Holes In Globular Clusters Research Articles

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