Roeland van der Marel HST’s Search for Intermediate-Mass Black Holes (IMBHs) in Globular Clusters

Roeland van der Marel - Space Telescope Science Institute 2 Outline u IMBHs in the Universe? I Theory I Observational Signatures u IMBHs in Globular Clusters? u IMBH in Omega Cen? I Anderson & vdMarel I (2010, ApJ in press) - HST observations I vdMarel & Anderson II (2010, ApJ, in press) - models u Outlook & Conclusions

Roeland van der Marel - Space Telescope Science Institute 3 Known Black Holes (BHs) in the Universe u Stellar mass BHs (  3-15 M  ): I Endpoint of the life of massive stars I Observable in X-ray binaries I  in every galaxy u Supermassive BHs (  M  ): I Generate the nuclear activity of active galaxies and quasars I ~1 in every galaxy

Roeland van der Marel - Space Telescope Science Institute 4 Intermediate-Mass Black Holes (IMBHs) u Intermediate mass BHs: I Mass range ~ M  u Questions: I Is there a reason why they should exist? I Is there evidence that they exist? u Status and Progress: I These questions can be meaningfully addressed I No consensus yet

Roeland van der Marel - Space Telescope Science Institute 5 Possible Mechanisms for IMBH Formation u Primordial u From Population III stars u As part of Supermassive BH formation u Dense star cluster evolution

Roeland van der Marel - Space Telescope Science Institute 6 What processes might reveal IMBHs? u Dynamics  influence on other objects (low-luminosity/late-type galaxies) u Accretion  X-rays (ULXs) u Gravitational lensing  brightening / distortion of background objects (LMC/bulge) u Progenitors  output products (metals, background light, …) u Space-time distortion  Gravitational Waves (LIGO/LISA?)

Roeland van der Marel - Space Telescope Science Institute 7 Dynamical Evolution of Star Clusters u Many physical processes in a dense stellar environment can in principle give runaway BH growth u Negative heat capacity of gravity  core collapse u Binary heating normally halts core collapse in systems with N * < Rees (1984)

Roeland van der Marel - Space Telescope Science Institute 8 A Scenario for IMBH Formation in Star Clusters u When core collapse sets in, energy equipartition is not maintained  the most massive stars sink to the center first u Calculations show that an IMBH can form due to runaway collisions (Portegies Zwart & McMillan) I Requires initial T relax < 25 Myr or present T relax < 100 Myr GRAPE 6

Roeland van der Marel - Space Telescope Science Institute 9 Possible IMBH Masses in Globular Clusters? u Theoretical Formation Scenarios I M BH /M ~ 0.1% - 1% u BH mass vs. velocity dispersion correlation I M BH /M ~ % u Expected masses for typical clusters I M BH ~ M  Tremaine et al. (2002)

Roeland van der Marel - Space Telescope Science Institute 10 Accretion Constraints in Globular Clusters u Globular clusters are gas-poor u Any accretion likely to be radiatively inefficient u Only very small accretion signatures expected u Radio observations provide more stringent constraints than X-ray observations u MBH constraints require various assumptions and extrapolations about gas content and accretion physics u Upper limits for 11 clusters provide (rather uncertain) upper limits just below the M-  relation (Maccarone & Servillat 2008) u 1 radio/X-ray detection discussed below

Roeland van der Marel - Space Telescope Science Institute 11 Density Profile Constraints in Globular Clusters u Equilibrium cusp around an IMBH has  ~ r (Bahcall & Wolf 1976)  projected mass density cusp slope u Light does not follow mass after core collapse (mass segregation) (Baumgardt et al. 2005; Trenti 2006)  projected light density cusp slope -0.1 to -0.3  large r core / r half u HST archival analysis shows such intermediate cusp slopes common in GCs (Noyola & Gebhardt 2006) u Intermediate cusp slopes found also without IMBH in post core-collapse (Trenti et al. 2009)

Roeland van der Marel - Space Telescope Science Institute 12 Mass Segregation Constraints in Globular Clusters u The presence of an IMBH reduces the amount of mass segregation after core-collapse (Gill et al. 2008) I The IMBH scatters heavy stars that sink to the center back to larger radii u HST/ACS data of NGC 2298 show more mass segregation (from LF at different radii) than expected with an IMBH (Pasquato et al. 2009)  M BH /Mclus < 1%

Roeland van der Marel - Space Telescope Science Institute 13 Dynamical Detection: Sphere of Influence u Stars directly affected by an IMBH are within the sphere of influence: r BH ~ G M BH /  2 u For typical values r BH ≤ 1 arcsec u Dynamical signatures I  ~ r -1/2 I Stars moving with v > v esc u Observational probes I 1) Line-of-sight motions (Doppler) I 2) proper motions (imaging) u Many stars need to be studied, in a crowded region, to detect this  Hubble Space Telescope ideally suited

