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 $z=4$. It explains naturally the short binary coalescence timescale, $\sim 10$ 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 $1-10$ pc inside Circumnuclear Gas Disks (CNDs). In CNDs only massive black holes with masses below $10^7 M_{\odot}$ can be significantly perturbed. They decay to sub-pc separations in up to $\sim 10^8$ 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 $R < 25$ 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 $z\geq{}2$. 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 ($M_{vir} > 10^8 M_{\odot}$) that fall into the main host before $z=2$. 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 $\gamma = 0.6$). The simulations are then run to $z=0$ with as many as 54 million particles and resolution as small as $\sim 4$ 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 $v/\sigma < 0.5$ 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