1,608 research outputs found
Massive black hole binaries: dynamical evolution and observational signatures
The study of the dynamical evolution of massive black hole pairs in mergers
is crucial in the context of a hierarchical galaxy formation scenario. The
timescales for the formation and the coalescence of black hole binaries are
still poorly constrained, resulting in large uncertainties in the expected rate
of massive black hole binaries detectable in the electromagnetic and
gravitational wave spectra. Here we review the current theoretical
understanding of the black hole pairing in galaxy mergers, with a particular
attention to recent developments and open issues. We conclude with a review of
the expected observational signatures of massive binaries, and of the
candidates discussed in literature to date.Comment: 4 Figures. Accepted for publication in Advances in Astronom
Unstable fields in Kerr spacetimes
We show that both the interior region of a Kerr black
hole and the Kerr naked singularity admit unstable solutions of the
Teukolsky equation for any value of the spin weight. For every harmonic number
there is at least one axially symmetric mode that grows exponentially in time
and decays properly in the radial directions. These can be used as Debye
potentials to generate solutions for the scalar, Weyl spinor, Maxwell and
linearized gravity field equations on these backgrounds, satisfying appropriate
spatial boundary conditions and growing exponentially in time, as shown in
detail for the Maxwell case. It is suggested that the existence of the unstable
modes is related to the so called "time machine" region, where the axial
Killing vector field is time-like, and the Teukolsky equation, restricted to
axially symmetric fields, changes its character from hyperbolic to elliptic
The initial value problem for linearized gravitational perturbations of the Schwarzschild naked singularity
The coupled equations for the scalar modes of the linearized Einstein
equations around Schwarzschild's spacetime were reduced by Zerilli to a 1+1
wave equation with a potential , on a field . For smooth metric
perturbations is singular at , the
mode harmonic number, and has a second order pole at . This is
irrelevant to the black hole exterior stability problem, where , and
, but it introduces a non trivial problem in the naked singular case
where , and the singularity appears in the relevant range of
. We solve this problem by developing a new approach to the evolution of the
even mode, based on a {\em new gauge invariant function}, -related
to by an intertwiner operator- that is a regular function of the
metric perturbation {\em for any value of }. This allows to address the
issue of evolution of gravitational perturbations in this non globally
hyperbolic background, and to complete the proof of the linear instability of
the Schwarzschild naked singularity, by showing that a previously found
unstable mode is excitable by generic initial data. This is further illustrated
by numerically solving the linearized equations for suitably chosen initial
data.Comment: typos corrected, references adde
A systematic search for massive black hole binaries in SDSS spectroscopic sample
We present the results of a systematic search for massive black hole binaries
in the Sloan Digital Sky Survey spectroscopic database. We focus on bound
binaries, under the assumption that one of the black holes is active. In this
framework, the broad lines associated to the accreting black hole are expected
to show systematic velocity shifts with respect to the narrow lines, which
trace the rest-frame of the galaxy. For a sample of 54586 quasars and 3929
galaxies at redshifts 0.1<z<1.5 we brute-force model each spectrum as a mixture
of two quasars at two different redshifts. The spectral model is a data-driven
dimensionality reduction of the SDSS quasar spectra based on a matrix
factorization. We identified 32 objects with peculiar spectra. Nine of them can
be interpreted as black hole binaries. This doubles the number of known black
hole binary candidates. We also report on the discovery of a new class of
extreme double-peaked emitters with exceptionally broad and faint Balmer lines.
For all the interesting sources, we present detailed analysis of the spectra,
and discuss possible interpretations.Comment: 10 pages, 2 figures, accepted for publication in Ap
Linear stability of Einstein-Gauss-Bonnet static spacetimes. Part II: vector and scalar perturbations
We study the stability under linear perturbations of a class of static
solutions of Einstein-Gauss-Bonnet gravity in dimensions with spatial
slices of the form \Sigma_{\k}^n \times {\mathbb R}^+, \Sigma_{\k}^n an
manifold of constant curvature \k. Linear perturbations for this class of
space-times can be generally classified into tensor, vector and scalar types.
