Black Hole Cluster Research Articles

Hypervelocity stars from the Andromeda galaxy

Hypervelocity stars (HVSs) discovered in the Milky Way (MW) halo are thought to be ejected from near the massive black hole (MBH) at the galactic centre. In this paper we investigate the spatial and velocity distributions of the HVSs which are expected to be similarly produced in the Andromeda galaxy (M31). We consider three different HVS production mechanisms: (i) the disruption of stellar binaries by the galactocentric MBH; (ii) the ejection of stars by an in-spiraling intermediate mass black hole; and (iii) the scattering of stars off a cluster of stellar-mass black holes orbiting around the MBH. While the first two mechanisms would produce large numbers of HVSs in M31, we show that the third mechanism would not be effective in M31. We numerically calculate 1.2*10^6 trajectories of HVSs from M31 within a simple model of the Local Group and hence infer the current distribution of these stars. Gravitational focusing of the HVSs by the MW and the diffuse Local Group medium leads to high densities of low mass (~ solar mass) M31 HVSs near the MW. Within the virialized MW halo, we expect there to be of order 1000 HVSs for the first mechanism and a few hundred HVSs for the second mechanism; many of these stars should have distinctively large approach velocities (< -500 km/s). In addition, we predict ~5 hypervelocity RGB stars within the M31 halo which could be identified observationally. Future MW astrometric surveys or searches for distant giants could thus find HVSs from M31.

Hypervelocity stars and the environment of Sgr A*

Abstract Hypervelocity stars (HVSs) are a natural consequence of the presence of a massive nuclear black hole (Sgr A*) in the Galactic Centre. Here we use the Brown et al. sample of unbound and bound HVSs together with numerical simulations of the propagation of HVSs in the Milky Way halo to constrain three plausible ejection mechanisms: (i) the scattering of stars bound to Sgr A* by an in-spiralling intermediate-mass black hole (IMBH); (ii) the disruption of stellar binaries in the tidal field of Sgr A*; and (iii) the two-body scattering of stars off a cluster of stellar-mass black holes orbiting Sgr A*. We compare the predicted radial and velocity distributions of HVSs with the limited-statistics data set currently available, and show that the IMBH model appears to produce a spectrum of ejection velocities that is too flat. Future astrometric and deep wide-field surveys of HVSs should shed unambiguous light on the stellar ejection mechanism and probe the Milky Way potential on scales as large as 200 kpc.

Initial Populations of Black Holes in Star Clusters

Using an updated population synthesis code we study the formation and evolution of black holes (BHs) in young star clusters following a massive starburst. This study continues and improves on the initial work described by Belczynski, Sadowski & Rasio (2004). In our new calculations we account for the possible ejections of BHs and their progenitors from clusters because of natal kicks imparted by supernovae and recoil following binary disruptions. The results indicate that the properties of both retained BHs in clusters and ejected BHs (forming a field population) depend sensitively on the depth of the cluster potential. In particular, most BHs ejected from binaries are also ejected from clusters with central escape speeds Vesc 50 km/s. BHs from single star evolution are also affected significantly: about half of the BHs originating from primordial single stars are ejected from clusters with Vesc < 50 km/s. Our results lay a foundation for theoretical studies of the formation of BH X-ray binaries in and around star clusters, including possible ultra-luminous sources, as well as merging BH--BH binaries detectable with future gravitational-wave observatories.

AGN Heating, Thermal Conduction, and the Sunyaev‐Zel'dovich Effect in Galaxy Groups and Clusters

