Roeland van der Marel Intermediate-Mass Black Holes ...

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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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


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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


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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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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


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Anisotropy Constraints   Two-body relaxation expected to lead to isotropy

  Appears validated by our new PM work


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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


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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

Roeland van der Marel Intermediate-Mass Black Holes ...

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