462 research outputs found
Multiple regimes and coalescence timescales for massive black hole pairs ; the critical role of galaxy formation physics
We discuss the latest results of numerical simulations following the orbital
decay of massive black hole pairs in galaxy mergers. We highlight important
differences between gas-poor and gas-rich hosts, and between orbital evolution
taking place at high redshift as opposed to low redshift. Two effects have a
huge impact and are rather novel in the context of massive black hole binaries.
The first is the increase in characteristic density of galactic nuclei of
merger remnants as galaxies are more compact at high redshift due to the way
dark halo collapse depends on redshift. This leads naturally to hardening
timescales due to 3-body encounters that should decrease by two orders of
magnitude up to . It explains naturally the short binary coalescence
timescale, Myr, found in novel cosmological simulations that follow
binary evolution from galactic to milliparsec scales. The second one is the
inhomogeneity of the interstellar medium in massive gas-rich disks at high
redshift. In the latter star forming clumps 1-2 orders of magnitude more
massive than local Giant Molecular Clouds (GMCs) can scatter massive black
holes out of the disk plane via gravitational perturbations and direct
encounters. This renders the character of orbital decay inherently stochastic,
often increasing orbital decay timescales by as much as a Gyr. At low redshift
a similar regime is present at scales of pc inside Circumnuclear Gas
Disks (CNDs). In CNDs only massive black holes with masses below can be significantly perturbed. They decay to sub-pc separations in
up to yr rather than the in just a few million years as in a smooth
CND. Finally implications for building robust forecasts of LISA event rates are
discussedComment: 13 pages, 3 Figures, Invited Paper to appear in the Proceedings of
the 11th International LISA Symposium, IOP Journal of Physics: Conference
Serie
Protoplanetary disk fragmentation with varying radiative physics, initial conditions and numerical techniques
We review recent results of SPH simulations of gravitational instability in
gaseous protoplanetary disks,emphasizing the role of thermodynamics in both
isolated and binary systems. Contradictory results appeared in the literature
regarding disk fragmentation at tens of AU from the central star are likely due
to the different treatment of radiation physics as well as reflecting different
initial conditions. Further progress on the subject requires extensive
comparisons between different codes with the requirement that the same initial
conditions are adopted. It is discussed how the local conditions of the disks
undergoing fragmentation at AU in recent SPH simulations are in rough
agreement with the prediction of analytical models, with small differences
being likely related to the inability of analytical models to account for the
dynamics and thermodynamics of three-dimensional spiral shocks. We report that
radically different adaptive hydrodynamical codes, SPH and adaptive mesh
refinement (AMR), yield very similar results on disk fragmentation at
comparable resolution in the simple case of an isothermal equation of state. A
high number of refinements in AMR codes is necessary but not sufficient to
correctly follow fragmentation, rather an initial resolution of the grid high
enough to capture the wavelength of the strongest spiral modes when they are
still barely nonlinear is essential. These tests represent a useful benchmark
and a starting point for a forthcoming code comparison with realistic radiation
physics.Comment: 13 pages, 4 figures, invited review, proceedings of the Conference
"Extreme Solar Systems", Santorini, Greece, June 25-29, 2007, slightly
extended version with bigger figure
Early evolution of clumps formed via gravitational instability in protoplanetary disks; precursors of Hot Jupiters?
Although it is fairly established that Gravitational Instability (GI) should
occur in the early phases of the evolution of a protoplanetary disk, the fate
of the clumps resulting from disk fragmentation and their role in planet
formation is still unclear. In the present study we investigate
semi-analytically their evolution following the contraction of a synthetic
population of clumps with varied initial structure and orbits coupled with the
surrounding disk and the central star. Our model is based on recently published
state-of-the-art 3D collapse simulations of clumps with varied thermodynamics.
