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Intermediate-Mass Black Holes: Formation Mechanisms and Observational Constraints

Intermediate-Mass Black Holes: Formation Mechanisms and Observational Constraints. Stellar mass BHs (  3-15 M  ) : Endpoint of the life of massive stars Observable in X-ray binaries 10 7 -10 9 in every galaxy Supermassive BHs ( 10 6 -10 9 M  ) :

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Intermediate-Mass Black Holes: Formation Mechanisms and Observational Constraints

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  1. Intermediate-Mass Black Holes:Formation Mechanisms and Observational Constraints

  2. 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

  3. Intermediate mass BHs: Mass range ~ 15 - 106 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

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

  5. Primordial Black Hole Formation • BHs may form primordially • Requires unusual pressureconditions (collapse of cosmic strings, spontaneous symmetry breaking, etc.) • Not predicted in standard cosmologies • BH mass horizon mass at formation time: • Planck Time (10-43 sec)  MBH = Planck Mass (10-5 g) • Quark-Hadron phase transition (10-5 sec)  MBH = 1 M • 1 sec  MBH = 105 M Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  6. Primordial Black Holes:Hawking Radiation • Primordial BHs withM < 1015 g would have evaporated by now • Hawking radiation is unimportant for BHs of astronomical interest Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  7. Present-Day Evolution of Massive Stars • Presently the IMF extends to ~200 M • Stars of initial mass  25-200 Mshed most of their mass before exploding, yielding BHs with masses MBH ≲15 M • Consistent with BH massesdynamically inferred for X-ray binaries • The dozen or so BH candidates inX-ray binaries have masses 3-15 M •  Stellar evolution is not presently producing IMBHs Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  8. Population III evolution of Massive Stars • At zero metallicity: • IMF may have been top-heavy • Little main-sequence mass loss • Fate of star depends on mass: • < 140 M: SN  BH or IMBH • 140-260 M: e-e+ instability  explosion, no remnant • 260 - 105M: Main Seq  no SN  IMBH • > 105M: post-Newtonian instability, no Main SeqIMBH Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  9. 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

