Massive gas-rich galaxy discs at z ~ 1-3 host massive star-forming clumps with typical baryonic masses in the range 10 super( 7)-10 super( 8) M... which can affect the orbital decay and concurrent ...growth of supermassive black hole (BH) pairs. Using a set of high-resolution simulations of isolated clumpy galaxies hosting a pair of unequal-mass BHs, we study the interaction between massive clumps and a BH pair at kiloparsec scales, during the early phase of the orbital decay. We find that both the interaction with massive clumps and the heating of the cold gas layer of the disc by BH feedback tend to delay significantly the orbital decay of the secondary, which in many cases is ejected and then hovers for a whole gigayear around a separation of 1-2 kpc. In the envelope, dynamical friction is weak and there is no contribution of disc torques: these lead to the fastest decay once the orbit of the secondary BH has circularized in the disc mid-plane. In runs with larger eccentricities the delay is stronger, although there are some exceptions. We also show that, even in discs with very sporadic transient clump formation, a strong spiral pattern affects the decay time-scale for BHs on eccentric orbits. We conclude that, contrary to previous belief, a gas-rich background is not necessarily conducive to a fast BH decay and binary formation, which prompts more extensive investigations aimed at calibrating event-rate forecasts for ongoing and future gravitational-wave searches, such as with Pulsar Timing Arrays and the future evolved Laser Interferometer Space Antenna. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
The orbital decay of a perturber within a larger system plays a key role in the dynamics of many astrophysical systems—from nuclear star clusters or globular clusters in galaxies, to massive ...black holes in galactic nuclei, to dwarf galaxy satellites within the dark matter halos of more massive galaxies. For many decades, there have been various attempts to determine the underlying physics and timescales of the drag mechanism, ranging from the local dynamical friction approach to descriptions based on the back-reaction of global modes induced in the background system. We present ultra-high-resolution
N
-body simulations of massive satellites orbiting a Milky Way-like galaxy (with > 10
8
particles), that appear to capture both the local
“wake”
and the global
“mode”
induced in the primary halo. We address directly the mechanism of orbital decay from the combined action of local and global perturbations and specifically analyze where the bulk of the torque originates.
Abstract
We present the results of the first fully cosmological hydrodynamical simulations studying the merger-driven model for massive black hole (BH) seed formation via direct collapse. Using the ...zoom-in technique as well as particle splitting, we achieve a final spatial resolution of 2 pc. We show that the major merger of two massive galaxies at redshift
z
∼ 8 results in the formation of a nuclear supermassive disk (SMD) of only 4 pc in radius, owing to a prodigious gas inflow sustained at 100–1000
M
⊙
yr
−1
. The core of the merger remnant is metal-rich, well above solar abundance, and the SMD reaches a gaseous mass of 3 × 10
8
M
⊙
in less than a million years after the merger, despite a concurrent prominent nuclear starburst. Dynamical heating as gas falls into the deepest part of the potential well, and heating and stirring by supernova blastwaves, generate a turbulent multiphase interstellar medium, with a gas velocity dispersion exceeding 100 km s
−1
. As a result, only moderate fragmentation occurs in the inner 10–20 pc, despite the temperature falling below 1000 K. The SMD is Jeans-unstable as well as bar-unstable and will collapse further adiabatically, becoming warm and ionized. We show that the SMD, following inevitable contraction, will become general-relativistic-unstable and directly form a supermassive BH of mass in the range 10
6
–10
8
M
⊙
, essentially skipping the stage of BH seed formation. These results confirm that mergers between the most massive galaxies at
z
∼ 8–10 can naturally explain the rapid emergence of bright high-redshift quasars.
Abstract
Using the Eris zoom-in cosmological simulation of assembly of a Milky Way analogue, we study the chemical enrichment of stars due to accretion of metal-enriched gas from the interstellar ...medium (ISM) during the Galaxy's development. We consider metal-poor and old stars in the Galactic halo and bulge through the use of stellar orbits, gas density and metallicity distributions in Eris. Assuming spherically symmetric Bondi–Hoyle accretion, we find that halo and bulge stars accrete metals at the rate of about 10−24 and 10−22 M⊙ yr−1, respectively, at redshifts z ≲ 3, but this accretion rate increases roughly a hundred-fold to about 10−20 M⊙ yr−1 at higher redshifts due to increased gas density. Bulge and halo stars accrete similar amounts of metals at high redshifts when kinematically distinct bulge and halo have not yet developed, and both sets of stars encounter a similar metal distribution in the ISM. Accretion alone can enrich main-sequence stars up to Fe/H ∼ −2 in extreme cases, with the median enrichment level due to accretion of about Fe/H ∼ −6 to −5. Because accretion mostly takes place at high redshifts, it is α-enriched to α/Fe ∼ 0.5. We find that accretive metal enrichment is sufficient to affect the predicted metallicity distribution function of halo stars at Fe/H < −5. This can hinder attempts to infer natal chemical environment of metal-poor stars from their observed enrichment. Peculiar enrichment patterns such as those predicted to arise from pair-instability supernovae could help in disentangling the natal and accreted metal content of stars.