We investigate in detail the role of active galactic nuclei (AGNs) in the physical state of the gas in galaxy groups and clusters, and the implications for anisotropy in the cosmic microwave background (CMB) from the Sunyaev-Zel'dovich (SZ) effect. We have recently shown that AGNs can significantly change the entropy of the intracluster medium (ICM) and explain the observations of excess entropy in groups and clusters. AGNs are assumed to deposit energy via buoyant bubbles that expand as they rise in the cluster atmosphere and do P dV work on the ICM. Here we include the effect of thermal conduction and find that the resulting profiles of temperature and entropy are consistent with observations. Unlike previously proposed models, our model predicts that isentropic cores are not an inevitable consequence of preheating. The model also reproduces the observational trend for the density profiles to flatten in lower mass systems. We deduce the energy (Eagn) required to explain the entropy observations as a function of mass of groups and clusters (Mcluster) and show that Eagn ∝ M with α ~ 1.5. We demonstrate that the entropy measurements, in conjunction with our model, can be translated into constraints on the cluster-black hole mass relation. The inferred relation is nonlinear and has the form Mbh ∝ M. This scaling is an analog and extension of a similar relation between the black hole mass and the galactic halo mass that holds on smaller scales. In addition, we study the implications of these results for the thermal SZ effect. We show that the central decrement of the CMB temperature is reduced due to the enhanced entropy of the ICM and that the decrement predicted from the plausible range of energy input from the AGN is consistent with available data on the SZ decrement. We also estimate the Poisson contribution to the angular power spectrum of the CMB from the SZ effect due to AGN heating. We show that AGN heating, combined with the observational constraints on entropy, leads to suppression of higher multipole moments in the power spectrum, and we find that this effect is stronger than previously thought. The suppression in the power spectrum in our model is due to depletion of gas from the central regions that is more efficient in low-mass clusters and groups than in massive clusters.

The dynamical state of a thick cloudy torus around an AGN

We present stationary models of obscuring dusty tori around active galactic nuclei. Vertical and radial structure are discussed separately based on a dynamical approach to a cloudy accretion flow. The vertical cloud distribution is described by a modified isothermal disk for arbitrary thickness of the torus with a wide funnel along the symmetry axis. The radial accretion flow is a consequence of cloud-cloud collisions. Accretion in the combined gravitational potential of central black hole and stellar cluster generates free energy, which is dissipated in collisions, and maintains the thickness of the torus. We focus on the largest clouds at the tidal shear limit, which will dominate the dynamics and the appearance of torus. Successful obscuration as required by unified schemes of AGN is achieved for large mass accretion rates above the Eddington limit of the central black hole. A complete quantitative treatment for the putative torus in the prototypical Seyfert 2 nucleus of NGC 1068 is given.

Orbital Capture of Stars by a Massive Black Hole via Exchanges with Compact Remnants

We propose a dynamical mechanism for capturing stars around a massive black hole (MBH) that is based on the accumulation of a very dense cluster of compact stellar remnants near the MBH. This study is motivated by the presence of ~10 young massive stars (M* ~ 3-15 M☉, spectral types ~B9 V-O8 V) less than 0.04 pc from the MBH in the Galactic center (GC). Their existence in the extreme environment so close to an MBH is a challenge for theories of star formation and stellar dynamics. We show that young stars, which formed far from the MBH and were then scattered into eccentric orbits, repeatedly cross a cluster of stellar black holes (SBHs) where they may undergo rare direct three-body exchanges with an MBH-SBH binary. The interaction between two objects of comparable mass ejects the SBH and captures the star on a tight orbit around the MBH. Such captures can naturally explain some trends observed in the orbits of the young stars. We derive the capture cross section, validate it by Monte Carlo simulations, and calculate the number of captured stars in the GC using the currently uncertain estimates of the numbers of SBHs in the inner 0.04 pc and of young stars in the inner few parsecs of the GC. We find that under favorable conditions, three-body exchange can account for ~25% of the observed stars, mostly at the fainter end of the observed range. We discuss additional effects that possibly increase the capture efficiency. Future detections of the dark mass around the MBH and deeper surveys of the central parsecs will establish whether or not there are enough SBHs and young stars there for exchange captures to singly account for the central young stars. We estimate that there are also ~35 lower mass stars (M* ~ 1-3 M☉, ~G2 V-A0 V) in the inner 0.04 pc similarly captured by exchanges with neutron stars (NSs). Ongoing replacement of compact remnants by main-sequence stars (SBHs by NS progenitors, NSs by white dwarf progenitors) may regulate the accumulation of compact remnants near the MBH.

The efficiency of the spiral-in of a black hole to the Galactic Centre

We study the efficiency at which a black hole or dense star cluster spirals in to the Galactic Centre. This process takes place on a dynamical friction time-scale, which depends on the value of the Coulomb logarithm (In Λ). We determine the accurate value of this parameter using the direct N-body method, a tree algorithm and a particle-mesh technique with up to two million plus one particles. The three different techniques are in excellent agreement. Our measurement for the Coulomb logarithm appears to be independent of the number of particles. We conclude that In A = 6.6 ′ 0.6 for a massive point particle in the inner few parsec of the Galactic bulge. For an extended object, such as a dense star cluster, In A is smaller, with a value of the logarithm argument A inversely proportional to the object size.