Roeland van der Marel - Space Telescope Science Institute 14 Globular Cluster G1 (Andromeda) u Gebhardt, Rich, Ho (2002, 2005): HST/STIS and Keck spectroscopy Most Massive M31 Cluster

Roeland van der Marel - Space Telescope Science Institute 15 Stellar Motions from Integrated Light (Concept) Without BH With BH

Roeland van der Marel - Space Telescope Science Institute 16 G1: Results u Increase in velocity dispersion towards center I M BH ~ 1.8 x 10 4 M  I ~2  detection ; r BH ~ arcsec I True dynamical significance disputed (Baumgardt et al. 2003) u Faint X-ray (Pooley & Rappaport 2006; Kong 2007) and radio emission (Ulvestad et al.) within ~1” I Consistent with IMBH, but alternatives not ruled out u Possible nucleus of disrupted galaxy I General implications for GCs unclear

Roeland van der Marel - Space Telescope Science Institute 17 Globular Cluster M15 u Well-studied Milky Way Cluster at ~10 kpc u High central density  Core-collapsed Guhathakurta et al. (1996) Sosin & King (1997)

Roeland van der Marel - Space Telescope Science Institute 18 u 64 HST/STIS velocities in central few arcsec (vdMarel et al. 2002) u + ~1800 ground-based velocities (e.g., Gebhardt et al. 2000) M15: Data Discrete Velocities V=13.7 V=18.1

Roeland van der Marel - Space Telescope Science Institute 19 M15: Results u Increase in velocity dispersion towards center u Jeans Models, constant (M/L) *  M dark= 3.2 (+2.2,-2.2) x 10 3 M  u Explanations I IMBH? (Gerssen et al. 2002) I Mass segregation (Dull et al. 2003; Baumgardt et al. 2003) u Activity? I No X-ray counterpart (Ho et al. 2003) I No radio counterpart (Maccarone et al. 2004) u Rapid rotation near center unexplained …

Roeland van der Marel - Space Telescope Science Institute 20 Globular Cluster Omega Cen u Massive Milky Way GC; large core u Disrupted satellite nucleus? [Spitzer] [HST WFC3 SM4 ERO]

Roeland van der Marel - Space Telescope Science Institute 21 Omega Cen: Data Ground-based IFU u Two Gemini/GMOS 5x5 arcsec fields [bright stars excluded] (Noyola, Gebhardt & Bergm.2008) I Center :  = 23 ± 2 km/s I 14” off-center :  = 19 ± 2 km/s u Dynamical models I M BH = 30, ,000 (± 10000) M  I Mass segregation unlikely to explain this u HST archival imaging I Central density cusp  = 0.08 ± 0.03 u No radio or X-ray detections [HST] [Gemini]

Roeland van der Marel - Space Telescope Science Institute 22 Proper Motions vs. Line-of-Sight Velocities u Proper motion advantages I Only imaging required, no spectra  Less observing time needed  Multiplexing: all stars studied simultaneously I More (fainter) stars can be studied  Allows better determination of , closer to cluster enter I Two velocity components observed for each star  Measures velocity anisotropy, constrains models u Proper motion disadvantages I Significant time baselines needed I Very small effect to measure I High telescope stability and calibration accuracy required

Roeland van der Marel - Space Telescope Science Institute 23 Proper Motion Measurement u 1 km/s at 5 kpc  ACS/WFC pixel / 5 year  Hubble Space Telescope u Sophisticated techniques developed (e.g., Anderson & King 2000) I ePSF (effective PSF) fitting I Linear transformations between epochs (breathing/focus) u Other applications I Cluster/field star separation  cleaner CMDs I Local Group Dynamics (LMC/SMC, M31?, ….) wrt background quasars or galaxies (Kallivayalil, Sohn, ….)

Roeland van der Marel - Space Telescope Science Institute 24 Omega Cen HST study: Observations & Catalogs u Three Epochs of ACS/WFC data u Photometric Data : 1.2 x 10 6 stars u Proper Motions : 1.7 x 10 5 stars (43% high quality) u Completeness via artificial star photometry [ (PI: Cool)] [ (Anderson)] [ (Sarajedini)] [B,R,H  ] [V, H  ] [V,I] [approx 10x10 arcmin]

Roeland van der Marel - Space Telescope Science Institute 25 Omega Cen HST study: CMD & Proper Motions Multiple Stellar Pops: No PM differences Field Stars zoom PM Catalog Limit ~0.35 M  B-R B PMy PMx

Roeland van der Marel - Space Telescope Science Institute 26 Omega Cen HST study: Visualization u Construct 3D model of cluster using (for “Hubble 3D” IMAX) I Observed photometry, colors, positions, colors I King model augmentation at large radii u Sequence shown here: zoom to 10’, 3’, 1’, observed PMs [SM4 ERO] [simulated reconstruction]

