Black Holes In Globular Clusters Research Articles

The presence of intermediate-mass black holes in globular clusters and their connection with extreme horizontal branch stars

By means of a multimass isotropic and spherical model that includes the self-consistent treatment of a central intermediate-mass black hole (IMBH), the influence of this black hole on the morphological and physical properties of globular clusters is investigated. In agreement with recent numerical studies, it is found that a cluster (with mass M) hosting an IMBH (with mass M.) shows, outside the region of gravitational influence of the black hole, a core-like profile resembling a King profile with concentration c M. The mass range estimate 12-4.8 < log(M./M) < - 1.1 c - 0.69, depending on the morphological parameters, is deduced by considering a wide range of models. Applying this estimate to a set of 39 globular clusters, it is found that NGC 2808, NGC 6388, M80, M13, M62, M54 and Gl (in M31) probably host an IMBH. For them, the scaling laws M. ∼ 0.09(M/M ⊙ ) 0.7 M ⊙ and M. ∼ 50(σ obs /km s -1 ) 1.2 M ⊙ are identified from weighted least-squares fits. An important result of this 'collective' study is that a strong correlation exists between the presence of an extreme blue horizontal branch and the presence of an IMBH. In particular, the presence of a central IMBH in M13 and NGC 6388 could explain why these clusters possess extreme horizontal-branch stars, in contrast to their 'second parameter' counterparts M3 and 47 Tuc.

Modelling Individual Globular Clusters

AbstractAstronomers have constructed models of globular clusters for over 100 years. These models mainly fall into two categories: (i) static models, such as King's model and its variants, and (ii) evolutionary models. Most attention has been given to static models, which are used to estimate mass-to-light ratios and mass segregation, and to combine data from proper motions and radial velocities. Evolutionary models have been developed for a few objects using the gaseous model, the Fokker-Planck model, Monte Carlo models and N-body models. These models have had a significant role in the search for massive black holes in globular clusters, for example.In this presentation the problems associated with these various techniques will be summarised, and then we shall describe new work with Giersz's Monte Carlo code, which has been enhanced recently to include the stellar evolution of single and binary stars. We describe in particular recent attempts to model the nearby globular cluster M4, including predictions on the spatial distribution of binary stars and their semi-major axis distribution, to illustrate the effects of about 12 Gyr of dynamical evolution. We also discuss work on an approximate way of predicting the “initial” conditions for such modelling.

Millisecond pulsars around intermediate-mass black holes in globular clusters

Globular clusters (GCs) are expected to be breeding grounds for the formation of single or binary intermediate-mass black holes (IMBHs) of ≳100 M⊙, but a clear signature of their existence is still missing. In this context, we study the process of dynamical capture of a millisecond pulsar (MSP) by a single or binary IMBH, simulating various types of single–binary and binary–binary encounters. It is found that [IMBH, MSP] binaries form over cosmic time in a cluster, at rates ≲10−11 yr−1, via encounters of wide-orbit binary MSPs off the single IMBH, and at a lower pace, via interactions of (binary or single) MSPs with the IMBH orbited by a typical cluster star. The formation of an [IMBH, MSP] system is strongly inhibited if the IMBH is orbited by a stellar mass black hole (BH): in this case, the only viable path is through the formation of a rare stable hierarchical triplet with the MSP orbiting exterior to the [IMBH, BH] binary. The [IMBH, MSP] binaries that form are relatively short-lived, ≲108−109 yr, since their orbits decay via emission of gravitational waves. The detection of an [IMBH, MSP] system has a low probability of occurrence, when inferred from the current sample of MSPs in GCs. If next-generation radio telescopes, like Square Kilometre Array (SKA), will detect an order of magnitude larger population of MSP in GCs, at least one [IMBH, MSP] is expected. Therefore, a complete search for low-luminosity MSPs in the GCs of the Milky Way with SKA will have the potential of testing the hypothesis that IMBHs of the order of 100 M⊙ are commonly hosted in GCs. The discovery will unambiguously prove that BHs exist in the still uncharted interval of masses around ≳100 M⊙.

