2,193 research outputs found
Massive Binary Black Holes in the Cosmic Landscape
Binary black holes occupy a special place in our quest for understanding the
evolution of galaxies along cosmic history. If massive black holes grow at the
center of (pre-)galactic structures that experience a sequence of merger
episodes, then dual black holes form as inescapable outcome of galaxy assembly.
But, if the black holes reach coalescence, then they become the loudest sources
of gravitational waves ever in the universe. Nature seems to provide a pathway
for the formation of these exotic binaries, and a number of key questions need
to be addressed: How do massive black holes pair in a merger? Depending on the
properties of the underlying galaxies, do black holes always form a close
Keplerian binary? If a binary forms, does hardening proceed down to the domain
controlled by gravitational wave back reaction? What is the role played by gas
and/or stars in braking the black holes, and on which timescale does
coalescence occur? Can the black holes accrete on flight and shine during their
pathway to coalescence? N-Body/hydrodynamical codes have proven to be vital
tools for studying their evolution, and progress in this field is expected to
grow rapidly in the effort to describe, in full realism, the physics of stars
and gas around the black holes, starting from the cosmological large scale of a
merger. If detected in the new window provided by the upcoming gravitational
wave experiments, binary black holes will provide a deep view into the process
of hierarchical clustering which is at the heart of the current paradigm of
galaxy formation. They will also be exquisite probes for testing General
Relativity, as the theory of gravity. The waveforms emitted during the
inspiral, coalescence and ring-down phase carry in their shape the sign of a
dynamically evolving space-time and the proof of the existence of an horizon.Comment: Invited Review to appear on Advanced Science Letters (ASL), Special
Issue on Computational Astrophysics, edited by Lucio Maye
LISA double black holes: Dynamics in gaseous nuclear discs
We study the inspiral of double black holes, with masses in the LISA window
of detectability, orbiting inside a massive circum-nuclear disc. Using
high-resolution SPH simulations, we follow the black hole dynamics in the early
phase when gas-dynamical friction acts on the black holes individually, and
continue our simulation until they form a close binary. We find that in the
early sinking the black holes lose memory of their initial orbital eccentricity
if they co-rotate with the gaseous disc, forming a binary with a low
eccentricity, consistent with zero within our numerical resolution limit. The
cause of circularization resides in the rotation present in the gaseous
background where dynamical friction operates. Circularization may hinder
gravitational waves from taking over and leading the binary to coalescence. In
the case of counter-rotating orbits the initial eccentricity does not decrease,
and the black holes may bind forming an eccentric binary. When dynamical
friction has subsided, for equal mass black holes and regardless their initial
eccentricity, angular momentum loss, driven by the gravitational torque exerted
on the binary by surrounding gas, is nevertheless observable down to the
smallest scale probed. In the case of unequal masses, dynamical friction
remains efficient down to our resolution limit, and there is no sign of
formation of any ellipsoidal gas distribution that may further harden the
binary. During inspiral, gravitational capture of gas by the black holes occurs
mainly along circular orbits: eccentric orbits imply high relative velocities
and weak gravitational focusing. Thus, AGN activity may be excited during the
black hole pairing process and double active nuclei may form when
circularization is completed, on distance-scales of tens of pcs.Comment: Minor changes, accepted to MNRAS (11 pags, 14 figs). Movies (.avi)
are available at http://pitto.mib.infn.it/~haardt/MOVIES
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
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
A path to radio-loudness through gas-poor galaxy mergers and the role of retrograde accretion
In this proceeding we explore a pathway to radio-loudness under the
hypothesis that retrograde accretion onto giant spinning black holes leads to
the launch of powerful jets, as seen in radio loud QSOs and recently in
LAT/Fermi and BAT/Swift Blazars. Counter-rotation of the accretion disc
relative to the BH spin is here associated to gas-poor galaxy mergers
progenitors of giant (missing-light) ellipticals. The occurrence of retrograde
accretion enters as unifying element that may account for the
radio-loudness/galaxy morphology dichotomy observed in AGN.Comment: To appear in the proceedings of the conference "Accretion and
Ejection in AGN: A global view, June 22-26 2009 - Como, Italy
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