We use 1-kpc resolution cosmological Adaptive Mesh Refinement (AMR) simulations of a Virgo-like galaxy cluster to investigate the effect of feedback from supermassive black holes on the mass ...distribution of dark matter, gas and stars. We compared three different models: (i) a standard galaxy formation model featuring gas cooling, star formation and supernovae feedback, (ii) a 'quenching' model for which star formation is artificially suppressed in massive haloes and finally (iii) the recently proposed active galactic nucleus (AGN) feedback model of Booth and Schaye. Without AGN feedback (even in the quenching case), our simulated cluster suffers from a strong overcooling problem, with a stellar mass fraction significantly above observed values in M87. The baryon distribution is highly concentrated, resulting in a strong adiabatic contraction (AC) of dark matter. With AGN feedback, on the contrary, the stellar mass in the brightest cluster galaxy (BCG) lies below observational estimates and the overcooling problem disappears. The stellar mass of the BCG is seen to increase with increasing mass resolution, suggesting that our stellar masses converge to the correct value from below. The gas and total mass distributions are in better agreement with observations. We also find a slight deficit (∼10 per cent) of baryons at the virial radius, due to the combined effect of AGN-driven convective motions in the inner parts and shock waves in the outer regions, pushing gas to Mpc scales and beyond. This baryon deficit results in a slight adiabatic expansion of the dark matter distribution that can be explained quantitatively by AC theory.
We analyze the present day structure and assembly history of a high-resolution hydrodynamic simulation of the formation of a Milky-Way-(MW)-like disk galaxy, from the "Eris" simulation suite, ...dissecting it into cohorts of stars formed at different epochs of cosmic history. The younger age cohorts populate disks of progressively longer radial scale lengths and shorter vertical scale heights. At a given radius, the vertical density profiles and velocity dispersions of stars vary smoothly as a function of age, and the superposition of old, vertically extended and young, vertically compact cohorts gives rise to a double-exponential profile like that observed in the MW. Turning to formation history, we find that the trends of spatial structure and kinematics with stellar age are largely imprinted at birth, or immediately thereafter. The predicted correlations of stellar age with spatial and kinematic structure are in good qualitative agreement with the correlations observed for mono-abundance stellar populations in the MW.
The Laser Interferometer Space Antenna (LISA) will detect gravitational-wave (GW) signals from merging supermassive black holes (BHs) with masses below 107 M . It is thus of paramount importance to ...understand the orbital dynamics of these relatively light central BHs, which typically reside in disk-dominated galaxies, in order to produce reliable forecasts of merger rates. To this aim, realistic simulations probing BH dynamics in unequal-mass disk galaxy mergers, into and beyond the binary hardening stage, are performed by combining smooth particle hydrodynamics and direct N-body codes. The structural properties and orbits of the galaxies are chosen to be consistent with the results of galaxy formation simulations. Stellar and dark matter distributions are triaxial down to the central 100 pc of the merger remnant. In all cases, a BH binary forms and hardens on timescales of at most 100 Myr, coalescing on another few-hundred-megayear timescale, depending on the characteristic density and orbital eccentricity. Overall, the sinking of the BH binary takes no more than ∼0.5 Gyr after the merger of the two galaxies is completed, but it can be much faster for very plunging orbits. Comparing with previous numerical simulations following the decay of BHs in massive early-type galaxies at z ∼ 3, we confirm that the characteristic density is the most crucial parameter determining the overall BH merging timescale, despite the structural diversity of the host galaxies. Our results lay down the basis for robust forecasts of LISA event rates in the case of merging BHs.
The Hubble sequence is a common classification scheme for the structure of galaxies. Despite the tremendous usefulness of this diagnostic, we still do not fully understand when, where, and how this ...morphological ordering was put in place. Here, we investigate the morphological evolution of a sample of 22 high-redshift (z ≥ 3) galaxies extracted from the Argo simulation. Argo is a cosmological zoom-in simulation of a group-sized halo and its environment. It adopts the same high-resolution (~10... M..., ~100 pc) and sub-grid physical model that was used in the Eris simulation but probes a sub-volume almost 10 times bigger with as many as 45 million gas and star particles in the zoom-in region. Argo follows the early assembly of galaxies with a broad range of stellar masses (log M.../M... ~ 8-11 at z ~ 3), while resolving properly their structural properties. We recover a diversity of morphologies, including late-type/irregular disc galaxies with flat rotation curves, spheroid dominated early-type discs, and a massive elliptical galaxy, already established at z ... 3. We identify major mergers as the main trigger for the formation of bulges and the steepening of the circular velocity curves. Minor mergers and non-axisymmetric perturbations (stellar bars) drive the bulge growth in some cases. The specific angular momenta of the simulated disc components fairly match the values inferred from nearby galaxies of similar M... once the expected redshift evolution of disc sizes is accounted for. We conclude that morphological transformations of high-redshift galaxies of intermediate mass are likely triggered by processes similar to those at low redshift and result in an early build-up of the Hubble sequence. (ProQuest: ... denotes formulae/symbols omitted.)
