Search

We present a method for comparing the Hβ emission-line profiles of observed supermassive black hole (SBHB) candidates and models of subparsec SBHBs in circumbinary disks. Using the approach based on principal component analysis, we infer the values of the binary parameters for the spectroscopic SBHB candidates and evaluate the parameter degeneracies, representative of the uncertainties intrinsic to such measurements. We find that as a population, the SBHB candidates favor the average value of the semimajor axis corresponding to log(a/M) ≈ 4.20 ± 0.42 and comparable mass ratios, q > 0.5. If the SBHB candidates considered are true binaries, this result would suggest that there is a physical process that allows initially unequal-mass systems to evolve toward comparable mass ratios (e.g., accretion that occurs preferentially onto the smaller of the black holes) or point to some, yet unspecified, selection bias. Our method also indicates that the SBHB candidates equally favor configurations in which the mini-disks are coplanar or misaligned with the binary orbital plane. If confirmed for true SBHBs, this finding would indicate the presence of a physical mechanism that maintains misalignment of the mini-disks down to subparsec binary separations (e.g., precession driven by gravitational torques). The probability distributions of the SBHB parameters inferred for the observed SBHB candidates and our control group of AGNs are statistically indistinguishable, implying that this method can in principle be used to interpret the observed emission-line profiles once a sample of confirmed SBHBs is available but cannot be used as a conclusive test of binarity. [ABSTRACT FROM AUTHOR]

The discovery of a binary comprising a black hole (BH) and a millisecond pulsar (MSP) would yield insights into stellar evolution and facilitate exquisitely sensitive tests of general relativity. Globular clusters (GCs) are known to harbour large MSP populations and recent studies suggest that GCs may also retain a substantial population of stellar mass BHs. We modelled the formation of BH+MSP binaries in GCs through exchange interactions between binary and single stars. We found that in dense, massive clusters most of the dynamically formed BH+MSP binaries will have orbital periods of 2–10 d, regardless of the mass of the BH, the number of BHs retained by the cluster, and the nature of the GC's binary population. The size of the BH+MSP population is sensitive to several uncertain parameters, including the BH mass function, the BH retention fraction, and the binary fraction in GCs. Based on our models, we estimate that there are 0.6 ± 0.2 dynamically formed BH+MSP binaries in the Milky Way GC system, and place an upper limit on the size of this population of ∼10. Interestingly, we find that BH+MSP binaries will be rare even if GCs retain large BH populations. [ABSTRACT FROM AUTHOR]

We model the formation of black hole–neutron star (BH–NS) binaries via dynamical interactions in globular clusters. We find that in dense, massive clusters, 16–61 per cent of the BH–NS binaries formed by interactions with existing BH binaries will undergo mergers driven by the emission of gravitational radiation. If the BHs are retained by the cluster after merging with an NS, the BHs acquire subsequent NS companions and undergo several mergers. Thus, the merger rate depends critically upon whether or not the BH is retained by the cluster after the merger. Results from numerical relativity suggest that kick imparted to a ∼7 M⊙ BH after it merges with an NS will greatly exceed the cluster's escape velocity. In this case, the models suggest that the majority of BH–NS mergers in globular clusters occur within 4 Gyr of the cluster's formation and would be unobservable by Advanced LIGO. For more massive BHs, on the other hand, the post-merger kick is suppressed and the BH is retained. Models with 35 M⊙ BHs predict Advanced LIGO detection rates in the range 0.04–0.7 yr−1. On the pessimistic end of this range, BH–NS mergers resulting from binary–single star interactions in globular clusters could account for an interesting fraction of all BH–NS mergers. On the optimistic end, this channel may dominate the rate of detectable BH– NS mergers. [ABSTRACT FROM AUTHOR]

ABSTRACT We model the emission lines generated in the photoionized debris of a tidally disrupted horizontal branch (HB) star. We find that at late times, the brightest optical emission lines are [N ii] λλ6548, 6583 and [O iii] λλ4959, 5007. Models of a red clump HB star undergoing mild disruption by a massive (50-100 M⊙) black hole (BH) yield an emission-line spectrum that is in good agreement with that observed in the NGC 1399 globular cluster hosting the ultraluminous X-ray source CXOJ033831.8−352604. We make predictions for the ultraviolet emission-line spectrum that can verify the tidal disruption scenario and constrain the mass of the BH. [ABSTRACT FROM AUTHOR]

We explore the growth of black holes that reside at the centres of spiral and field dwarf galaxies in a Local Group type of environment. We use merger trees from a cosmological N-body simulation known as Via Lactea 2 (VL-2) as a framework to test two merger-driven semi-analytic recipes for black hole growth that include dynamical friction, tidal stripping and gravitational wave recoil in over 20 000 merger tree realizations. First, we apply a Fundamental Plane limited (FPL) model to the growth of Sgr A*, which drives the central black hole to a maximum mass limited by the black hole Fundamental Plane after every merger. Next, we present a new model that allows for low-level prolonged gas accretion (PGA) during the merger. We find that both models can generate an Sgr A* mass black hole. We predict a population of massive black holes in local field dwarf galaxies - if the VL-2 simulation is representative of the growth of the Local Group, we predict up to 35 massive black holes () in Local Group field dwarfs. We also predict that hundreds of black holes fail to merge, and instead populate the Milky Way halo, with the most massive of them at roughly the virial radius. In addition, we find that there may be hundreds of massive black holes ejected from their hosts into the nearby intergalactic medium due to gravitational wave recoil. We discuss how the black hole population in the Local Group field dwarfs may help to constrain the growth mechanism for Sgr A*. [ABSTRACT FROM AUTHOR]

Primordial stars are likely to be very massive (≥ 30 M⊙), form in isolation, and will likely leave black holes as remnants in the centres of their host dark matter haloes. We expect primordial stars to form in haloes in the mass range 106–1010 M⊙. Some of these early black holes, formed at redshifts z≳ 10, could be the seed black holes for a significant fraction of the supermassive black holes (SMBHs) found in galaxies in the local Universe. If the black hole descendants of the primordial stars exist, their merger with nearby SMBHs may be a prime candidate for long wavelength gravitational wave detectors. We simulate formation and evolution of dark matter haloes in a Λ cold dark matter universe. We seed high-redshift dark matter haloes with early black holes, and explore the merger history of the host haloes and the implications of black hole kick velocities arising from their coalescence. The central concentration of low-mass early black holes in present-day galaxies is reduced if they experience even moderate kicks of tens of km s−1. Even such modest kicks allow the black holes to leave their parent halo, which consequently leads to dynamical friction being less effective on the low-mass black holes that were ejected, compared to those still embedded in their parent haloes. Therefore, merger rates with central SMBHs in the largest halo may be reduced by more than an order of magnitude. Using analytical and illustrative cosmological N-body simulations, we quantify the role of kicks on the merger rates of black holes formed from massive metal-free stars with SMBHs in present-day galaxies. [ABSTRACT FROM AUTHOR]

Explores the dynamical and observational consequences of the presence of primordial black-hole population in a globular cluster of stars. Segregation; Number of retained black-holes; Disruption of main-sequence and giant stellar binaries; Tidal interactions.