59 research outputs found
Evolution of Massive Black Hole Binaries in Rotating Stellar Nuclei and its Implications for Gravitational Wave Detection
According to the currently prevailing cosmological paradigm, mergers between galaxies are an important part of their evolution. Assuming also that most galaxies contain a supermassive black hole at their center, binary supermassive black holes (BSBH) should be common products of galactic mergers.
The subject of this dissertation is the dynamical evolution of a BSBH at the center of a galaxy. I calculate the rate of change of a binary\u27s orbital elements due to interactions with the stars of the galaxy by means of 3-body scattering experiments. My model includes a new degree of freedom - the orientation of the BSBH\u27s orbital plane - which is allowed to change due to interaction with the stars in a rotating nucleus. The binary\u27s eccentricity also evolves in an orientation-dependent manner. I find that the dynamics are qualitatively different compared to non-rotating nuclei: 1) The orbital orientation of a BSBH changes towards alignment with the plane of rotation of the nucleus. 2) The orbital eccentricity of a BSBH decreases for aligned BSBHs and increases for counter-aligned ones.
I then apply my model to calculate the effects of stellar environment on the gravitational wave background spectrum produced by BSBHs. Using the results of N-body/Monte-Carlo simulations, I account for the different rate of stellar interactions in spherical, axisymmetric and triaxial galaxies. I also consider the possibility that supermassive black hole masses are systematically lower than usually assumed. The net result of the new physical mechanisms included in my model is a spectrum for the stochastic gravitational wave background that has a significantly lower amplitude than in previous treatments, which could explain the discrepancy that currently exists between the models and the upper limits set by pulsar timing array observations
Orbital evolution of LIGO/Virgo binaries in stellar clusters driven by cluster tides, stellar encounters and general relativity
Origin of LIGO/Virgo gravitational wave events may involve production of
binaries with relativistic components in dense stellar systems - globular or
nuclear star clusters - and their subsequent evolution towards merger. Orbital
parameters of these binaries (the inner orbit) and their motion inside the
cluster (the outer orbit) evolve due to both external agents - random
encounters with cluster stars and cluster tides due to the smooth cluster
potential - and the internal ones - various sources of dissipation and
precession within the binary. We present a numerical framework - Binary
Evolution in Stellar Clusters (BESC) - that follows the evolution of the binary
inner and outer orbits accounting for all these effects simultaneously,
enabling efficient Monte Carlo studies. The secular effect of cluster tides is
computed in the singly-averaged approximation, without averaging over the outer
binary orbit. As to stellar encounters, we include the effects of both close
and distant flybys on the inner and outer orbits of the binary, respectively.
In particular, this allows us to explicitly account for the dynamical friction
sinking the binary towards the cluster centre. Also, given our focus on the
LIGO/Virgo sources, we include the general relativistic precession (which
suppresses cluster tides at high eccentricities) and the gravitational wave
emission (shrinking the binary orbit). We use BESC to illustrate a number of
characteristic binary evolutionary outcomes and discuss relative contributions
of different physical processes. BESC can also be used to study other objects
in clusters, e.g. blue stragglers, hot Jupiters, X-ray binaries, etc.Comment: 20 pages, 16 figures, submitted to MNRA
Binary intermediate-mass black hole mergers in globular clusters
We consider the formation of binary intermediate black holes (BIMBH) in
globular clusters (GC), which could happen either in situ or due to the mergers
between clusters. We simulate the evolution of the BIMBH orbit (and its
subsequent merger) due to stellar ejections. We also take into account the
evaporation of GCs due to the tidal field of the host galaxy and two-body
relaxation. Our results show that if at least of all GCs become BIMBH
hosts and the BIMBH masses are of the GC mass, at least one of the
inspiralling (or merging) BIMBHs will be detected by LISA during its 4-year
mission lifetime. Most of the detected BIMBHs come 1) from heavy GCs
(), as lower-mass GCs end up being disrupted before
their BIMBHs have time to merge, and 2) from redshifts , assuming that
most of GCs form around and given that the merger timescale for most
BIMBHs is Gyr. If the BIMBH to GC mass ratio is lower ()
but the fraction of BIMBH hosts among GCs is higher (), some of
their mergers will also be detected by LIGO, VIRGO, and KAGRA and the proposed
Einstein Telescope.Comment: 14 pages, 14 figures, 1 tabl
The impact of stripped Nuclei on the Super-Massive Black Hole number density in the local Universe
The recent discovery of super-massive black holes (SMBHs) in the centers of
high-mass ultra compact dwarf galaxies (UCDs) suggests that at least some UCDs
are the stripped nuclear star clusters of lower mass galaxies. Tracing these
former nuclei of stripped galaxies provides a unique way to track the assembly
history of a galaxy or galaxy cluster. In this paper we present a new method to
estimate how many UCDs host an SMBH in their center and thus are stripped
galaxy nuclei. We revisit the dynamical mass measurements that suggest many
UCDs have more mass than expected from stellar population estimates, which
recent observations have shown is due to the presence of an SMBH. We revise the
stellar population mass estimates using a new empirical relation between the
mass-to-light ratio (M/L) and metallicity, and use this to predict which UCDs
are most likely to host an SMBH. This enables us to calculate the fraction of
UCDs that host SMBHs across their entire luminosity range for the first time.
We then apply the SMBH occupation fraction to the observed luminosity function
of UCDs and estimate that in the Fornax and Virgo cluster alone there should be
stripped nuclei with SMBHs. This analysis shows that stripped
nuclei with SMBHs are almost as common in clusters as present-day galaxy
nuclei. We estimate the local SMBH number density in stripped nuclei to
, which represents a significant fraction (10-40\%)
of the SMBH density in the local Universe. These SMBHs hidden in stripped
nuclei will increase expected event rates for tidal disruption events and
SMBH-SMBH and SMBH-BH mergers. The existence of numerous stripped nuclei with
SMBHs are a direct consequence of hierarchical galaxy formation, but until now
their impact on the SMBH density had not been quantified.Comment: 15 pages, 8 Figures, accepted for publication in Ap
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