ABSTRACT
We present a revised version of Peters’ time-scale for the gravitational wave (GW)-induced decay of two point masses. The new formula includes the effects of the first-order post-Newtonian ...perturbation and additionally provides a simple fit to account for the Newtonian self-consistent evolution of the eccentricity. The revised time-scale is found by multiplying Peters’ estimate by two factors, $R(e_0)= 8^{1-\sqrt{1-e_0}}$ and Qf(p0) = exp (2.5(rS/p0)), where e0 and p0 are the initial eccentricity and periapsis, respectively, and rS the Schwarzschild radius of the system. Their use can correct errors of a factor of 1–10 that arise from using the original Peters’ formula. We apply the revised time-scales to a set of typical sources for existing ground-based laser interferometers and for the future Laser Interferometer Space Antenna (LISA), at the onset of their GW-driven decay. We argue that our more accurate model for the orbital evolution will affect current event- and detection-rate estimates for mergers of compact object binaries, with stronger deviations for eccentric LISA sources, such as extreme and intermediate mass-ratio inspirals. We propose the correction factors R and Qf as a simple prescription to quantify decay time-scales more accurately in future population synthesis models. We also suggest that the corrected time-scale may be used as a computationally efficient alternative to numerical integration in other applications that include the modelling of radiation reaction for eccentric sources.
ABSTRACT
Soon after the recent first ever detection of gravitational waves from merging black holes it has been suggested that their origin is primordial. Appealingly, a sufficient number of ...primordial black holes (PBHs) could also partially or entirely constitute the dark matter (DM) in our Universe. However, recent studies on PBHs in ultra-faint dwarf galaxies (UFDGs) suggest that they would dynamically heat up the stellar component due to two-body relaxation processes. From the comparison with the observed stellar velocity dispersions and the stellar half-light radii, it was claimed that only PBHs with masses $\lesssim 10\, {\rm M}_\odot$ can significantly contribute to the DM. In this work, we improve the latter constraints by considering the largest observational sample of UFDGs and by allowing the PBH masses to follow an extended (lognormal) distribution. By means of collisional Fokker–Planck simulations, we explore a wide parameter space of UFDGs containing PBHs. The analysis of the half-light radii and velocity dispersions resulting from the simulations leads to three general findings that exclude PBHs with masses $\sim \mathcal {O}(1\operatorname{-}100)\, {\rm M}_\odot {}$ from constituting all of the DM: (i) we identify a critical sub-sample of UFDGs that only allows for $\sim \mathcal {O}(1)\, {\rm M}_\odot$ PBH masses; (ii) for any PBH mass, there is an UFDG in our sample that disfavours it; (iii) the spatial extensions of a majority of simulated UFDGs containing PBHs are too large to match the observed.
We analyze stellar masses of clumps drawn from a compilation of star-forming galaxies at 1.1 < z < 3.6. Comparing clumps selected in different ways, and in lensed or blank field galaxies, we examine ...the effects of spatial resolution and sensitivity on the inferred stellar masses. Large differences are found, with median stellar masses ranging from for clumps in the often-referenced field galaxies to for fainter clumps selected in deep-field or lensed galaxies. We argue that the clump masses, observed in non-lensed galaxies with a limited spatial resolution of ∼1 kpc, are artificially increased due to the clustering of clumps of smaller mass. Furthermore, we show that the sensitivity threshold used for the clump selection affects the inferred masses even more strongly than resolution, biasing clumps at the low-mass end. Both improved spatial resolution and sensitivity appear to shift the clump stellar mass distribution to lower masses, qualitatively in agreement with clump masses found in recent high-resolution simulations of disk fragmentation. We discuss the nature of the most massive clumps, and we conclude that it is currently not possible to properly establish a meaningful clump stellar mass distribution from observations and to infer the existence and value of a characteristic clump mass scale.
