Roeland van der Marel
HST’s Search for HST’s Search for Intermediate-Mass Intermediate-Mass Black HolesBlack Holes (IMBHs)(IMBHs)in Globular Clusters in Globular Clusters
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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OutlineOutline
IMBHs in the Universe? Theory Observational Signatures
IMBHs in Globular Clusters? IMBH in Omega Cen?
Anderson & vdMarel I (2010, ApJ in press) - HST observations
vdMarel & Anderson II (2010, ApJ, in press) - models
Outlook & Conclusions
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Known Black Holes (BHs)Known Black Holes (BHs)in the Universein the Universe
Stellar mass BHs (3-15 M): Endpoint of the life of massive
stars Observable in X-ray binaries 107-109 in every galaxy
Supermassive BHs (106-109 M): Generate the nuclear activity of
active galaxies and quasars ~1 in every galaxy
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Intermediate-MassIntermediate-MassBlack Holes (IMBHs)Black Holes (IMBHs)
Intermediate mass BHs: Mass range ~ 102 - 105 M
Questions: Is there a reason why they should exist? Is there evidence that they exist?
Status and Progress: These questions can be meaningfully
addressed No consensus yet
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Possible Mechanisms for Possible Mechanisms for IMBH Formation IMBH Formation
Primordial From Population III stars As part of Supermassive BH
formation
Dense star cluster evolution
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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What processes might What processes might reveal IMBHs?reveal IMBHs?
Dynamics influence on other objects(low-luminosity/late-type galaxies)
Accretion X-rays (ULXs) Gravitational lensing brightening /
distortion of background objects (LMC/bulge)
Progenitors output products(metals, background light, …)
Space-time distortion Gravitational Waves(LIGO/LISA?)
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Dynamical Evolution of Star Dynamical Evolution of Star ClustersClusters
Many physical processes in a dense stellar environment can in principle give runaway BH growth
Negative heat capacity of gravity core collapse
Binary heating normally halts core collapse in systems with N* < 106-7
Rees (1984)
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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A Scenario for IMBH A Scenario for IMBH Formation in Star ClustersFormation in Star Clusters
When core collapse sets in, energy equipartition is not maintained the most massive stars sink to the center first
Calculations show that anIMBH can form due torunaway collisions (PortegiesZwart & McMillan) Requires initial Trelax < 25 Myr
or present Trelax < 100 MyrGRAPE 6
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Possible IMBH Masses in Possible IMBH Masses in Globular Clusters?Globular Clusters?
Theoretical Formation Scenarios MBH/M ~ 0.1% - 1%
BH mass vs. velocitydispersion correlation MBH/M ~ 0.1 - 0.2%
Expected masses for typical clusters MBH ~ 102 - 104 M
Tremaine et al. (2002)
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Accretion Constraints inAccretion Constraints inGlobular ClustersGlobular Clusters
Globular clusters are gas-poor Any accretion likely to be radiatively inefficient Only very small accretion signatures expected Radio observations provide more stringent
constraints than X-ray observations MBH constraints require various assumptions
and extrapolations about gas content and accretion physics
Upper limits for 11 clusters provide (rather uncertain) upper limits just below the M- relation(Maccarone & Servillat 2008)
1 radio/X-ray detection discussed below
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Density Profile Constraints Density Profile Constraints in Globular Clustersin Globular Clusters
Equilibrium cusp around an IMBH has ~ r-1.75 (Bahcall & Wolf 1976)
projected mass density cusp slope -0.75 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 rcore / rhalf
HST archival analysis shows suchintermediate cusp slopes commonin GCs (Noyola & Gebhardt 2006)
Intermediate cusp slopes found also without IMBH in post core-collapse(Trenti et al. 2009)
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Mass Segregation Constraints Mass Segregation Constraints in Globular Clustersin Globular Clusters
