Transcript N-Body Simulations of Star Clusters with IMBH

Intermediate-mass Black Holes in Star
Clusters
Holger Baumgardt
Astrophysical Computing Laboratory, RIKEN, Tokyo [email protected]
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 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.
Formation of IMBH's in star clusters
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.
Simulations 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 concentrations 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 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- mediatemass 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 Clusters
Observations 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 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 1E9/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.