Roeland van der Marel
Intermediate-Mass Black Holes (IMBHs) in Globular Clusters? HST Proper Motion Constraints
Why Study IMBHs in Globular Clusters (GCs) ?
IMBHs: IMBHs can probe a new BH mass range, between stellar (∼3-15 M¤)
and supermassive (∼106-109 M¤) BHs There are many ways in which IMBHs may have formed in the Universe
[e.g., vdM 2004 review]
GCs: There are plausible scenarios by which IMBHs may have formed in GCs
[e.g., Portegies Zwart & McMillan 2002]
(Some) GCs may be remant nuclei of disrupted dwarfs with possible IMBHs [e.g., Freeman 1993; Greene & Ho 2004]
Observational evidence for the presence of IMBHs has been reported for select GCs
Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Possible IMBH Masses in Globular Clusters?
Theoretical Formation Scenarios MBH/M ~ 0.1% - 1%
BH mass vs. velocity dispersion correlation MBH/M ~ 0.1 - 0.2%
Expected masses for typical clusters MBH ~ 102 - 104 M¤
Tremaine et al. (2002)
GCs
Indicators & Tracers Radio emission
Many upper limits (incl. M15, Omega Cen, M54) and a few ambiguous detections (incl. G1, NGC 6266) [e.g., Maccarone & Seveillat 2008; Bash et al. 2008; Wrobel et al. 2011; Strader et al. 2012, Miller Jones et al. 2013]
X-ray emission Many upper limits (incl. M15, Omega Cen) some detections but not unique
IMBH signatures (incl. G1) [e.g., Ho et al. 2003; Miller-Jones et al. 2013; Haggard talk]
ULXs detected in some GCs, but IMBH connection unclear [e.g., Zepf et al. 2008]
Density profile cusps Intermediate cusp slopes possibly from IMBHs [e.g., Noyola & Baumgardt 2011]
but not a unique signature [Vesperini & Trenti 2010]
Mass segregation/Equipartition signatures IMBH reduces these [e.g., Gill et al. 2008; Umbreit & Rasio 2012; Trenti & van der
Marel 2013] but only few data-model comparisons so far
Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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IMBH Gravitational Potential: Stellar Dynamics
Sphere of influence: stars directly affected by an IMBH are within rBH ~ G MBH / σ2 rBH ≤ few arcsec
Dynamical signatures σ ~ r-1/2 (hydrostatic equilibrium) Stars moving with v > vesc
Observational probes 1) Line-of-sight (LOS) motions (spectra using Doppler effect) 2) proper motions (PM) (imaging at different times)
Limitation: Dark mass concentration is not necessarily IMBH
Line-of-Sight Velocities: Methods & Results
Individual velocities Bright stars only (spectra required); blending/crowding near center
Integrated Light Weighted towards bright stars è shot noise important in data analysis
Dark Mass/IMBH findings: M15: (3.9 ± 2.2) x 103
M¤ [van der Marel et al. 2002; Gerssen et al. 2002] G1: (1.8 ± 0.5) x 104
M¤ [Gebhardt et al. 2002, 2004] Omega Cen: (4.7 ± 1.0) x 104
M¤ [Noyola et al. 2008, 2010; Jalali et al. 2011] M54: ≤ 9.4 x 103
M¤ [Ibata et al. 2009] NGC6388: (1.7 ± 0.9) x 104
M¤ [Lutzgendorf et al. 2011] NGC1904 (3 ± 1) x 103
M¤ [Lutzgendorf et al. 2012] NGC6266: (2 ± 1) x 103
M¤ [Lutzgendorf et al. 2012]
Caveats: few-sigma significance, not yet much supporting evidence, some contradictory evidence
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Proper Motions: Method
Advantages Individual stellar velocities of high accuracy to faint levels
Less ambiguity in interpreting measurements Possibility to probe for fast-moving stars inside sphere of influence
Large N (104-105 stars) multiplexing with full 2D coverage Two components of motion: anisotropy measured
Disadvantage Difficult to constrain solid-body rotation (differential rotation OK)
Complexity Requires telescope stability, high spatial resolution, long time baselines,
state-of-the-art calibration and software: Hubble Space Telescope Small displacements: 1 km/s at 5 kpc ⇒ 0.004 ACS/WFC pixel / 5 year
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Proper Motions: Results
M15 [McNamara et al. 2003], N=714, WFPC2-WFPC2 Mdark implied, probably not IMBH [van den Bosch et al. 2006]
