Intermediate-mass Black Holes in Star Clusters

Intermediate-mass Black Holes in Star Intermediate-mass Black Holes in Star ClustersClusters

Holger Baumgardt

Astrophysical Computing Laboratory, RIKEN, Tokyo holger@riken.jp

new address: University of Bonn, Germany

in collaboration with

Jun Makino, Simon Portegies Zwart,

Piet Hut, Steve McMillan, Toshikazu Ebisuzaki

NBODY4 + GRAPE6

Formation of IMBH's in star clustersFormation of IMBH's in star clusters

Observations indicate that there might be a connection between ULX and star clusters. Matsumoto et al. (2001) for example found a bright X-ray source at the center of the starburst galaxy M82 with an Eddington luminosity corresponding to a black hole of several hundred solar masses. Optical follow-up observations showed that the position of this source coincides with that of a young luminous star cluster.

McCrady et al. (2003) used the HST and Keck telescopes to determine the density profile and total masses for a number of young star clusters in M82.

MGG-11 was the most concentrated (half-light

radius 1.2 pc) and second heaviest cluster in

their sample (M= 3.5*10**5 Msun).

In addition it appears to have a top-heavy IMF.

Formation of IMBH's in star clustersFormation of IMBH's in star clusters

Simulations of Black Hole FormationSimulations of Black Hole Formation

We followed the evolution of MGG-11 by N-body simulations of star clusters containing N= 130.000 stars, and starting from King models with initial concen- trations in the range 3.0 < Wo < 12.0. We found that heavy mass stars sink into the cluster center as a result of dynamical friction. For central concentrations Wo>9.0, this happened fast enough that runaway merging of stars occurs in the center.

Simulations of Black Hole FormationSimulations of Black Hole Formation

The runaway merging leads to the formation of a single object of more than 1000 Msun within a few Myrs. This object could form an inter- mediate-mass black hole which creates the X-ray radiation. Simulations of other clusters show that only MGG-11 is concentrated enough to undergo runaway merging, in agreement with the fact that only MGG-11 contains an ULX.

(from Portegies Zwart et al. 2004)

Black Holes in Globular ClustersBlack Holes in Globular Clusters

Observations of M15Observations of M15

The radial velocities of stars show an increase of the vel. dispersion towards the center.

If one estimates the observed velocity

dispersion from the cluster light profile with a

constant M/L ratio, one obtains a mismatch in

the inner parts.

This was seen as evidence for an IMBH of

about 2000 Msun in the center of M15.

(from Gerssen et al. 2002)

N-body simulations of M15N-body simulations of M15

Density profile Velocity dispersion

Evolution of Star Clusters with Black Holes

Our simulations have shown that star clusters with high enough densities can form black holes through run-away merging of stars.

In addition, the simulations done so far have shown that a black

hole in M15 is not necessary to explain the observations, but they

do not rule it out.

We therefore also made simulations of star clusters which start

with an IMBH at their center.

Evolution of clusters with an IMBH

Cluster expansion Projected radii

Evolution of clusters with an IMBH

3D Density profile:

Projected Density Profile

The projected density distribution of bright stars has a constant density core and would appear like a standard King profile cluster to observers. HST observations of the central velocity dispersion would reveal the IMBH for galactic GCs.

Dynamical Processes in the Center

Black hole ejection The innermost stars

Gravitational radiation from IMBH

Scaling our result to larger particle numbers, we find that each IMBH in a globular cluster will undergo at least one merging event with another BH. Assuming a 10% IMBH fraction, there is only a 10% chance that any galactic GC presently emits detectable amounts of GR. Within 1 Gpc, about 5 events should be detectable per year.

Disruption of stars by the IMBH

IMBHs in the densest globular clusters have disruption rates of up to 1E-6/yr. Most clusters with high disruption rates are however core collapse clusters. IMBH in clusters with large enough radii have disruption rates of only 1E-9/yr. IMBHs would therefore probably be invisible in X-rays most of the time.

Disruption of Stars by the IMBH

Most stars disrupted by the IMBH are main-sequence stars and giants. Disruption of neutron stars are rare. During the simulations, no black holes merged with the IMBH. Stars disrupted by the IMBH move on very elongated orbits, so the fate of the stellar material is uncertain.