Black Holes in Globular Clusters

Karl Gebhardt (UT)

Spatially-Resolved Kinematics in Globular Clusters

Problems: • HST has limited spectral and spatial resolution

• transition between using integrated-light or individual velocities

• obtaining proper motions is very painful but very powerful; accuracy is similar to the signal; hard to get system rotation without reference

• radial velocities have excellent accuracy but only give 1D information

Resolution : use AO with large ground-based observatories

Current BH/sigma correlation using published results

Results to Date from Globular Clusters

• M15 has been painful for many years

Gerssen et al. 2003 claim a 2000 Msun BH from radial vels

Baumgardt et al. 2003 say no evidence

McNamara et al. 2003 say no evidence from proper motions

Re-analysis give 1-2e3 Msun BH

• G1

Gebhardt, Rich, & Ho claim 2e4 Msun BH

Baumgardt et al. 2003 say no evidence

New data still suggest 1-2e4 Msun BH

• NGC 6752

Drukkier et al. 2003 claim large M/L increase from pm’s

Xie et al. 2003 claim little M/L increase from radial vels

Colpi et al. 2003 use pulsars to argue for something massive

G1 with WFPC G1 with HRC

STIS spectra for G1 Keck spectra for G1

Velocity and dispersion profiles for G1 from KeckHIRES observations.

Black lines are simpleisotropic BH models usingthe surface brightnessprofile to estimate the massdensity. BHs are zero,1e3, 1e4, 5e4 Msun.

G1 M/L profile assuming isotropy

The surface brightness andvelocity dispersion profileprovide a non-parametricestimate of the M/L, assuming isotropy (as in KG & Fischer 95).

Baumgardt et al. find that the central M/L is at least constant and slightly decreases.

Comparison of radial velocities (dotted line) with the proper motion data from McNamara (red and green)for M15.

Comparison with Baumgardt et al theoretical dispersion profile, including a BH of 1700 Msun.Dotted line comes from radial vels and solid black line is average from pm from McNamara et al.

Models with adding a BH to Baumgardt profilefor the 100% and 0% ns population

Preliminary results fromorbit-based models fittingradial velocities and propermotions for both distance andBH mass.

0 Msun100015002000

N185 shows an interesting dispersion profile. Below are the data from Held et al. and isotropic black hole models from 0 to 2e7 Msun.

log r (arcsec)

VG1G280G78

Comparison of surfacebrightness profilesfrom HST/ACS images.

Surface brightness profile for M54 from HST and from the ground:

Distribution of central SB slopes for the HST sample of GCs

Noyola & Gebhardt (04) measure surface brightness profiles from HST

imaging. Next step is kinematics.

BH/sigma correlation using isotropic models

Future for Black Hole Studies in GCs

• AO IR (CO 2.3um) IFU/FP studies are the future and will lead a new era for black hole measurements

• omega Cen will be a trivial test case to measure a black hole if it is consistent with BH/sigma

• these capabilities exists (in the fall for both VLT and Keck) now so the future is here

Gemini-S GMOS IFU observations of the central region in omega Cen (Noyola, Gebhardt, Bergmann). This is an overlay on an HST image.

N4486A has a 9th mag star 2.5” from nucleus. Keck AO NIR spectroscopy gave 0.045” PSF with Strehl from 40-50%.

Next step is to measure CO-bandhead kinematics behind AO. This will provide better spatial resolution than HST.

Conclusions

• About 40 nearby black holes with well measured masses; this will grow to about 100 soon, including many globulars.

• Globular clusters show interesting kinematics and density profiles near their centers. G1 and M15 black hole models are most consistent with the current data.

• Future for black studies is very promising. IFU in optical regions will open up studies in largest galaxies and in globular clusters.

• For many clusters (e.g., oCen, 47Tuc), it should be easy to have excellent constraints on BH masses.

Comparison of local “nuclei”

Deep VLA observations showno emission at the center ofM15 (from M. Goss). Also,Ho et al. (03) find no x-rayemission. But, the gas contentmay be too low.

This image is a 1.5 hourexposure in X band centeredon the cluster.

Rotation in Galactic clusters

Central Rotation in Galactic clusters

Reliability of orbit-superposition models:

•tests on analytic models (Richstone et al. 04, Gebhardt et al. 04, Verolme et al. 03)

•stellar dynamical models for N4258 give BH of 3e7 Msun and maser mass is 3.9e7

•N3379 stellar and gas models give same BH mass (works if galaxy has well-defined disk)

•different codes used on same galaxies give the same BH mass

•correlations with hosts give same trend using either gas or stellar determinations

Results for orbit-superposition models do not depend on number of orbits:

However, results do depend on the quality and quantity of the data: