547 research outputs found
Birth of massive black hole binaries
If massive black holes (BHs) are ubiquitous in galaxies and galaxies
experience multiple mergers during their cosmic assembly, then BH binaries
should be common albeit temporary features of most galactic bulges.
Observationally, the paucity of active BH pairs points toward binary lifetimes
far shorter than the Hubble time, indicating rapid inspiral of the BHs down to
the domain where gravitational waves lead to their coalescence. Here, we review
a series of studies on the dynamics of massive BHs in gas-rich galaxy mergers
that underscore the vital role played by a cool, gaseous component in promoting
the rapid formation of the BH binary. The BH binary is found to reside at the
center of a massive self-gravitating nuclear disc resulting from the collision
of the two gaseous discs present in the mother galaxies. Hardening by
gravitational torques against gas in this grand disc is found to continue down
to sub-parsec scales. The eccentricity decreases with time to zero and when the
binary is circular, accretion sets in around the two BHs. When this occurs,
each BH is endowed with it own small-size (< 0.01 pc) accretion disc comprising
a few percent of the BH mass. Double AGN activity is expected to occur on an
estimated timescale of < 1 Myr. The double nuclear point-like sources that may
appear have typical separation of < 10 pc, and are likely to be embedded in the
still ongoing starburst. We note that a potential threat of binary stalling, in
a gaseous environment, may come from radiation and/or mechanical energy
injections by the BHs. Only short-lived or sub-Eddington accretion episodes can
guarantee the persistence of a dense cool gas structure around the binary
necessary for continuing BH inspiral.Comment: To appear in "2007 STScI Spring Symposium: Black Holes", eds. M.
Livio & A. M. Koekemoer, Cambridge University Press, 25 pages, 12 figure
Biases in mass estimates of dSph galaxies
Using a high resolution N-body simulation of a two-component dwarf galaxy
orbiting in the potential of the Milky Way, we study two effects that lead to
significant biases in mass estimates of dwarf spheroidal galaxies. Both are due
to the strong tidal interaction of initially disky dwarfs with their host. The
tidal stripping of dwarf stars leads to the formation of strong tidal tails
that are typically aligned with the line of sight of an observer positioned
close to the host center. The stars from the tails contaminate the kinematic
samples leading to a velocity dispersion profile increasing with the projected
radius and resulting in an overestimate of mass. The tidal stirring of the
dwarf leads to the morphological transformation of the initial stellar disk
into a bar and then a spheroid. The distribution of stars in the dwarf remains
non-spherical for a long time leading to an overestimate of its mass if it is
observed along the major axis and an underestimate if it seen in the
perpendicular direction.Comment: 5 pages, 3 figures, contribution to the proceedings of "Hunting for
the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds.
V.P. Debattista & C.C. Popescu, AIP Conference Series, in pres
On the inspiral of Massive Black Holes in gas-rich galaxy mergers
We present a study on the dynamics of massive BHs in galaxy mergers, obtained
from a series of high-resolution N-Body/SPH simulations. The presence of a
gaseous component is essential for the rapid formation of an eccentric
(Keplerian) BH binary, that resides at the center of a massive (~10^9 Msun)
turbulent nuclear disc. Using physically and/or numerically motivated recipes,
we follow the accretion history of the BHs during the merger. The mass of the
BHs increases as large central inflows of gas occur inside each galaxy, and
their mass ratio varies with time. Given the encountered strong degeneracy
between numerical resolution and physical assumptions, we suggest here three
possible paths followed by the galaxies and the BHs during a merger in order to
fulfill the M-sigma relation : Adjustment, Symbiosis, and BH Dominance. In an
extremely high resolution run, we resolved the turbulent gas pattern down to
parsec scales, and found that BH feedback is expected to be effective near the
end of the merger. We then trace the BH binary orbit down to a scale of 0.1 pc
modeling the nuclear disc as an equilibrium Mestel disc composed either of gas,
gas and stars, or just stars. Under the action of dynamical friction against
the rotating gaseous and/or stellar background the orbit circularizes. When
this occurs, each BH is endowed with its own small-size (~0.01 pc) accretion
disc comprising a few percent of the BH mass. Double AGN activity is expected
to occur on an estimated timescale of ~10 Myrs, comparable to the inspiral
time. The double nuclear point--like sources that may appear have typical
separations of ~10 pc, and are likely to be embedded in the still ongoing
starburst.Comment: 10 pages, 5 figures, Proceedings of the Conference "The Multicoloured
Landscape of Compact Objects and their Explosive Origins", Cefalu` 200
The stellar structure and kinematics of dwarf spheroidal galaxies formed by tidal stirring
Using high-resolution N-body simulations we study the stellar properties of
dwarf spheroidal galaxies resulting from the tidally induced morphological
transformation of disky dwarfs on a cosmologically motivated eccentric orbit
around the Milky Way. Dwarf galaxy models initially consist of an exponential
stellar disk embedded in an extended spherical dark matter halo. Depending on
the initial orientation of the disk with respect to the orbital plane,
different final configurations are obtained. The least evolved dwarf is
triaxial and retains a significant amount of rotation. The more evolved dwarfs
are prolate spheroids with little rotation. We show that the final density
distribution of stars can be approximated by a simple modification of the
Plummer law. The kinematics of the dwarfs is significantly different depending
on the line of sight which has important implications for mapping the observed
stellar velocity dispersions of dwarfs to subhalo circular velocities. When the
dwarfs are observed along the long axis, the measured velocity dispersion is
higher and decreases faster with radius. In the case where rotation is
significant, when viewed perpendicular to the long axis, the effect of minor
axis rotation is detected, as expected for triaxial systems. We model the
velocity dispersion profiles and rotation curves of the dwarfs by solving the
Jeans equations for spherical and axisymmetric systems and adjusting different
sets of free parameters. We find that the mass is typically overestimated when
the dwarf is seen along the long axis and underestimated when the observation
is along the short or intermediate axis. The effect of non-sphericity cannot
however bias the inferred mass by more than 60 percent in either direction,
even for the most strongly stripped dwarf which is close to disruption.Comment: 17 pages, 15 figures, revised version accepted for publication in Ap
Orbital Decay of Supermassive Black Hole Binaries in Clumpy Multiphase Merger Remnants
We simulate an equal-mass merger of two Milky Way-size galaxy discs with
moderate gas fractions at parsec-scale resolution including a new model for
radiative cooling and heating in a multi-phase medium, as well as star
formation and feedback from supernovae. The two discs initially have a
supermassive black hole (SMBH) embedded in
their centers. As the merger completes and the two galactic cores merge, the
SMBHs form a a pair with a separation of a few hundred pc that gradually
decays. Due to the stochastic nature of the system immediately following the
merger, the orbital plane of the binary is significantly perturbed.
Furthermore, owing to the strong starburst the gas from the central region is
completely evacuated, requiring ~Myr for a nuclear disc to rebuild.
Most importantly, the clumpy nature of the interstellar medium has a major
impact on the the dynamical evolution of the SMBH pair, which undergo
gravitational encounters with massive gas clouds and stochastic torquing by
both clouds and spiral modes in the disk. These effects combine to greatly
delay the decay of the two SMBHs to separations of a few parsecs by nearly two
orders of magnitude, yr, compared to previous work. In mergers of
more gas-rich, clumpier galaxies at high redshift stochastic torques will be
even more pronounced and potentially lead to stronger modulation of the orbital
decay. This suggests that SMBH pairs at separations of several tens of parsecs
should be relatively common at any redshift.Comment: submitted to MNRAS; Comments very welcom
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