Detection Rate Estimates of Gravity Waves Emitted during Parabolic Encounters of Stellar Black Holes in Globular Clusters

The rapid advance of gravitational wave (GW) detector facilities makes it very important to estimate the event rates of possible detection candidates. We consider an additional possibility of GW bursts produced during parabolic encounters (PEs) of stellar-mass compact objects in globular clusters (GCs). We estimate the rate of successful detections for specific detectors: the initial Laser Interferometric Gravitational-Wave Observatory (InLIGO), the French-Italian gravitational wave antenna VIRGO, the near-future Advanced-LIGO (AdLIGO), the space-based Laser Interferometric Space Antenna (LISA), and the Next Generation LISA (NGLISA). Simple GC models are constructed to account for the compact object mass function, mass segregation, number density distribution, and velocity distribution. We both calculate encounters classically and account for general relativistic corrections by extrapolating the results for infinite mass ratios. We also include the cosmological redshift of waveforms and event rates. We find that typical PEs with masses m1 = m2 = 40 M☉ are detectable with matched filtering over a signal-to-noise ratio S/N = 5 within a distance dL ~ 200 Mpc for InLIGO and VIRGO, z = 1 for AdLIGO, 0.4 Mpc for LISA, and 1 Gpc for NGLISA. We estimate single data stream detection rates of 5.5 × 10-5 yr-1 for InLIGO, 7.2 × 10-5 yr-1 for VIRGO, 0.063 yr-1 for AdLIGO, 2.9 × 10-6 yr-1 for LISA, and 1.0 yr-1 for NGLISA, for reasonably conservative assumptions. These estimates are subject to uncertainties in the GC parameters, most importantly the total number and mass distribution of BHs in the cluster core. In reasonably optimistic cases, we get ≳1 detection for AdLIGO per year. We expect that a coincident analysis using multiple detectors and accounting for GW recoil capture significantly increases the detection rates. The regular detection of GWs during PEs would provide a unique observational probe for constraining the stellar BH mass function of dense clusters.

The fingerprint of binary intermediate-mass black holes in globular clusters: suprathermal stars and angular momentum alignment

We explore the signatures that a binary intermediate mass black hole (IMBH) imprints on the velocity and on the angular momentum of globular cluster stars. Simulating 3-body encounters between a star and a binary IMBH, we find that the binary IMBH generates a family of few hundreds of stars (~100-300) which remain bound to the globular cluster (GC) and have velocity significantly higher than the dispersion velocity. For this reason we term them ''supra-thermal'' stars. We also notice that, after the interaction, a considerable fraction (55-70%) of stars tend to align their orbital angular momentum with the angular momentum of the binary IMBH, introducing an anisotropy in the angular momentum distribution of cluster stars. We simulate the dynamical evolution of these supra-thermal stars before thermalization, and find that these stars tend to cluster at a distance of few core radii from the GC center. We conclude that the detectability of such signatures appears problematic with present telescopes.

Formation of Massive Black Holes in Globular Clusters

The hydrodynamic formation of massive black holes (BHs) in globular clusters is considered. In particular, we examine the possibility of BH formation induced by the radiation drag that is exerted on the interstellar matter by stellar radiation in globular clusters. The radiation drag extracts angular momentum from interstellar gas and thus allows the gas to accrete onto the cluster center. By incorporating the realistic chemical evolution of globular clusters, we scrutinize the efficiency of the radiation drag to assess the total accreted mass. As a result, we find that if a globular cluster is more massive than ≈6 × 106 M☉ in the present-day stellar component, a massive BH with >260 M☉ can form within it. But the BH-to-cluster mass ratio is considerably smaller than the BH-to-bulge mass ratio (≈10-3) found in galactic bulges. The results are not sensitive to the assumed stellar initial mass function and star formation rate in the cluster, as long as the resulting color-magnitude relation, metallicity, and mass-to-luminosity ratio satisfy those observed in globular clusters. Hence, the putative linear relation between BH mass and bulge mass (MBH-Mbulge relation) cannot be extrapolated to globular cluster systems. In the present regime, we discuss the BH formation in M15, G1, ω Cen, the M33 nucleus, and the compact X-ray sources in M82. Finally, we discuss observational indications of the formation process of massive BHs in globular clusters. We find that the final phase of BH growth due to the radiation drag can be observed as ultraluminous X-ray sources (ULXs) with ~1041 ergs s-1.

Binary Disruption by Massive Black Holes in Globular Clusters

A massive (~102-104 M☉) black hole at the center of a globular cluster will tidally disrupt binaries that pass sufficiently close. Such an encounter results in the capture of one binary component by the black hole and ejection of the other star. A 103 M☉ black hole disrupts one binary every 1-10 Myr; captured stars orbit with initial periods of days to decades. Although 100 stars are captured in only 1 Gyr, the number with periods of <10 yr at any one time is probably less than 10, regulated by stellar interactions and tidal disruption by the black hole. Binary disruption and stellar capture should be critically examined with detailed numerical simulations, in order to corroborate the analytic results given here and to weigh possible observational consequences.