The presence of an IMBH reduces the amount of mass segregation after core-collapse (Gill et al. 2008) The IMBH scatters heavy stars
that sink to the center back to larger radii
HST/ACS data of NGC 2298 show more mass segregation (from LF at different radii) than expected with an IMBH (Pasquato et al. 2009)
MBH/Mclus < 1%
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Dynamical Detection:Dynamical Detection:Sphere of InfluenceSphere of Influence
Stars directly affected by an IMBH are within thesphere of influence: rBH ~ G MBH / 2
For typical valuesrBH ≤ 1 arcsec
Dynamical signatures ~ r-1/2
Stars moving with v > vesc
Observational probes 1) Line-of-sight motions (Doppler) 2) proper motions (imaging)
Many stars need to be studied, in a crowded region, to detect this Hubble Space Telescope ideally suited
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Globular ClusterGlobular ClusterG1 (Andromeda)G1 (Andromeda)
Gebhardt, Rich, Ho (2002, 2005):HST/STIS and Keck spectroscopy
Most MassiveM31 Cluster
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Stellar Motions from Stellar Motions from Integrated Light (Concept)Integrated Light (Concept)
Without BH
With BH
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G1:G1: Results Results
Increase in velocitydispersion towards center MBH ~ 1.8 x 104
M
~2 detection ; rBH ~ 0.035 arcsec True dynamical significance
disputed (Baumgardt et al. 2003) Faint X-ray (Pooley & Rappaport 2006; Kong
2007) and radio emission (Ulvestad et al.) within ~1” Consistent with IMBH, but alternatives not ruled out
Possible nucleus of disrupted galaxy General implications for GCs unclear
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Globular ClusterGlobular ClusterM15M15
Well-studied Milky Way Cluster at ~10 kpc
High central density Core-collapsedGuhathakurta et al. (1996)Sosin & King (1997)
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64 HST/STIS velocities in central few arcsec(vdMarel et al. 2002)
+ ~1800 ground-based velocities (e.g., Gebhardt et al. 2000)
M15: DataM15: DataDiscrete VelocitiesDiscrete Velocities
V=13.7
V=18.1
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M15: ResultsM15: Results
Increase in velocitydispersion towards center
Jeans Models, constant (M/L)* Mdark= 3.2 (+2.2,-2.2) x 103
M Explanations
IMBH? (Gerssen et al. 2002) Mass segregation
(Dull et al. 2003; Baumgardt et al. 2003) Activity?
No X-ray counterpart (Ho et al. 2003) No radio counterpart (Maccarone et al. 2004)
Rapid rotation near center unexplained …
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Globular Cluster Globular Cluster Omega CenOmega Cen
Massive Milky Way GC; large core Disrupted satellite nucleus?
[Spitzer]
[HST WFC3 SM4 ERO]
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Omega Cen: DataOmega Cen: DataGround-based IFUGround-based IFU
Two Gemini/GMOS 5x5 arcsecfields [bright stars excluded](Noyola, Gebhardt & Bergm.2008) Center : = 23 ± 2 km/s 14” off-center : = 19 ± 2 km/s
Dynamical models MBH = 30,000 - 40,000 (± 10000) M Mass segregation unlikely to explain this
HST archival imaging Central density cusp = 0.08 ± 0.03
No radio or X-ray detections
[HST]
[Gemini]
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Proper Motions vs.Proper Motions vs.Line-of-Sight VelocitiesLine-of-Sight Velocities
Proper motion advantages Only imaging required, no spectra
Less observing time needed Multiplexing: all stars studied simultaneously
More (fainter) stars can be studied Allows better determination of , closer to cluster enter
Two velocity components observed for each star Measures velocity anisotropy, constrains models
Proper motion disadvantages Significant time baselines needed Very small effect to measure High telescope stability and calibration accuracy
required
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Proper Motion Proper Motion MeasurementMeasurement
1 km/s at 5 kpc 0.004 ACS/WFC pixel / 5 year
Hubble Space Telescope Sophisticated techniques developed (e.g.,
Anderson & King 2000) ePSF (effective PSF) fitting Linear transformations between epochs
(breathing/focus) Other applications
Cluster/field star separation cleaner CMDs Local Group Dynamics (LMC/SMC, M31?, ….) wrt
background quasars or galaxies (Kallivayalil, Sohn, ….)
