models we find that typically 10.
We find two separate regimes for the efficient dynamical hardening of BHBs:
(1) spherical star clusters with high central densities, low velocity
dispersions and no significant Keplerian component; and (2) migration traps in
disks around SMBHs lacking any significant spherical stellar component in the
vicinity of the migration trap, which is expected due to effective orbital
inclination reduction of any spherical population by the disk. We also find a
weak correlation between the ratio of the second-order velocity moment to
velocity dispersion in galactic nuclei and the rate of BHB mergers, where this
ratio is a proxy for the ratio between the rotation- and dispersion-supported
components. Because disks enforce planar interactions that are efficient in
hardening BHBs, particularly in migration traps, they have high merger rates
that can contribute significantly to the rate of BHB mergers detected by the
advanced Laser Interferometer Gravitational-Wave Observatory.Comment: 13 pages, 9 figures, accepted for publication in MNRA
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
Optical and JWST Mid-IR Emission Line Diagnostics for Simultaneous IMBH and Stellar Excitation in z~0 Dwarf Galaxies
Current observational facilities have yet to conclusively detect intermediate mass black holes (IMBHs) that fill in the evolutionary
gap between early universe seed black holes and supermassive black
holes. Dwarf galaxies present an opportunity to reveal active IMBHs amidst
persistent star formation. We introduce photoionization simulations tailored to
address key physical uncertainties: coincident vs. non-coincident mixing of
IMBH and starlight excitation, open vs. closed surrounding gas cloud
geometries, and different AGN SED shapes. We examine possible AGN emission line
diagnostics in the optical and mid-IR, and find that the diagnostics are often
degenerate with respect to the investigated physical uncertainties. In spite of
these setbacks, and in contrast to recent work, we are able to show that [O
III]/H typically remains bright for dwarf AGN powered by IMBHs down to
. Dwarf AGN are predicted to have inconsistent star-forming and
Seyfert/LINER classifications using the most common optical diagnostics. In the
mid-IR, [O IV] 25.9m and [Ar II] 6.98m are less sensitive to physical
uncertainties than are optical diagnostics. Based on these emission lines, we
provide several mid-IR emission line diagnostic diagrams with demarcations for
separating starbursts and AGN with varying levels of activity. The diagrams are
valid over a wide range of ionization parameters and metallicities out to
, so will prove useful for future JWST observations of local dwarf
AGN in the search for IMBHs. We make our photoionization simulation suite
freely available.Comment: 24 pages, 13 figures, accepted to Ap
The Candidate Intermediate-Mass Black Hole in the Globular Cluster M54
Ibata et al. reported evidence for density and kinematic cusps in the
Galactic globular cluster M54, possibly due to the presence of a 9400
solar-mass black hole. Radiative signatures of accretion onto M54's candidate
intermediate-mass black hole (IMBH) could bolster the case for its existence.
Analysis of new Chandra and recent Hubble Space Telescope astrometry rules out
the X-ray counterpart to the candidate IMBH suggested by Ibata et al. If an
IMBH exists in M54, then it has an Eddington ratio of L(0.3-8 keV) / L(Edd) <
1.4 x 10^(-10), more similar to that of the candidate IMBH in M15 than that in
G1. From new imaging with the NRAO Very Large Array, the luminosity of the
candidate IMBH is L(8.5 GHz) < 3.6 x 10^29 ergs/s (3 sigma). Two background
active galaxies discovered toward M54 could serve as probes of its intracluster
medium.Comment: 4 pages; 2 figures; emulateapj.cls; to appear in A
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
Orbital Migration of Interacting Stellar Mass Black Holes in Disks around Supermassive Black Holes
The merger rate of stellar-mass black hole binaries (sBHBs) inferred by the
Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) suggests
the need for an efficient source of sBHB formation. Active galactic nucleus
(AGN) disks are a promising location for the formation of these sBHBs, as well
as binaries of other compact objects, because of powerful torques exerted by
the gas disk. These gas torques cause orbiting compact objects to migrate
towards regions in the disk where inward and outward torques cancel, known as
migration traps. We simulate the migration of stellar mass black holes in an
example of a model AGN disk, using an augmented N-body code that includes
analytic approximations to migration torques, stochastic gravitational forces
exerted by turbulent density fluctuations in the disk, and inclination and
eccentricity dampening produced by passages through the gas disk, in addition
to the standard gravitational forces between objects. We find that sBHBs form
rapidly in our model disk as stellar-mass black holes migrate towards the
migration trap. These sBHBs are likely to subsequently merge on short
time-scales. The process continues, leading to the build-up of a population of
over-massive stellar-mass black holes. The formation of sBHBs in AGN disks
could contribute significantly to the sBHB merger rate inferred by LIGO.Comment: 18 pages, 13 figures, Accepted to Ap
ALMA detection of a disc-dominated [C II] emission line at z = 4.6 in the luminous QSO J1554+1937
We present observations and analysis of an unusual [C ii] emission line in the very luminous quasi-stellar object (QSO) SDSS J155426.16+193703.0 at z ~ 4.6. The line is extremely broad (full width at half-maximum 735 km s−1) and seems to have a flat-topped or double-peaked line profile. A velocity map of the line shows a gradient across the source that indicates large-scale rotation of star-forming gas. Together, the velocity map and line profile suggest the presence of a massive rotating disc with a dynamical mass Mdyn ≳5×1010 M☉. Using the assumption of a rotating disc origin, we employ an empirical relation between galaxy disc circular velocity and bulge velocity dispersion (σ) to estimate that σ > 310 km s−1, subject to a correction for the unknown disc inclination. This result implies that this source is consistent with the local M–σ relation, or offset at most by an order of magnitude in black hole mass. In contrast, the assumption of a bulge origin for the [C ii] emission line would lead to a conclusion that the black hole is nearly two orders of magnitude more massive than predicted by the M–σ relation, similar to previous findings for other high-redshift QSOs. As disc rotation may be a common origin for [C ii] emission at high redshifts, these results stress that careful consideration of dynamical origins is required when using observations of this line to derive properties of high-redshift galaxies
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