Intermediate-Mass Black Holes (IMBHs) in Globular Clusters? HST Proper Motion Constraints. Roeland van der Marel

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1 Intermediate-Mass Black Holes (IMBHs) in Globular Clusters? HST Proper Motion Constraints Roeland van der Marel

2 Why Study IMBHs in Globular Clusters (GCs)? IMBHs: IMBHs can probe a new BH mass range, between stellar ( 3-15 M ) and supermassive ( M ) BHs GCs: There are many ways in which IMBHs may have formed in the Universe [e.g., vdm 2004 review] 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 2

which IMBHs may have formed in the Universe [e.g.

3 Possible IMBH Masses in Globular Clusters? Theoretical Formation Scenarios M BH /M ~ 0.1% - 1% BH mass vs. velocity dispersion correlation M BH /M ~ % Expected masses for typical clusters GCs M BH ~ M Tremaine et al. (2002) 3

4 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 4

5 IMBH Gravitational Potential: Stellar Dynamics Sphere of influence: stars directly affected by an IMBH are within r BH ~ G M BH / σ 2 r BH few arcsec Dynamical signatures σ ~ r -1/2 (hydrostatic equilibrium) Stars moving with v > v esc 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 5

moving with v > v esc Observational probes 1) Line-of-sight (LOS) motions (spectra using Doppler effect) 2)

6 Individual velocities Line-of-Sight Velocities: Methods & Results 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 10 3 M [van der Marel et al. 2002; Gerssen et al. 2002] G1: (1.8 ± 0.5) x 10 4 M [Gebhardt et al. 2002, 2004] Omega Cen: (4.7 ± 1.0) x 10 4 M [Noyola et al. 2008, 2010; Jalali et al. 2011] M54: 9.4 x 10 3 M [Ibata et al. 2009] NGC6388: (1.7 ± 0.9) x 10 4 M [Lutzgendorf et al. 2011] NGC1904 (3 ± 1) x 10 3 M [Lutzgendorf et al. 2012] NGC6266: (2 ± 1) x 10 3 M [Lutzgendorf et al. 2012] Caveats: few-sigma significance, not yet much supporting evidence, some contradictory evidence 6

2002, 2004] Omega Cen: (4.7 ± 1.0) x 10 4 M [Noyola et al. 2008, 2010; Jalali et al. 2011] M54: 9.4 x 10 3 M [Ibata et al. 2009] NGC6388: (1.7 ± 0.9) x 10 4 M [Lutzgendorf et al.

7 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 ( 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 ACS/WFC pixel / 5 year 7

measured Disadvantage Difficult to constrain solid-body rotation (differential rotation OK) Complexity Requires telescope stability, high spatial

8 Proper Motions: Results M15 [McNamara et al. 2003], N=714, WFPC2-WFPC2 M dark implied, probably not IMBH [van den Bosch et al. 2006] NGC 6266 [McNamara et al. 2012], N=886, WFPC2-WFPC2 < few x 10 3 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 10 4 M Several of these GCs have been suggested to host IMBHs based on V LOS data; Omega Cen results strongly contradictory 8

2012], N=886, WFPC2-WFPC2 < few x 10 3 M 47 Tuc [McLaughlin et al.

9 Proper Motions: A New HST Survey 23 GCs with multiple epochs of HST ACS or WFC3 data [Bellini, van der Marel, Anderson ] 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 9

accuracy Applications Milky Way GC population: distances, 3D velocities (absolute PMs) GC stellar populations: clean CMDs; kinematics

10 Example 1: NGC 6681 (separating GC, Sgr dsph, bulge) Massari, Bellini, vdm et al. in prep.] 10

11 Example 2: NGC 6752 (σ versus mass and radius) 11

12 Omega Cen Massive Milky Way GC; large core Disrupted satellite nucleus? [Spitzer] [HST WFC3 SM4 ERO] 12

13 Omega Cen HST PM study: Dispersion Profile Proper motion dispersion profile consistent with being flat in the central ~20 Yields IMBH upper limit 13

14 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 10 6 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 center 12 arcsec away) method biases towards larger IMBH mass 14

(2010): ~1 arcsec accuracy Large-scale center of number density (N = 1.

15 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? 15

16 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 10 4 M ruled out at >99.9% confidence 16

wings [van der Marel 1994; Drukier & Bailyn 2003] The more massive the IMBH, the more

17 Anisotropy Constraints Two-body relaxation expected to lead to isotropy Appears validated by our new PM work 17

18 General Considerations IMBHs with M/M GC 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 M BH upward 18

19 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 19

Important differences in some cases Preliminary indications IMBHs may exist IMBHs scarce at

Ellipticals. Elliptical galaxies: Elliptical galaxies: Some ellipticals are not so simple M89 E0

Ellipticals. Elliptical galaxies: Elliptical galaxies: Some ellipticals are not so simple M89 E0 Elliptical galaxies: Ellipticals Old view (ellipticals are boring, simple systems)! Ellipticals contain no gas & dust! Ellipticals are composed of old stars! Ellipticals formed in a monolithic collapse,