643 research outputs found
Cold Dark Matter Substructure and Galactic Disks
We perform a set of high-resolution, dissipationless N-body simulations to
investigate the influence of cold dark matter (CDM) substructure on the
dynamical evolution of thin galactic disks. Our method combines cosmological
simulations of galaxy-sized CDM halos to derive the properties of substructure
populations and controlled numerical experiments of consecutive subhalo impacts
onto initially-thin, fully-formed disk galaxies. We demonstrate that close
encounters between massive subhalos and galactic disks since z~1 should be
common occurrences in LCDM models. In contrast, extremely few satellites in
present-day CDM halos are likely to have a significant impact on the disk
structure. One typical host halo merger history is used to seed controlled
N-body experiments of subhalo-disk encounters. As a result of these accretion
events, the disk thickens considerably at all radii with the disk scale height
increasing in excess of a factor of 2 in the solar neighborhood. We show that
interactions with the subhalo population produce a wealth of distinctive
morphological signatures in the disk stars including: conspicuous flares; bars;
low-lived, ring-like features in the outskirts; and low-density, filamentary
structures above the disk plane. We compare a resulting dynamically-cold,
ring-like feature in our simulations to the Monoceros ring stellar structure in
the MW. The comparison shows quantitative agreement in both spatial
distribution and kinematics, suggesting that such observed complex stellar
components may arise naturally as disk stars are excited by encounters with
subhalos. These findings highlight the significant role of CDM substructure in
setting the structure of disk galaxies and driving galaxy evolution.Comment: 10 pages, 4 figures. To appear in the proceedings of the IAU
Symposium No. 254 "The Galaxy Disk in Cosmological Context", Copenhagen 9-13
June 2008, Denmark, (Eds.) J. Andersen, J. Bland-Hawthorn & B. Nordstrom,
Cambridge University Pres
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
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
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
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
Rapid Formation of Supermassive Black Hole Binaries in Galaxy Mergers with Gas
Supermassive black holes (SMBHs) are a ubiquitous component of the nuclei of
galaxies. It is normally assumed that, following the merger of two massive
galaxies, a SMBH binary will form, shrink due to stellar or gas dynamical
processes and ultimately coalesce by emitting a burst of gravitational waves.
However, so far it has not been possible to show how two SMBHs bind during a
galaxy merger with gas due to the difficulty of modeling a wide range of
spatial scales. Here we report hydrodynamical simulations that track the
formation of a SMBH binary down to scales of a few light years following the
collision between two spiral galaxies. A massive, turbulent nuclear gaseous
disk arises as a result of the galaxy merger. The black holes form an eccentric
binary in the disk in less than a million years as a result of the
gravitational drag from the gas rather than from the stars.Comment: Accepted for publication in Science, 40 pages, 7 figures,
Supplementary Information include
Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical Satellite Accretion
(Abridged) We conduct a series of high-resolution, dissipationless N-body
simulations to investigate the cumulative effect of substructure mergers onto
thin disk galaxies in the context of the LCDM paradigm of structure formation.
Our simulation campaign is based on a hybrid approach. Substructure properties
are culled directly from cosmological simulations of galaxy-sized cold dark
matter (CDM) halos. In contrast to what can be inferred from statistics of the
present-day substructure populations, accretions of massive subhalos onto the
central regions of host halos, where the galactic disk resides, since z~1
should be common occurrences. One host halo merger history is subsequently used
to seed controlled numerical experiments of repeated satellite impacts on an
initially-thin Milky Way-type disk galaxy. We show that these accretion events
produce several distinctive observational signatures in the stellar disk
including: a ring-like feature in the outskirts; a significant flare; a central
bar; and faint filamentary structures that (spuriously) resemble tidal streams.
The final distribution of disk stars exhibits a complex vertical structure that
is well-described by a standard ``thin-thick'' disk decomposition. We conclude
that satellite-disk encounters of the kind expected in LCDM models can induce
morphological features in galactic disks that are similar to those being
discovered in the Milky Way, M31, and in other disk galaxies. These results
highlight the significant role of CDM substructure in setting the structure of
disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys
and astrometric satellites may be able to distinguish between competing
cosmological models by testing whether the detailed structure of galactic disks
is as excited as predicted by the CDM paradigm.Comment: Accepted version to appear in ApJ, 24 pages, 8 figures, LaTeX (uses
emulateapj.cls). Comparison between the simulated ring-like features and the
Monoceros ring stellar structure in the Milky Way performed; conclusions
unaltere
Tidal stirring of Milky Way satellites: a simple picture with the integrated tidal force
Most of dwarf spheroidal galaxies in the Local Group were probably formed via
environmental processes like the tidal interaction with the Milky Way. We study
this process via N-body simulations of dwarf galaxies evolving on seven
different orbits around the Galaxy. The dwarf galaxy is initially composed of a
rotating stellar disk and a dark matter halo. Due to the action of tidal forces
it loses mass and the disk gradually transforms into a spheroid while stellar
motions become increasingly random. We measure the characteristic scale-length
of the dwarf, its maximum circular velocity, mass, shape and kinematics as a
function of the integrated tidal force along the orbit. The final properties of
the evolved dwarfs are remarkably similar if the total tidal force they
experienced was the same, independently of the actual size and eccentricity of
the orbit.Comment: 5 pages, 2 figures, contribution to the proceedings of JENAM 2010 in
Lisbon, Symposium 2 "Environment and the formation of galaxies: 30 years
later", comments welcom
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