Roeland van der Marel - Space Telescope Science Institute 27 Omega Cen HST study: Center Determination u Used both contour methods and “pie-slice” methods u Incompleteness corrected where necessary u Also analyzed 2MASS images [Stellar density] [Proper Motion Dispersion] Resulting Center Accuracy ~ 1 arcsec

Roeland van der Marel - Space Telescope Science Institute 28 Omega Cen HST study: Center Confusion u Traditional estimates &Noyola et al. pointing 12” away from true center u Cause: few bright stars dominate light [Harris] [van Leeuwen] [Noyola] [HST PM] [HST stars] [2MASS] [Noyola off-center IFU field]

Roeland van der Marel - Space Telescope Science Institute 29 Omega Cen HST study: Density Profile u Models with a core or with a shallow cusp (  ~ 0.05) both provide an acceptable fit

Roeland van der Marel - Space Telescope Science Institute 30 u Proper motion dispersion profile consistent with being flat in the central ~20” u No difference in PM dispersion between two Noyola et al. IFU fields (both 19.0  1.5 km/s) Omega Cen HST study: PM Dispersion Profile

Roeland van der Marel - Space Telescope Science Institute 31 Omega Cen HST study: New IMBH assessment u HST data augmented with ground-based data: I Important for constraining larger-radii kinematics I Line-of-sight velocities: 8 different studies I Proper motions: van Leeuwen (2000) [50 years!] u Spherical Jeans Models: I Simple, but sufficient (more detailed techniques: vdVen 06) I Little rotation, ellipticity near cluster center I LOS, PM-radial, PM-tangential predicted separately

Roeland van der Marel - Space Telescope Science Institute 32 Omega Cen HST study: Model Parameters u Anisotropy:  tan /  r = 0.94  0.01 (center) = 1.24  0.10 (large radii) u M/L: 2.6  0.1 (V-band solar units) u D: 4.7  0.1 kpc I Consistent with photometric values ~ 5.0  0.2 kpc

Roeland van der Marel - Space Telescope Science Institute 33 Omega Cen HST study: IMBH constraints u Core model: I M BH  7400 M  u Cusp model: I M BH = (8700 ± 2900) M  u Big density difference in 3D u In 2D projection both models fit the density/brightness data u IMBH not required in  Cen (  M  ) (  M  )

Roeland van der Marel - Space Telescope Science Institute 34 Omega Cen HST study: Ultra-Rapid Stars? u Big core: most stars observed near center are not close in 3D I ~100 stars within 3” projected aperture I only 1-6% are within 3” in 3D u No fast moving stars observed (  60 km/s), but few expected for reasonable IMBH mass

Roeland van der Marel - Space Telescope Science Institute 35 Omega Cen HST study: Equipartition? u PM dispersion measured as function of main sequence mass:  ~ m 0.2 u Equipartition predicts E ~ m  2 = constant:  ~ m 0.5 u N-body simulations (Trenti & vdM, in prep.): I Omega Cen should have reached it equilibrium  vs. m relation, despite long relaxation time (~9 Gyr) I Equilibrium does not represent equipartition I Typical IMFs may not be able to reach equipartition (Vishniac 1978) due to Spitzer (1969) instability

Roeland van der Marel - Space Telescope Science Institute 36 Other Existing Proper Motion Studies u M15 (McNamara et al. 2003) I 704 stars, HST/WFC2 I Consistent with line-of-sight work I Models of combined data set do not resolve IMBH vs. mass segregation degeneracy u 47 Tuc (McLaughlin et al. 2006) I 14,366 stars, HST/WFPC2 and HST/ACS I MBH < M  (upper limit) I Velocity dispersion of 23 blue stragglers (30  10% smaller than RGB stars) provided evidence for mass segregation, but  (m) relationship not studied

Roeland van der Marel - Space Telescope Science Institute 37 Globular Cluster IMBH Demographics u Unresolved line-of-sight analysis (+radio/X-ray detection) I G1: M BH /M clus ~ 0.3%, roughly consistent with M BH -  u Radio non-detections I 11 (crude) upper limits somewhat below M BH -  u Proper motion dynamical analysis I 3 upper limits somewhat above M BH -  u Spatial mass segregation analysis I 1 upper limit somewhat above M BH -  u Tentative conclusion: IMBHs not very prevalent in GCs at the masses (near M BH -  ) that can currently be probed

Roeland van der Marel - Space Telescope Science Institute 38 Future Work u Radio I More deep observations I Future high-sensitivity instruments EVLA, SKA, etc. u HST Proper motions I Ongoing studies in HST programs by e.g. PIs Chandar, Ford, Chaname I 2 or 3 epochs in hand I 9 clusters I Improved modeling tools to fully use the rich information

Roeland van der Marel - Space Telescope Science Institute 39 Conclusions: u The existence of IMBHs in Globular Clusters I Is predicted by some theories I Can be observationally tested u HST proper motion studies I provide a unique tool for this subject I provide a wealth of information on globular cluster structure u Preliminary indications I IMBHs may exist I IMBHs scarce at currently accessible masses