NGC 6266 [McNamara et al. 2012], N=886, WFPC2-WFPC2 < few x 103
M¤
47 Tuc [McLaughlin et al. 2006], N=14,366, WFPC2-ACS
< 1500 M¤
Omega Cen [Anderson & vdM 2010; vdM & Anderson 2010], N=169,800, ACS-ACS < 1.2 x 104
M¤ Several of these GCs have been suggested to host IMBHs
based on VLOS data; Omega Cen results strongly contradictory
Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Proper Motions: A New HST Survey
23 GCs with multiple epochs of HST ACS or WFC3 data [Bellini, van der Marel, Anderson 2013++] Preliminary PM catalogs created; improvements being implemented N = 2000 to 293,000 per cluster (median: 57,000) Few km/s per star accuracy
Applications Milky Way GC population: distances, 3D velocities (absolute PMs) GC stellar populations: clean CMDs; kinematics for different stellar
types and populations GC dynamics: equipartition, mass segregation, rotating components,
anisotropy IMBHs: σ(r), fast moving stars
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Example 1: NGC 6681 (separating GC, Sgr dSph, bulge)
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Massari, Bellini, vdM et al. in prep.]
Example 2: NGC 6752 (σ versus mass and radius)
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Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Omega Cen
Massive Milky Way GC; large core Disrupted satellite nucleus?
[Spitzer]
[HST WFC3 SM4 ERO]
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Proper motion dispersion profile consistent with being flat in the central ~20”
Yields IMBH upper limit
Omega Cen HST PM study: Dispersion Profile
Omega Cen: Why Different IMBH Results from PM /LOS?
σ(r) measurements don’t agree Independent of where Omega Cen center is placed
Centers don’t agree Anderson & van der Marel (2010): ~1 arcsec accuracy
Large-scale center of number density (N = 1.2 x 106 stars) Large-scale center of 2MASS integrated light Large-scale center of PM dispersion field
Noyola et al. (2010) Small-scale center of LOS dispersion field 4 arcsec away (replaces Noyola et al. 2008 center 12 arcsec away) method biases towards larger IMBH mass
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Omega Cen: PM /LOS Comparison
Difference persists with latest Omega Cen PM catalog, incl. WFC3 data (doubles time baseline)
Distance scaling free parameter; σ(r) gradient is what matters
Explanation: shot noise? Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Omega Cen: Fast Moving Stars?
Stars close to an IMBH move fast (v ~ 1/√r) Projected velocity distribution
has broad wings [van der Marel 1994; Drukier & Bailyn 2003]
The more massive the IMBH, the more fast-moving stars are predicted
Omega Cen Of ~1000 stars at R<10 arcsec, none has 1D PM >60 km/s models: IMBH = 4 x 104
M¤ ruled out at >99.9% confidence
Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Anisotropy Constraints Two-body relaxation expected to lead to isotropy
Appears validated by our new PM work
Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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General Considerations
IMBHs with M/MGC ≤ 1% leave very subtle stellar dynamical signatures Modeling details matter (center, cusp slope, rotation, mass
segregation, anisotropy, etc.) Consensus requires agreement between LOS and PM data
Beware 1-2 sigma detections happen by chance 1/3 of the time ….. Any systematic error biases MBH upward
Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Roeland van der Marel - Space Telescope Science Institute [email protected] http://www.stsci.edu/~marel
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Conclusions
Many new HST PM datasets being created Spectacular quality Allow many unique studies
Both PM and LOS datasets now probe IMBHs in an interesting mass range Good agreement in some cases Important differences in some cases
Preliminary indications IMBHs may exist IMBHs scarce at currently accessible masses
Insufficient consensus on any specific GC to conclude IMBHs convincingly detected