ABSTRACT
Accretion at sustained or episodic super-Eddington (SE) rates has been proposed as a pathway to grow efficiently light seeds produced by Pop-III stars. We investigate if SE accretion can be ...sustained onto a black hole (BH) with MBH ∼ 103 M⊙ in the centre of a gas-rich proto-galaxy at z = 15. We perform high-resolution smoothed-particle hydrodynamical simulations, including two different sub-grid models for SE accretion, one based on the slim disc paradigm, and one inspired by recent radiation-magnetohydrodynamical simulations by Jiang and collaborators. Radiative feedback has the form of a thermal dump to surrounding gas particles, with the radiative efficiency being set according to the different SE accretion models. We find that, in all simulations, star formation, BH feedback, and interactions between clumps and the BH rapidly quench accretion after ∼1 Myr, irrespective of the sub-grid model used for accretion. Quenching is stronger in the model based on the simulations of Jiang and collaborators relative to the slim disc model because of its higher radiative efficiency. The SE growth phase is always very brief, lasting a few 0.1 Myr. In the most optimistic case, the BH reaches a mass of ∼104 M⊙. We extrapolate the final BH masses from z = 15 to z ∼ 6, assuming subsequent galaxy mergers will replenish the gas reservoir and trigger new cycles of SE accretion. We find that at most BH seeds would grow to ∼106 M⊙, comparable to the mass of massive BHs in spiral galaxies such as the Milky Way, but falling short of the mass of the high-redshift quasars.
ABSTRACT We present novel 3D multi-scale smoothed particle hydrodynamics (SPH) simulations of gas-rich galaxy mergers between the most massive galaxies at z ∼ 8-10, designed to scrutinize the direct ...collapse formation scenario for massive black hole seeds proposed in Mayer et al. The simulations achieve a resolution of 0.1 pc, and include both metallicity-dependent optically thin cooling and a model for thermal balance at high optical depth. We consider different formulations of the SPH hydrodynamical equations, including thermal and metal diffusion. When the two merging galaxy cores collide, gas infall produces a compact, optically thick nuclear disk with densities exceeding 10−10 g cm3. The disk rapidly accretes higher angular momentum gas from its surroundings reaching ∼5 pc and a mass of 109 M in only a few 104 years. Outside 2 pc it fragments into massive clumps. Instead, supersonic turbulence prevents fragmentation in the inner parsec region, which remains warm (∼3000-6000 K) and develops strong non-axisymmetric modes that cause prominent radial gas inflows (>104 M yr−1), forming an ultra-dense massive disky core. Angular momentum transport by non-axisymmetric modes should continue below our spatial resolution limit, quickly turning the disky core into a supermassive protostar which can collapse directly into a massive black hole of mass 108-109 M via the relativistic radial instability. Such a "cold direct collapse" explains naturally the early emergence of high-z QSOs. Its telltale signature would be a burst of gravitational waves in the frequency range of 10−4-10−1 Hz, possibly detectable by the planned eLISA interferometer.
Abstract
We present results from the ‘Ponos’ simulation suite on the early evolution of a massive, Mvir(z = 0) = 1.2 × 1013 M⊙ galaxy. At z ≳ 6, before feedback from a central supermassive black hole ...becomes dominant, the main galaxy has a stellar mass ∼2 × 109 M⊙ and a star formation rate ∼20 M⊙ yr−1. The galaxy sits near the expected main sequence of star-forming galaxies at those redshifts, and resembles moderately star-forming systems observed at z > 5. The high specific star formation rate results in vigorous heating and stirring of the gas by supernovae feedback, and the galaxy develops a thick and turbulent disc, with gas velocity dispersion ∼40 km s−1, rotation to dispersion ratio ∼2, and with a significant amount of gas at ∼105 K. The Toomre parameter always exceeds the critical value for gravito-turbulence, Q ∼ 1.5–2, mainly due to the contribution of warm/hot gas inside the disc. Without feedback, a nearly gravito-turbulent regime establishes with similar gas velocity dispersion and lower Q. We propose that the ‘hot and turbulent’ disc regime seen in our simulations, unlike the ‘cold and turbulent’ gravito-turbulent regime of massive clumpy disc galaxies at z ∼ 1–2, is a fundamental characterization of the main-sequence galaxies at z ≳ 6, as they can sustain star formation rates comparable to those of low-mass starbursts at z = 0. This results in no sustained coherent gas inflows through the disc, and in fluctuating and anisotropic mass transport, possibly postponing the assembly of the bulge and causing the initial feeding of the central black hole to be highly intermittent.
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