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Omega Cen HST study:Omega Cen HST study: Observations & CatalogsObservations & Catalogs
Three Epochs of ACS/WFC data
Photometric Data : 1.2 x 106 stars Proper Motions : 1.7 x 105 stars (43%
high quality) Completeness via artificial star
photometry
[2002.5 (PI: Cool)] [2005.0 (Anderson)] [2006.6 (Sarajedini)][B,R,H] [V, H] [V,I]
[approx10x10 arcmin]
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Omega Cen HST study:Omega Cen HST study:CMD & Proper MotionsCMD & Proper Motions
MultipleStellarPops:No PMdifferences
FieldStars
zoomPMCatalogLimit~0.35 M
B-R
B
PMy
PMx
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Omega Cen HST study:Omega Cen HST study:VisualizationVisualization
Construct 3D model of cluster using (for “Hubble 3D” IMAX) Observed photometry, colors, positions, colors King model augmentation at large radii
Sequence shown here: zoom to 10’, 3’, 1’, observed PMs
[SM4 ERO] [simulated reconstruction]
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Omega Cen HST study:Omega Cen HST study:Center DeterminationCenter Determination
Used both contour methods and “pie-slice” methods Incompleteness corrected where necessary Also analyzed 2MASS images
[Stellar density] [Proper Motion Dispersion]
ResultingCenterAccuracy~ 1 arcsec
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Omega Cen HST study:Omega Cen HST study:Center ConfusionCenter Confusion
Traditional estimates&Noyola et al. pointing12” away fromtrue center
Cause: few bright starsdominate light
[Harris]
[van Leeuwen]
[Noyola]
[HST PM][HST stars][2MASS]
[Noyola off-center IFU field]
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Omega Cen HST study:Omega Cen HST study:Density ProfileDensity Profile
Models with a core or with a shallow cusp( ~ 0.05) both provide an acceptable fit
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Proper motion dispersion profile consistent with being flat in the central ~20”
No difference in PM dispersion between two Noyola et al. IFU fields (both 19.0 1.5 km/s)
Omega Cen HST study:Omega Cen HST study:PM Dispersion ProfilePM Dispersion Profile
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Omega Cen HST study:Omega Cen HST study:New IMBH assessmentNew IMBH assessment
HST data augmented with ground-based data: Important for constraining larger-radii kinematics Line-of-sight velocities: 8 different studies Proper motions: van Leeuwen (2000) [50 years!]
Spherical Jeans Models: Simple, but sufficient (more detailed techniques:
vdVen 06) Little rotation, ellipticity near cluster center LOS, PM-radial, PM-tangential predicted separately
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Omega Cen HST study:Omega Cen HST study:Model ParametersModel Parameters
Anisotropy: tan / r = 0.94 0.01 (center)= 1.24 0.10 (large
radii)
M/L: 2.6 0.1 (V-band solar units) D: 4.7 0.1 kpc
Consistent with photometric values ~ 5.0 0.2 kpc
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Omega Cen HST study:Omega Cen HST study:IMBH constraintsIMBH constraints
Core model: MBH 7400 M
Cusp model: MBH =
(8700 ± 2900) M
Big densitydifference in 3D
In 2D projectionboth models fit the density/brightness data
IMBH not required in Cen ( 12000 M @1) ( 18000 M @3)
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Omega Cen HST study:Omega Cen HST study:Ultra-Rapid Stars?Ultra-Rapid Stars?
Big core: most stars observed near center are not close in 3D ~100 stars within 3”
projected aperture only 1-6% are within 3” in
3D No fast moving stars
observed (60 km/s), but few expected for reasonable IMBH mass
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Omega Cen HST study:Omega Cen HST study:Equipartition?Equipartition?
PM dispersion measured as function of main sequence mass: ~ m0.2
Equipartition predicts E ~ m 2 = constant: ~ m0.5
N-body simulations(Trenti & vdM, in prep.): Omega Cen should have reached it
equilibrium vs. m relation, despite long relaxation time (~9 Gyr)
Equilibrium does not represent equipartition
Typical IMFs may not be able to reach equipartition (Vishniac 1978) due to Spitzer (1969) instability
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Other Existing Proper Other Existing Proper Motion StudiesMotion Studies
M15 (McNamara et al. 2003) 704 stars, HST/WFC2 Consistent with line-of-sight work Models of combined data set do not resolve IMBH vs.
mass segregation degeneracy 47 Tuc (McLaughlin et al. 2006)
14,366 stars, HST/WFPC2 and HST/ACS MBH < 1000-1500 M (upper limit) Velocity dispersion of 23 blue stragglers (30
10% smaller than RGB stars) provided evidence for mass segregation, but (m) relationship not studied
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Globular ClusterGlobular ClusterIMBH DemographicsIMBH Demographics
Unresolved line-of-sight analysis (+radio/X-ray detection) G1: MBH/Mclus ~ 0.3%, roughly consistent with MBH-
Radio non-detections 11 (crude) upper limits somewhat below MBH-
Proper motion dynamical analysis 3 upper limits somewhat above MBH-
Spatial mass segregation analysis 1 upper limit somewhat above MBH-
Tentative conclusion: IMBHs not very prevalent in GCs at the masses (near MBH-) that can currently be probed
Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel
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Future WorkFuture Work
Radio More deep observations Future high-sensitivity instruments EVLA,
SKA, etc. HST Proper motions
Ongoing studies in HST programs by e.g. PIs Chandar, Ford, Chaname
2 or 3 epochs in hand 9 clusters Improved modeling tools to fully use the rich
information
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Conclusions:Conclusions:
The existence of IMBHs in Globular Clusters Is predicted by some theories Can be observationally tested
HST proper motion studies provide a unique tool for this subject provide a wealth of information on
globular cluster structure Preliminary indications
IMBHs may exist IMBHs scarce at currently accessible
masses