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HST’s Search for Intermediate-Mass Black Holes (IMBHs) in Globular Clusters

HST’s Search for Intermediate-Mass Black Holes (IMBHs) in Globular Clusters. Outline. IMBHs in the Universe? Theory Observational Signatures IMBHs in Globular Clusters? IMBH in Omega Cen? Anderson & vdMarel I (2010, ApJ in press) - HST observations

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HST’s Search for Intermediate-Mass Black Holes (IMBHs) in Globular Clusters

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  1. HST’s Search for Intermediate-Mass Black Holes(IMBHs)in Globular Clusters

  2. Outline • 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 Institute marel@stsci.edu http://www.stsci.edu/~marel

  3. 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 ofactive galaxies and quasars ~1 in every galaxy Known Black Holes (BHs)in the Universe Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  4. 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 Intermediate-MassBlack Holes (IMBHs) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  5. Possible Mechanisms for IMBH Formation • Primordial • From Population III stars • As part of Supermassive BH formation • Dense star cluster evolution Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  6. What processes might 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 Institute marel@stsci.edu http://www.stsci.edu/~marel

  7. Dynamical Evolution of Star Clusters • 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 Institute marel@stsci.edu http://www.stsci.edu/~marel

  8. A Scenario for IMBH Formation 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 Myror present Trelax < 100 Myr GRAPE 6 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  9. Possible IMBH Masses in 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) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  10. Accretion Constraints inGlobular 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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  11. Density Profile Constraints in 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 foundalso without IMBH in post core-collapse(Trenti et al. 2009) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  12. Mass Segregation Constraints in 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% Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  13. Dynamical Detection:Sphere 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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  14. Globular ClusterG1 (Andromeda) • Gebhardt, Rich, Ho (2002, 2005):HST/STIS and Keck spectroscopy Most MassiveM31 Cluster Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  15. Stellar Motions from Integrated Light (Concept) Without BH With BH Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  16. G1: Results • Increase in velocitydispersion towards center • MBH ~1.8x 104M • ~2 detection ; rBH ~ 0.035 arcsec • True dynamical significancedisputed (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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  17. Globular ClusterM15 • Well-studied Milky Way Cluster at ~10 kpc • High central density  Core-collapsed Guhathakurta et al. (1996) Sosin & King (1997) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  18. M15: DataDiscrete Velocities • 64 HST/STIS velocities in central few arcsec(vdMarel et al. 2002) • + ~1800 ground-based velocities (e.g., Gebhardt et al. 2000) V=13.7 V=18.1 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  19. M15: Results • Increase in velocitydispersion towards center • Jeans Models, constant (M/L)* Mdark= 3.2 (+2.2,-2.2)x 103M • 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 … Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  20. Globular Cluster Omega Cen • Massive Milky Way GC; large core • Disrupted satellite nucleus? [Spitzer] [HST WFC3 SM4 ERO] Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  21. Omega Cen: DataGround-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] Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel [Gemini]

  22. Proper Motions vs.Line-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 Institute marel@stsci.edu http://www.stsci.edu/~marel

  23. Proper Motion Measurement • 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 Institute marel@stsci.edu http://www.stsci.edu/~marel

  24. Omega Cen HST study: Observations & 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 [approx10x10 arcmin] [2002.5 (PI: Cool)] [2005.0 (Anderson)] [2006.6 (Sarajedini)] [B,R,H] [V, H] [V,I] Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  25. Omega Cen HST study:CMD & Proper Motions MultipleStellarPops: No PM differences PMx PMy zoom PM CatalogLimit~0.35 M B FieldStars B-R Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  26. Omega Cen HST study:Visualization [SM4 ERO] [simulated reconstruction] • 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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  27. Omega Cen HST study:Center Determination [Stellar density] [Proper Motion Dispersion] • Used both contour methods and “pie-slice” methods • Incompleteness corrected where necessary • Also analyzed 2MASS images ResultingCenterAccuracy~ 1 arcsec Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  28. Omega Cen HST study:Center Confusion [Harris] • Traditional estimates&Noyola et al. pointing12” away fromtrue center • Cause: few bright starsdominate light [van Leeuwen] [Noyola] [2MASS] [HST stars] [HST PM] [Noyola off-center IFU field] Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  29. Omega Cen HST study:Density Profile • Models with a core or with a shallow cusp( ~ 0.05) both provide an acceptable fit Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  30. Omega Cen HST study:PM Dispersion Profile • 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) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  31. Omega Cen HST study:New 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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  32. Omega Cen HST study:Model 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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  33. Omega Cen HST study:IMBH 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) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  34. Omega Cen HST study: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 Institute marel@stsci.edu http://www.stsci.edu/~marel

  35. Omega Cen HST study: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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  36. Other Existing Proper Motion 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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  37. Globular ClusterIMBH 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 Institute marel@stsci.edu http://www.stsci.edu/~marel

  38. Future 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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  39. 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 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

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