Radio emission as a test of the existence of intermediate-mass black holes in globular clusters and dwarf spheroidal galaxies

We take the established relation between black hole mass, X-ray luminosity, and radio luminosity and show that intermediate mass black holes, such as those predicted to exist at the centers of globular clusters, will be easily identifiable objects in deep radio observations. We show that the radio observations will be far more senstive than any possible X-ray observations. We also discuss the likely optical photometric and spectroscopic appearance of such systems in the event that radio detections are made.

Formation of Massive Black Holes in Dense Star Clusters. I. Mass Segregation and Core Collapse

We study the early dynamical evolution of young, dense star clusters using Monte Carlo simulations for systems with up to N~10^7 stars. Rapid mass segregation of massive main-sequence stars and the development of the Spitzer instability can drive these systems to core collapse in a small fraction of the initial half-mass relaxation time. If the core collapse time is less than the lifetime of the massive stars, all stars in the collapsing core may then undergo a runaway collision process leading to the formation of a massive black hole. Here we study in detail the first step in this process, up to the occurrence of core collapse. We have performed about 100 simulations for clusters with a wide variety of initial conditions, varying systematically the cluster density profile, stellar IMF, and number of stars. We also considered the effects of initial mass segregation and stellar evolution mass loss. Our results show that, for clusters with a moderate initial central concentration and any realistic IMF, the ratio of core collapse time to initial half-mass relaxation time is typically ~0.1, in agreement with the value previously found by direct N-body simulations for much smaller systems. Models with even higher central concentration initially, or with initial mass segregation (from star formation) have even shorter core-collapse times. Remarkably, we find that, for all realistic initial conditions, the mass of the collapsing core is always close to ~10^-3 of the total cluster mass, very similar to the observed correlation between central black hole mass and total cluster mass in a variety of environments. We discuss the implications of our results for the formation of intermediate-mass black holes in globular clusters and super star clusters, ultraluminous X-ray sources, and seed black holes in proto-galactic nuclei.

Formation of Massive Black Holes in Globular Clusters

The hydrodynamic formation of massive black holes (BHs) in globular clusters is considered. In particular, we examine the possibility of BH formation induced by the radiation drag that is exerted on the interstellar matter by stellar radiation in globular clusters. The radiation drag extracts angular momentum from interstellar gas and thus allows the gas to accrete onto the cluster center. By incorporating the realistic chemical evolution of globular clusters, we scrutinize the efficiency of the radiation drag to assess the total accreted mass. As a result, we find that if a ½�

Can High-Velocity Stars Reveal Black Holes in Globular Clusters?

We estimate the number of individual, fast-moving stars observable in globular clusters under the assumption that the clusters contain massive central black holes which follow the galactic black-hole mass vs. sigma relationship. We find that radial velocity measurements are unlikely to detect such stars, but that proper motion studies could reveal such stars, if they exist, in the most likely clusters. Thus, HST proper motion studies can test this hypothesis in a few nearby clusters.

ITAL]Hubble Space Telescope[/ITAL] Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. II. Kinematic Analysis and Dynamical Modeling

We have used the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) to obtain high spatial resolution spectroscopy of the central region of the dense globular cluster M15. The observational strategy and data reduction were described in Paper I of this series. Here we analyze the extracted spectra with a cross-correlation technique to determine the line-of-sight velocities of individual stars. Our final STIS velocity sample contains 64 stars, two-thirds of which have their velocity measured for the first time. The new data set triples the number of stars with measured velocities in the central projected R ? 1'' of M15 and doubles the number in the central R ? 2''. We combine our data with existing ground-based data to obtain nonparametric estimates of the radial profiles of the projected rotation velocity, velocity dispersion, and rms velocity ?rms. The results differ from earlier work in the central few arcseconds in that we find that ?rms rises to ~14 km s-1, somewhat higher than the values of 10?12 km s-1 inferred previously from ground-based data. To interpret the results we construct dynamical models based on the Jeans equation for a spherical system. If the velocity distribution is isotropic, then M15 must have a central concentration of nonluminous material. If this is due to a single black hole, then a fit to the full velocity information as function of radius implies that its mass is MBH = (3.9 ? 2.2) ? 103 M?. The existence of intermediate-mass black holes in globular clusters is consistent with several scenarios for globular cluster evolution proposed in the literature. The inferred mass for M15 is consistent with the extrapolation of the relation between MBH and ?rms that has been established for galaxies. Therefore, these results may have important implications for our understanding of the evolution of globular clusters, the growth of black holes, the connection between globular cluster and galaxy formation, and the nature of the recently discovered ultraluminous X-ray sources in nearby galaxies. Instead of a single intermediate-mass black hole, M15 could have a central concentration of dark remnants (e.g., neutron stars) due to mass segregation. However, we argue that the best-fitting Fokker-Planck models that have previously been constructed for M15 do not predict a central mass concentration that is sufficient to explain the observed kinematics. To fit the M15 data without any central dark mass concentration one must assume that the velocity distribution is significantly radially anisotropic near the center, which contradicts predictions from both Fokker-Planck and N-body calculations.