  10. 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

  11. IMBHs and Supermassive Black Hole Formation • Supermassive BH formation: • Direct collapse into a BH • Requires that H2 cooling is suppressed • Accretion onto a seed IMBH • Merging of IMBHs • IMBHs sink to galaxy centers through dynamical friction • The galaxies in which IMBHs reside merge hierarchically • Consquences: • A substantial population of IMBHs may exist in galaxy halos • BHs in some galaxy centers may not have grown supermassive Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  12. How much mass could there be in IMBHs? • Supernovae, WMAP, etc: •  = 1, m = 0.3 • Big Bang Nucleosynthesis: • b = 0.04 • Inventory of luminous material: • v = 0.02 • Dark matter: • Non-baryonic: m - b = 0.26 • Baryonic: b - v = 0.02 (IMBHs in Dark Halos?) • Supermassive BHs: SMBH = 10-5.7 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  13. Where Could IMBHs be Hiding? • Galaxies Disks/Spheroids/Halos? • Galactic nuclei ? • Centers of Star Clusters ? Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  14. What processes might reveal IMBHs? • Gravitational lensing  brightening / distortion of background objects • Dynamics  influence on other objects • Progenitors metals, light, … • Accretion  X-rays • Space-time distortion Gravitational Waves(LIGO/LISA?) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  15. Finding Black HolesThrough Microlensing • Halo BHs produce microlensing: Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  16. Galactic Halo Black Holes:LMC Microlensing • Microlensing timescale ~ 140 (MBH /M)1/2 days • Observations: • efficiency smallfor timescales of a few years • ~1 long-duration event expectedfor a halo made of100 MIMBHs • None detected • Conclusion (MACHO team): • Galactic Halo notfully composed of BHs withMBH  1 - 30 M Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  17. Dynamical Constraints on IMBHs in Dark Halos • Are dark halos made entirely of IMBHs? • dynamical interactions observational consequences • Limits on viable BH masses: • BH accumulation in the galaxy center by dynamical friction MBH ≲106 M (stringent) • disk heating MBH ≲106 M (stringent) • heating of small dark-matterdominated systems MBH ≲103-4 M (?) • globular cluster disruption MBH ≲103-5 M (?) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  18. Limits on IMBHs from Population III stars • Background Light Limits: • All Pop III stars (below 105M ) shine bright during their main-sequence life • Contribution to extragalactic background light (IR) uncertain (dust reprocessing) • Barely consistent with  = 0.02 • Metal Enrichment Limits: • Pop III stars with MBH < 260 M shed most metals at the end of their life cannot contribute more than  = 10-4 • Pop III stars with MBH > 260 M do not go supernova  no  limit Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  19. How many Pop III IMBH remnants could there be? • Madau & Rees (2001): • Assume: one IMBH formed in each minihalo that was collapsing at z=20 from a 3 peak • Then: IMBH similar to SMBH = 10-5.7 • IMBHs would reside ingalaxies and be sinkingtowards their centers Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  20. Finding Individual IMBHs • Is there evidence for individual IMBHs? • Bulge-star microlensing • Galaxy centers • Globular clusters • Ultra-Luminous X-ray sources Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  21. Individual Black Holes From Bulge-Star Microlensing • Seven long-timescale events were detected that show parallax: • Allows mass estimate • Three lenses haveM > 3 M and L < 1 L  Possible BHs • First such BHs detected outside binaries! Bennett et al. (2000) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  22. An IMBH from Bulge-Star Microlensing? • MACHO-99-BLG-22 could be an IMBH if the lens is in the disk (most likely) or a stellar-mass BH if it is in the bulge. • Caveat: phase-space distribution function of lenses assumed known. Bennett et al. (2002) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  23. BHs in Galaxy Centers • BHs in galaxy centers can be found and weighed using dynamics of starsor gas Brown et al. (1999) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  24. Measuring Stellar Motions in External Galaxies Without BH With BH Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  25. Other Examples of KnownSuper-massive BHs NGC 7052 NGC 6240 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  26. IMBHs in Galaxy Centers? • BH mass vs. velocitydispersion correlation: • Ferrarese & Merritt;Gebhardt et al. • hot stellar systems •  >70 km/s • Do all galaxies have BHs? • Do IMBHs exist in galaxy centers with  < 50 km/s? Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  27. Black Hole constraints in Low Dispersion Systems • AGN activity: • Some very late-type galaxies are active,e.g., NGC 4395 (Sm), POX52 (dE) • BH mass estimated at MBH ~ 105 M • Stellar kinematics: only MBH upper limits • Irregulars ?? Dwarf Spheroidals ?? • Dwarf Ellipticals (Geha, Guhathakurta & vdM) •  = 20-50 km/s; MBH < 107 M • Late-Type spirals (IC 342 Boeker, vdM & Vacca) •  = 33 km/s; MBH <105.7 M Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  28. Case Study: IC 342 •  = 33 km/s MBH <105.7 M(upper limit). Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  29. Central Star Clusters in Late Type Galaxies • Late-type galaxies generally have nuclear star clusters • M ~ 106M • Barely resolved (<0.1”) • BH measurement: • Requires spatial resolution of cluster • restricted to HST data for Local Group galaxies Boeker et al. (2002) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  30. M33 • Nucleus/star cluster dominates central few arcsec • HST/STIS: • Gebhardt et al.,Merritt et al. •  = 24 km/s • MBH <1500-3000 M(upper limit) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  31. Globular Clusters:G1 (Andromeda) • Gebhardt, Rich, Ho (2002): HST/STIS data Unusually Massive Cluster Nucleus Disrupted Satellite Galaxy? Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  32. G1: Models • Gebhardt et al: Same technique as for galaxies: • Potential characterized by M/L (profile) and MBH • Find orbit superposition that best fits data • No time evolution • Baumgardt et al: • Use N-body simulations • Vary initial conditions to best fit data • Time evolution due to collisions and stellar evolution • Scaling with N complicated Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  33. Gebhardt et al.:MBH =2.0 (+1.4,-0.8)x 104M Baumgardt et al.:no black hole G1: Results Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  34. G1: Interpretation • Agreement: Mass segregation not important in G1 • (M/L)* ~ constant • Disagreement: IMBH needed to fit the data? • Quoted IMBH sphere of influence: 0.035 arcsec • Subtle, but detectable: compare to M33 • Similar distance and dispersion • BH mass upper limit 6 times smaller than G1 detection • Sphere of influence < 0.006 arcsec • Reason for Disagreement: • higher-order moments? • Very difficult measurement ……. Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  35. Globular Clusters: M15 • High central density • 1800 stars with known ground-based velocities Guhathakurta et al. (1996) Sosin & King (1997) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  36. M15: HST/STIS Project • vdM et al., Gerssen et al. (2002) V=13.7 V=18.1 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  37. M15: Observations & Reduction • Observations: • 0.1 arcsec slit • 45-60 min at 18 slit positions • G430M grating (around Mg b) • Spectral pixel size ~16 km/s • Calibration complications: • HST motion • Correct for position of star in slit (WFPC2 Catalog) • Statistical correction for blending Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  38. M15: Results • HST/STIS: 64 stellar velocities • Combine with ground-based data • R < 1 arcsec: sample tripled • R < 2 arcsec: sample doubled • Non-parametric kinematic profiles • Near the center: • Surprisingly large rotation •  = 14 km/s Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  39. M15: Evidence for Central Dark Mass • Jeans Models with constant (M/L)* MBH = 3.2 (+2.2,-2.2)x 103M • The inferred central (M/L) increase could be due to an IMBH or to mass segregation Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  40. M15: Models with Core Collapse & Mass Segregation • Fokker Planck Models (Dull et al. 1997,2003) • Results: • No BH: statistically consistent with data • BH does improve fit: MBH = 1.7 (+2.7,-1.7)x 103M • N-body models(Baumgardt et al.): similar results Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  41. M15: Interpretation • Central dark mass concentration could be mass segregation, but this does have uncertainties: • Neutron stars (1.4 M) • pulsar kick velocities indicate most probably escape • Heavy white dwarfs (1.0-1.4 M) • Have cooled too long to be observable • Local white dwarf population centers strongly on ~0.6 M, with rather few white dwarfs >1 M • High-mass IMF+evolution poorly constrained observationally • IMBH not ruled out • Large rotationunexplained … • But: no X-ray counterpart (Ho et al. 2003) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  42. Importance of (possible) IMBHs in Globular Clusters • New link between formation and evolution of galaxies, globular clusters and central BHs? • Do the seeds in supermassive BHs come from globular cluster IMBHs? Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  43. IMBHs in Globular Clusters:What’s Next? • Study nearby clusters with (non-collapsed) cores • Understand rotation • Study proper motions with HST • Study more M31 globular clusters with HST • Improve models and data-model comparison Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  44. Ultra-LuminousX-ray Sources • Many nearby galaxies have `Ultra-Luminous’X-ray sources (ULX) • LX > 1039 ergs/sec(if assumed isotropic) • Brighter than the Eddington limit for a normal X-ray binary • Fainter than Seyfert nuclei • Point sources M82 Kaaret et al. (2001) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  45. Generic Properties of ULXs • Off-center w.r.t. host galaxy not AGN related • No radio counterparts • Often variablenot young X-ray SNe • Bondi accrretion from dense ISM insufficient • Periodicity sometimes observed • State transitions sometimes observed  ULXs are compact objects accreting from a binary companion Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  46. Accretion Models:Isotropic Emission? • Isotropic emission requires that the accreting objects is an IMBH (102-104 M) • Problems: • How does an IMBH-star binary form? • Late-stage acquisition of the binary companion Dense stellar environment • Observations: not aunique correspondencewith star clusters • Companion star consumedin 106-7 years Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  47. Frequency of Occurrence • Average ~1 ULX per 4 galaxies • Strong correlation withstar formation • Antennae: 17 ULXs • Cartwheel: 20 ULXs • Suggests association with HXRBs? • Not always associated withstar forming regions • ULXs exist in some ellipticals, generally in globular clusters • Suggests association with LMXBs? • Luminosity Function continuous Zezas & Fabbiano (2002) Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  48. Accretion Models:Anisotropic emission? • Normal binary in unusual accretion mode: • Thin accretion disk with radiation-driven inhomogeneities? • Short-lived anisotropicsuper-Eddington stage;[think SS433 and Galactic micro-quasars] • Relativistic Beaming? • Difficult to explain most luminous ULXs • LX = 1040-41 ergs/sec • 1 per 100 galaxies Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  49. ULXs: Spectral Information • ULX spectra well fit by multi-color disk black body model (or sometimes a single power-law) • Inner-disk T ~ 1-2 keV similar to XRBs • XMM-Newton spectra have revealed soft components in several sources (NGC 1313 X-1, M81 X-9) with T < 200 eV T  M-1/4  IMBH Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

  50. ULX: What’s next? • Optical counterparts • few reported • Systematic study underway (Colbert, Ptak, Roye, vdM) • ULX Catalog • HST Archive • Timing • Spectra density breaks, QPOs • Associated with inner stable orbit?  f  M-1 Roeland van der Marel - Space Telescope Science Institute marel@stsci.edu http://www.stsci.edu/~marel

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