In a previous paper, tensor perturbations were analyzed. In this paper we study
vector and scalar perturbations. We show that vector perturbations can be
analyzed in general using an S-deformation approach and do not introduce
instabilities. On the other hand, we show by analyzing an explicit example
that, contrary to what happens in Einstein gravity, scalar perturbations may
lead to instabilities in black holes with spherical horizons when the
Gauss-Bonnet string corrections are taken into account.Comment: 16 pages, 6 figure
SDSSJ092712.65+294344.0: a candidate massive black hole binary
In this Letter we explore the hypothesis that the quasar
SDSSJ092712.65+294344.0 is hosting a massive black hole binary embedded in a
circumbinary disc. The lightest, secondary black hole is active, and gas
orbiting around it is responsible for the blue-shifted broad emission lines
with velocity off-set of 2650 km/s, relative to the galaxy rest frame. As the
tidal interaction of the binary with the outer disc is expected to excavate a
gap, the blue-shifted narrow emission lines are consistent with being emitted
from the low-density inhomogeneous gas of the hollow region. From the
observations we infer a binary mass ratio q ~ 0.3, a mass for the primary of M1
~ 2 billion Msun and a semi-major axis of 0.34 pc, corresponding to an orbital
period of 370 years. We use the results of cosmological merger trees to
estimate the likely-hood of observing SDSSJ092712.65+294344.0 as recoiling
black hole or as a binary. We find that the binary hypothesis is preferred
being one hundred times more probable than the ejection hypothesis. If
SDSSJ092712.65+294344.0 hosts a binary, it would be the one closest massive
black hole binary system ever discovered.Comment: Accepted for publication in MNRAS Letter
Gravitational instabilities in Kerr space-times
In this paper we consider the possible existence of unstable axisymmetric
modes in Kerr space times, resulting from exponentially growing solutions of
the Teukolsky equation. We describe a transformation that casts the radial
equation that results upon separation of variables in the Teukolsky equation,
in the form of a Schr\"odinger equation, and combine the properties of the
solutions of this equations with some recent results on the asymptotic
behaviour of spin weighted spheroidal harmonics to prove the existence of an
infinite family of unstable modes. Thus we prove that the stationary region
beyond a Kerr black hole inner horizon is unstable under gravitational linear
perturbations. We also prove that Kerr space-time with angular momentum larger
than its square mass, which has a naked singularity, is unstable.Comment: 9 pages, 4 figures, comments, references and calculation details
added, asymptotic expansion typos fixe
Black hole accretion versus star formation rate: theory confronts observations
We use a suite of hydrodynamical simulations of galaxy mergers to compare
star formation rate (SFR) and black hole accretion rate (BHAR) for galaxies
before the interaction ('stochastic' phase), during the `merger' proper,
lasting ~0.2-0.3 Gyr, and in the `remnant' phase. We calculate the bi-variate
distribution of SFR and BHAR and define the regions in the SFR-BHAR plane that
the three phases occupy. No strong correlation between BHAR and galaxy-wide SFR
is found. A possible exception are galaxies with the highest SFR and the
highest BHAR. We also bin the data in the same way used in several
observational studies, by either measuring the mean SFR for AGN in different
luminosity bins, or the mean BHAR for galaxies in bins of SFR. We find that the
apparent contradiction or SFR versus BHAR for observed samples of AGN and star
forming galaxies is actually caused by binning effects. The two types of
samples use different projections of the full bi-variate distribution, and the
full information would lead to unambiguous interpretation. We also find that a
galaxy can be classified as AGN-dominated up to 1.5 Gyr after the merger-driven
starburst took place. Our study is consistent with the suggestion that most
low-luminosity AGN hosts do not show morphological disturbances.Comment: MNRAS Letters, in pres
Growing black holes and galaxies: black hole accretion versus star formation rate
We present a new suite of hydrodynamical simulations and use it to study, in
detail, black hole and galaxy properties. The high time, spatial and mass
resolution, and realistic orbits and mass ratios, down to 1:6 and 1:10, enable
us to meaningfully compare star formation rate (SFR) and BH accretion rate
(BHAR) timescales, temporal behaviour and relative magnitude. We find that (i)
BHAR and galaxy-wide SFR are typically temporally uncorrelated, and have
different variability timescales, except during the merger proper, lasting
~0.2-0.3 Gyr. BHAR and nuclear (<100 pc) SFR are better correlated, and their
variability are similar. Averaging over time, the merger phase leads typically
to an increase by a factor of a few in the BHAR/SFR ratio. (ii) BHAR and
nuclear SFR are intrinsically proportional, but the correlation lessens if the
long-term SFR is measured. (iii) Galaxies in the remnant phase are the ones
most likely to be selected as systems dominated by an active galactic nucleus
(AGN), because of the long time spent in this phase. (iv) The timescale over
which a given diagnostic probes the SFR has a profound impact on the recovered
correlations with BHAR, and on the interpretation of observational data.Comment: Accepted for publication in MNRA
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