Various evolutionary mechanisms are taken into account, and their effect is
explored both individually and in combination with others: migration and tidal
disruption, mass accretion, gap opening and disk viscosity. It is found that,
in general, at least 50% of the initial clumps survive tides, leaving behind
potential gas giant progenitors after ~10^5 yr of evolution in the disk. The
rest might be either disrupted or produce super-Earths and other low mass
planets provided that a solid core can be assembled on a sufficiently short
timescale, a possibility that we do not address in this paper. Extrapolating to
million year timescales, all our surviving protoplanets would lead to close-in
gas giants. This outcome might in part reflect the limitations of the migration
model adopted, and is reminiscent of the analogous result found in
core-accretion models in absence of fine-tuning of the migration rate. Yet it
suggests that a significant fraction of the clumps formed by gravitational
instability could be the precursors of Hot Jupiters
The Argo Simulation: I. Quenching of Massive Galaxies at High Redshift as a Result of Cosmological Starvation
Observations show a prevalence of high redshift galaxies with large stellar
masses and predominantly passive stellar populations. A variety of processes
have been suggested that could reduce the star formation in such galaxies to
observed levels, including quasar mode feedback, virial shock heating, or
galactic winds driven by stellar feedback. However, the main quenching
mechanisms have yet to be identified. Here we study the origin of star
formation quenching using Argo, a cosmological, hydrodynamical zoom-in
simulation that follows the evolution of a massive galaxy at . This
simulation adopts the same sub-grid recipes of the Eris simulations, which have
been shown to form realistic disk galaxies, and, in one version, adopts also a
mass and spatial resolution identical to Eris. The resulting galaxy has
properties consistent with those of observed, massive (M_* ~ 1e11 M_sun)
galaxies at z~2 and with abundance matching predictions. Our models do not
include AGN feedback indicating that supermassive black holes likely play a
subordinate role in determining masses and sizes of massive galaxies at high z.
The specific star formation rate (sSFR) of the simulated galaxy matches the
observed M_* - sSFR relation at early times. This period of smooth stellar mass
growth comes to a sudden halt at z=3.5 when the sSFR drops by almost an order
of magnitude within a few hundred Myr. The suppression is initiated by a
leveling off and a subsequent reduction of the cool gas accretion rate onto the
galaxy, and not by feedback processes. This "cosmological starvation" occurs as
the parent dark matter halo switches from a fast collapsing mode to a slow
accretion mode. Additional mechanisms, such as perhaps radio mode feedback from
an AGN, are needed to quench any residual star formation of the galaxy and to
maintain a low sSFR until the present time.Comment: 20 pages, 12 figures, 2 tables, accepted for publication in MNRA
The evolution of dwarf galaxy satellites with different dark matter density profiles in the ErisMod simulations. I. The early infalls
We present the first simulations of tidal stirring of dwarf galaxies in the
Local Group carried out in a cosmological context. We use the ErisDARK
simulation of a MW-sized galaxy to identify some of the most massive subhalos
() that fall into the main host before .
Subhalos are replaced before infall with high-resolution models of dwarf
galaxies comprising a faint stellar disk embedded in a dark matter halo. The
set of models contains cuspy halos as well as halos with "cored" profiles (with
asymptotic inner slope ). The simulations are then run to
with as many as 54 million particles and resolution as small as pc
using the N-Body code ChaNGa. The stellar components of all satellites are
significantly affected by tidal stirring, losing stellar mass and undergoing a
morphological transformation towards a pressure supported spheroidal system.
However, while some remnants with cuspy halos maintain significant rotational
flattening and disk-like features, all the shallow halo models achieve
and round shapes typical of dSph satellites of the MW and M31.
Mass loss is also enhanced in the latter, and remnants can reach luminosities
and velocity dispersions as low as those of Ultra Faint Dwarfs (UFDs). We argue
that cuspy progenitors must be the exception rather than the rule among
satellites of the MW since all the MW and M31 satellites in the luminosity
range of our remnants are dSphs, a result matched only in the simulation with
"cored" models
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