hst’s search for intermediate-mass black holes (imbhs) in globular clusters

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


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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


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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


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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


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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?)


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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)


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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


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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)


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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


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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, ….)


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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


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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


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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

hst’s search for intermediate-mass black holes (imbhs) in globular clusters

Documents

mass segregation constraints

mass range

universestellar mass

possible imbh masses

corecollapse gill

post corecollapse trenti

imbh scatters heavy

stringent constraints