Millisecond Pulsars as Probes of Mass Segregation in the Galactic Center

We propose a simple test for the existence of a cluster of black hole remnants around Sgr A* that is based on a small sample of any type of Galactic center objects, provided they are substantially less massive than the black holes and constitute part of an old (≳1 Gyr) population. The test relies on the fact that, in the presence of such a cluster of heavy remnants and because of energy equipartition, lower mass objects would be expelled from the central regions and settle into a distribution very different from the cusp expected to be induced by the supermassive black hole alone. We show that with a sample of just 50 objects and using only their angular positions on the sky relative to Sgr A*, it is possible to clearly differentiate between a distribution consistent with the presence of the cluster of black holes and a power-law cusp distribution. We argue that millisecond pulsars might currently be the best candidate to perform this test, because of the large uncertainties involved in the age determination of less exotic objects. In addition, by measuring their first and second period derivatives, millisecond pulsars offer the rare opportunity to determine the complete phase-space information of the objects. We show that this extra information improves the detection of mass segregation by about 30%.

The formation of primary galactic nuclei during phase transitions in the early universe

A new mechanism describing the formation of protogalaxies is proposed, based on the second-order phase transition in the inflation stage and the domain wall formation upon the end of inflation. The presence of closed domain walls with the size markedly exceeding the cosmological horizon at the instant of their formation and the wall collapse in the postinflation epoch (when the wall size becomes comparable with the cosmological horizon) lead to the formation of massive black hole clusters that can serve as nuclei for the future galaxies. The black hole mass distributions obtained do not contradict the available experimental data. The number of black holes with M ∼ 100 solar masses (M ⊙) and above is comparable with the number of galaxies in the visible Universe. Development of the proposed approach gives grounds for a principally new scenario of galaxy formation in the model of a hot Universe.

A Cluster of Black Holes at the Galactic Center

If the stellar population of the bulge contains black holes formed in the final core collapse of ordinary stars with M \ga 30 M_{\odot}, then about 25,000 stellar mass black holes should have migrated by dynamical friction into the central parsec of the Milky Way, forming a black hole cluster around the central supermassive black hole. These black holes can be captured by the central black hole when they randomly reach a highly eccentric orbit due to relaxation, either by direct capture (when their Newtonian peribothron is less than 4 Schwarzschild radii), or after losing orbital energy through gravitational waves. The overall depletion timescale is ~ 30 Gyr, so most of the 25,000 black holes remain in the central cluster today. The presence of this black hole cluster would have several observable consequences. First, the low-mass, old stellar population should have been expelled from the region occupied by the black hole cluster due to relaxation, implying a core in the profile of solar-mass red giants with a radius of ~ 2 pc (i.e., 1'). The observed central density cusp (which has a core radius of only a few arc seconds) should be composed primarily of young (\la 1 Gyr) stars. Second, flares from stars being captured by supermassive black holes in other galaxies should be rarer than usually expected because the older stars will have been expelled from the central regions by the black hole clusters of those galaxies. Third, the young (\la 2 Gyr) stars found at distances ~ 3 - 10 pc from the Galactic center should be preferentially on highly eccentric orbits. Fourth, if future high-resolution $K$-band images reveal sources microlensed by the Milky Way's central black hole, then the cluster black holes could give rise to secondary (``planet-like'') perturbations on the main event.

Evolution of multiple black hole nuclei in merged galaxies

AbstractThe evolution of three and four black hole clusters which arise in mergers of small galaxy groups have been simulated. Typically, the chaotic dynamics of these multiple black hole systems leads to a decay via the slingshot process. Orbits of 100,000 initial configurations have been calculated and the final configurations have been classified in seven different categories. Examples of the orbits in each category are illustrated.

X‐ray variability study of ROSAT sources

AbstractThe Röntgensatellite ROSAT has been in orbit since June 1,1990. After a 6‐month all‐sky survey ˜ 6,500 pointed observations have been performed. In total more than 150,000 X‐ray sources have been detected. Among the various object classes observed are comets, stars, white dwarfs, cataclysmic variables, neutron stars, black hole candidates, supernova remnants, nearby galaxies, AGN, and clusters of galaxies. Many of these objects are known to be time variable sources. Various catalogues of sources found in the ROSAT All‐Sky Survey and in the pointed observations are used as the basis for studying the temporal behaviour of ROSAT sources. This paper deals with the description, production and content of the various ROSAT catalogues with particular emphasis on the usefulness for the study of source variability.