Four‐Body Effects in Globular Cluster Black Hole Coalescence

In the high-density cores of globular clusters, multibody interactions are expected to be common, with the result that black holes in binaries are hardened by interactions. It was shown by Sigurdsson and Hernquist, Kulkarni, Hut, and McMillan, and others in 1993 that 10 M☉ black holes interacting exclusively by three-body encounters do not merge in the clusters themselves, because recoil kicks the binaries out of the clusters before the binaries are tight enough to merge. Here we consider a new mechanism, involving four-body encounters. Numerical simulations by a number of authors suggest that roughly 20%-50% of binary-binary encounters will eject one star but leave behind a stable hierarchical triple. If the orbital plane of the inner binary is strongly tilted with respect to the orbital plane of the outer object, a secular Kozai resonance, first investigated in the context of asteroids in the solar system, can increase the eccentricity of the inner body significantly. We show that in a substantial fraction of cases, the eccentricity is driven to a high enough value that the inner binary will merge by gravitational radiation, without a strong accompanying kick. Thus, the merged object remains in the cluster; depending on the binary fraction of black holes and the inclination distribution of newly formed hierarchical triples, this mechanism may allow massive black holes to accumulate through successive mergers in the cores of globular clusters. It may also increase the likelihood that stellar-mass black holes in globular clusters will be detectable by their gravitational radiation.

Production of intermediate-mass black holes in globular clusters

The discovery of numerous non-nuclear X-ray point sources with luminosities L>10^39 erg/s in several starburst galaxies has stimulated speculation about their nature and origin. The strong variability seen in several sources points to massive black holes as the central engines. If the flux is isotropic, the luminosities range up to roughly 10^41 erg/s, implying masses of M>10^3 Msun if the luminosity is sub-Eddington. Here we explore a model for these sources. We suggest that in some tens of percent of globular clusters a very massive black hole, M>50 Msun, is formed. This black hole sinks in 50 Msun black hole has enough inertia that it remains bound to the cluster. We suggest that 10^3 Msun black holes may be common in the centers of dense globular clusters, and may therefore exist in some tens of percent of current globulars. If the cluster later merges with its host galaxy, accretion from young star clusters in molecular clouds by the black hole can generate luminosity consistent with that observed. We also consider the detectability of massive black holes in globular clusters with gravitational wave detectors such as LISA and LIGO, and speculate on future observations that may test our predictions.

ASCA observation of ω Centauri

The number of compact stars such as white dwarfs, neutron stars, and black holes in globular clusters gives us information on binary formation rates and dynamical history of the clusters.

Black holes in globular clusters

Black holes in globular clusters

Gas in globular clusters - V. Formation of 10 M black holes in globular clusters

Gas in globular clusters - V. Formation of 10 M black holes in globular clusters

Scattering model for X-ray bursts - Massive black holes in globular clusters

A model for the X-ray bursts discovered from 3U 1820-30 (NGC 6624) is presented. The temporal and spectral variations observed in the bursts are interpreted as Compton scattering of a primary pulse in a hot cloud surrounding the X-ray source. The cloud parameters derived imply the presence of a collapsed core in the globular cluster source with a mass greater than several hundred solar masses.

Massive black holes in globular clusters?

FOUR X-ray sources have been identified1,2 with globular star clusters (NGC1851, NGC6441, NGC6624 and M15). These are the first X-ray sources to be optically identified with the oldest component (age∼1010 yr) of the stellar population of our Galaxy; previous identifications of galactic X-ray sources were mostly with bright, young, Population I stars or with relatively recent supernova remnants. Katz3, Clark4 and Canizares and Neighbours5 have discussed the apparent differences in frequency and population between the globular cluster X-ray sources and other galactic X-ray sources. They have assumed that the X-ray sources in globular clusters are, like the sources identified with Population I stars, binary stars in which a normal companion transfers mass on to a collapsed star, thereby producing X rays.

Observational Aspects of Black Holes in Globular Clusters

Observational Aspects of Black Holes in Globular Clusters