We present a new zoom-in hydrodynamical simulation, ‘ErisBH’, which features the same initial conditions, resolution, and sub-grid physics as the close Milky Way-analogue ‘Eris’ (Guedes et al. 2011), ...but it also includes prescriptions for the formation, growth and feedback of supermassive black holes. This enables a detailed study of black hole evolution and the impact of active galactic nuclei (AGN) feedback in a late-type galaxy. At z = 0, the main galaxy of ErisBH hosts a central black hole of 2.6 × 106 M⊙, which correlates to the bulge mass and the galaxy's central velocity dispersion similarly to what is observed in the Milky Way and in pseudobulges. During its evolution, the black hole grows mostly through mergers with black holes brought in by accreted satellite galaxies and very little by gas accretion (due to the modest amount of gas that reaches the central regions). AGN feedback is weak and it affects only the central
$1\text{--}2 \,\rm {kpc}$
. Yet, it limits the growth of the bulge, which results in a rotation curve that, in the inner ∼ 10 kpc, is flatter than that of Eris. We find that ErisBH is more prone to instabilities than Eris, due to its smaller bulge and larger disc. At z ∼ 0.3, an initially small bar grows to be of a few disc scalelengths in size. The formation of the bar causes a small burst of star formation in the inner few hundred pc, provides new gas to the central black hole and causes the bulge to have a boxy/peanut morphology by z = 0.
We use N-body simulations to study the effects that a divergent (i.e. ‘cuspy’) dark matter profile introduces on the tidal evolution of dwarf spheroidal galaxies (dSphs). Our models assume ...cosmologically motivated initial conditions where dSphs are dark-matter-dominated systems on eccentric orbits about a host galaxy composed of a dark halo and a baryonic disc. We find that the resilience of dSphs to tidal stripping is extremely sensitive to the cuspiness of the inner halo profile; whereas dwarfs with a cored profile can be easily destroyed by the disc component, those with cusps always retain a bound remnant, even after losing more than 99.99 per cent of the original mass. For a given halo profile, the evolution of the structural parameters as driven by tides is controlled solely by the total amount of mass lost. This information is used to construct a semi-analytic code that follows the tidal evolution of individual satellites as they fall into a more massive host, which allows us to simulate the hierarchical build-up of spiral galaxies assuming different halo profiles and disc masses. We find that tidal encounters with discs tend to decrease the average mass of satellite galaxies at all galactocentric radii. Of all satellites, those accreted before re-ionization (z≳ 6), which may be singled out by anomalous metallicity patterns, provide the strongest constraints on the inner profile of dark haloes. These galaxies move on orbits that penetrate the disc repeatedly and survive to the present day only if haloes have an inner density cusp. We show that the size–mass relationship established from Milky Way (MW) dwarfs strongly supports the presence of cusps in the majority of these systems, as cored models systematically underestimate the masses of the known ultra-faint dSphs. Our models also indicate that a massive M31 disc may explain why many of its dSphs with suitable kinematic data fall below the size–mass relationship derived from MW dSphs. We also examine whether our modelling can constrain the mass threshold below which star formation is suppressed in dark matter haloes. We find that luminous satellites must be accreted with masses above 108–109 M⊙ in order to explain the size–mass relation observed in MW dwarfs.
ABSTRACT
Peters’ formula is an analytical estimate of the time-scale of gravitational wave (GW)-induced coalescence of binary systems. It is used in countless applications, where the convenience of a ...simple formula outweighs the need for precision. However, many promising sources of the Laser Interferometer Space Antenna (LISA), such as supermassive black hole binaries and extreme mass-ratio inspirals (EMRIs), are expected to enter the LISA band with highly eccentric (e ≳ 0.9) and highly relativistic orbits. These are exactly the two limits in which Peters’ estimate performs the worst. In this work, we expand upon previous results and give simple analytical fits to quantify how the inspiral time-scale is affected by the relative 1.5 post-Newtonian (PN) hereditary fluxes and spin–orbit couplings. We discuss several cases that demand a more accurate GW time-scale. We show how this can have a major influence on quantities that are relevant for LISA event-rate estimates, such as the EMRI critical semimajor axis. We further discuss two types of environmental perturbations that can play a role in the inspiral phase: the gravitational interaction with a third massive body and the energy loss due to dynamical friction and torques from a surrounding gas medium ubiquitous in galactic nuclei. With the aid of PN corrections to the time-scale in vacuum, we find simple analytical expressions for the regions of phase space in which environmental perturbations are of comparable strength to the effects of any particular PN order, being able to qualitatively reproduce the results of much more sophisticated analyses.
Abstract
In the local Universe, black holes of $10^{5-6}\, {\rm M_\odot }$ are hosted in galaxies displaying a variety of stellar profiles and morphologies. These black holes are the anticipated ...targets of LISA, the Laser Interferometer Space Antenna that will detect the low-frequency gravitational-wave signal emitted by binary black holes in this mass interval. In this paper, we infer upper limits on the lifetime of binary black holes of $10^{5-6}\, {\rm M_\odot }$ and up to $10^8\, {\rm M_\odot }$, forming in galaxy mergers, exploring two underlying stellar density profiles, by Dehnen and by Prugniel & Simien, and by exploiting local scaling relations between the mass of the black holes and several quantities of their hosts. We focus on the phase of the dynamical evolution when the binary is transitioning from the hardening phase ruled by the interaction with single stars to the phase driven by the emission of gravitational waves. We find that different stellar profiles predict very distinct trends with binary mass, with lifetimes ranging between fractions of a Gyr to more than 10 Gyr, and with a spread of about one order of magnitude, given by the uncertainties in the observed correlations, which are larger in the low-mass tail of the observed black hole population.
PDF