The Nuclear Star Cluster of the Milky Way: Star Formation, Stellar Collisions and Central Dark Mass

High resolution near-infrared imaging and spectroscopy now gives detailed information about the structure, evolution and mass distribution in the nuclear star cluster of the Milky Way. The central parsec is powered by a cluster of luminous and helium rich, blue supergiants/Wolf-Rayet stars. The most likely scenario for the formation of the massive stars is a star formation burst a few million years ago at which time a dense gas cloud may have fallen into the center. The stellar density in the ∼ 0.3 pc radius central core is high enough that collisions with main sequence stars destroy the largest late type giant stars. Radial velocity measurements for about 300 early and late type stars between 0.1 and 5pc radius from the dynamic center now strongly favor the existence of a central dark mass of 2.5 − 3.3 × 106M⊙ (density (109M⊙pc−3, M/L2μm) ∼ 100M⊙/L⊙) within 0.1pc of the dynamic center. This central dark mass cannot be a cluster of neutron stars. It is either a compact cluster of stellar black holes or, most likely, a single massive black hole.

Active galactic nuclei. 5: X-ray variability and the black hole cluster paradigm

Active galactic nuclei. 5: X-ray variability and the black hole cluster paradigm

Evolution of galactic nuclei with 10-M black holes

A star with main-sequence mass greater than 25-30 M⊙ may collapse to a black hole of about 10 M⊙ at the final stage of the evolution. About an order of 1 per cent of stellar mass is likely to be in the form of such black holes in galaxies. The presence of even such a small number of 10-M⊙ black holes in a spherical stellar system mainly composed of low-mass main-sequence stars (i.e. M≲1 M⊙) can have significant effects on the dynamical evolution. We have examined the dynamics of two-component stellar systems composed of 0.7-M⊙ main-sequence stars, representing the old population of stars whose main-sequence lifetimes are longer than the Hubble time, and a small fraction of 10-M⊙ black holes. The dynamical friction leads to the segregation of black holes to the core, and the core collapse takes place among the black holes on a time-scale much shorter than that required for a single component cluster. Various physical processes occur as the density of the central cluster of black holes becomes higher: formation of binaries by three-body processes, heating by these binaries, gravitational radiation capture of black hole binary formation and subsequent merger, and tidal capture or disruption of stars by black holes. The ultimate evolution of the two-component stellar system depends on the role of three-body binaries formed among the black holes. In low-velocity-dispersion systems (i.e. ν≲100 km s-1, where v is the three-dimensional velocity dispersion), three-body binaries are eventually ejected from the stellar system after being hardened to a hardness of a few hundred. This picture has to be altered as the velocity dispersion changes. For a system with ν≳100 km s-1, binaries merge by gravitational radiation at some hardness instead of being ejected. The critical hardness, at which the collision time and the merger time become comparable, determines the efficiency of the binary as a heat source. The efficiency is found to be inversely proportional to the velocity dispersion. For clusters without a serious reduction in heating efficiency (i.e. velocity dispersion well below 500 km s-1), heating by three-body binaries has the effect of stopping the core-collapse. The cluster expands, but at a rate set by the half-mass relaxation time of the whole system, which is very long. Thus one obtains a nearly static two-component configuration: a central cluster of black holes surrounded by low-mass clusters. However, such a state would not last longer than the Hubble time if v≳50 km s-1, because most of the black holes would experience binary formation and subsequent mergers. Thus a seed black hole can easily form in the central parts of galaxies with even moderate initial conditions (i.e. νc≳100 km s-1).

Active galactic nuclei. IV - Supplying black hole clusters by tidal disruption and by tidal capture of stars

The extent to which individual holes in a cluster of black holes with a mass spectrum can liberate and accrete the resulting material by tidally disrupting stars they encounter, or by capturing stars as binary companions is studied. It is found that the smaller black holes in 'the halo' of such clusters can adequately supply themselves to the level M-dot sub h or greater than 0.0001(M-dot sub h) sub crit, and up to 0.05(M-dot sub h)sub crit for the smallest holes, by tidal disruption, as long as the cluster is embedded in a distribution of stars of relatively high density (not less than 0.1M sub cl/cu pc), and as long as the entire cluster of stars is not too compact (not less than 0.5 pc). Consideration is given to modifications this 'internal' mode of supply introduces in the spectrum emitted by such black hole clusters, and to the current status of their viability as models for AGN and QSOs in light of dynamical studies by Quinlan and Shapiro (1987, 1989).

The Few-Body Problem in Astrophysics

The few body problem is reviewed for the evolution of star clusters, the interacting galaxies, the clusters of galaxies and the black hole clusters

Active galactic nuclei. III - Accretion flow in an externally supplied cluster of black holes

This third paper in the series modeling QSOs and AGNs as clusters of accreting black holes studies the accretion flow within an externally supplied cluster. Significant radiation will be emitted by the cluster core, but the black holes in the outer halo, where the flow is considered spherically symmetric, will not contribute much to the overall luminosity of the source because of their large velocities relative to the infalling gas and therefore their small accretion radii. As a result, the scenario discussed in Paper I will refer to the cluster cores, rather than to entire clusters. This will steepen the high-frequency region of the spectrum unless inverse Compton scattering is effective. In many cases accretion flow in the central part of the cluster will be optically thick to electron scattering, resulting in a spectrum featuring optically thick radiative component in addition to power-law regimes. The fitting of these spectra to QSO and AGN observations is discussed, and application to 3C 273 is worked out as an example.

Active galactic nuclei. II - The acceleration of relativistic particles in a cluster of accreting black holes

An accreting cluster of black holes in an active galactic nucleus is a natural site for a system of shock structures with a hierarchy of sizes, corresponding to the distribution of masses in the cluster. Accreted gas containing some magnetic fields and supersonically falling onto the core forms shocks on the outside of each hole and these shocks are capable of accelerating relativistic particles. The energies reached in a single shock are size rather than acceleration time limited and are proportional to the mass of the hole with a proportionality constant being a function of the position of the hole within a cluster and the model of the cluster and the shock formation. These energies are adequate to explain the observed properties of synchrotron and inverse-Compton radiation from these objects. The resulting energy spectrum of particles in the cluster in "zeroth" approximation has the form of a doubly broken power law with indices of 2 and 3 on both extremes of the energy domain respectively, bridged by an index of ~2.5.

Active galactic nuclei. I - Central source as a cluster of accreting black holes

view Abstract Citations (14) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Active Galactic Nuclei. I. Central Source as a Cluster of Accreting Black Holes Pacholczyk, A. G. ; Stoeger, W. R. Abstract A generalization of an accreting supermassive black hole model for the central nuclear engine of QSOs and active galactic nuclei (AGNs) is suggested in terms of a dense cluster of many black holes and compact objects. In this scenario, the cluster possesses a mass spectrum, with each hole surrounded by an accretion disk. The fitting of the thermal radiation spectrum of such a cluster to the observed optical-X-ray power-law spectrum of QSOs and AGNs determines the total mass, number of black holes, and mass spectrum index for the cluster. Their dynamics are also discussed, and the consumption and evaporation rates of member holes are estimated under the approximation of many stellar-size holes in the field of a single large central black hole. Finally, the variability of a QSO or AGN modeled in terms of a cluster of accreting holes is investigated, and it is shown that such a model allows the same time scales of variability as, or even shorter ones than, a model involving a single hole of the same mass. The shorter time scales are allowed when rapid variations are limited to a longer waveband, as is observed for sources in contrast with the single-hole model. Publication: The Astrophysical Journal Pub Date: April 1986 DOI: 10.1086/164053 Bibcode: 1986ApJ...303...76P Keywords: Accretion Disks; Active Galactic Nuclei; Astronomical Models; Black Holes (Astronomy); Quasars; Galactic Structure; Periodic Variations; Astrophysics; BLACK HOLES; GALAXIES: NUCLEI; QUASARS full text sources ADS | data products SIMBAD (36) NED (34) Related Materials (5) Part 2: 1988ApJ...324..695P Part 3: 1989ApJ...343..563P Part 4: 1992ApJ...391..550S Part 5: 1994ApJ...434..435P Part 6: 2000ApJ...540..122L