Roeland van der Marel Intermediate-Mass Black Holes: Formation Mechanisms and Observational Constraints

Roeland van der Marel - Space Telescope Science Institute 2 Known Black Holes (BHs) in the Universe P Stellar mass BHs (  3-15 M  ): P Endpoint of the life of massive stars P Observable in X-ray binaries P  in every galaxy P Supermassive BHs (  M  ): P Generate the nuclear activity of active galaxies and quasars P ~1 in every galaxy

Roeland van der Marel - Space Telescope Science Institute 3 Intermediate-Mass Black Holes (IMBHs) P Intermediate mass BHs: P Mass range ~ M  P Questions: P Is there a reason why they should exist? P Is there evidence that they exist? P Status and Progress: P These questions can be meaningfully addressed P No consensus yet

Roeland van der Marel - Space Telescope Science Institute 4 Possible Mechanisms for IMBH Formation P Primordial P From Population III stars P In Dense Star Clusters P As part of Supermassive BH formation

Roeland van der Marel - Space Telescope Science Institute 5 Primordial Black Hole Formation P BHs may form primordially P Requires unusual pressure conditions (collapse of cosmic strings, spontaneous symmetry breaking, etc.) P Not predicted in standard cosmologies P BH mass  horizon mass at formation time: P Planck Time ( sec)  M BH = Planck Mass (10 -5 g) P Quark-Hadron phase transition (10 -5 sec)  M BH = 1 M  P 1 sec  M BH = 10 5 M 

Roeland van der Marel - Space Telescope Science Institute 6 Primordial Black Holes: Hawking Radiation P Primordial BHs with M < g would have evaporated by now P Hawking radiation is unimportant for BHs of astronomical interest

Roeland van der Marel - Space Telescope Science Institute 7 Present-Day Evolution of Massive Stars P Presently the IMF extends to ~200 M  P Stars of initial mass  M  shed most of their mass before exploding, yielding BHs with masses M BH ≲ 15 M  P Consistent with BH masses dynamically inferred for X-ray binaries P The dozen or so BH candidates in X-ray binaries have masses 3-15 M  P  Stellar evolution is not presently producing IMBHs

Roeland van der Marel - Space Telescope Science Institute 8 Population III evolution of Massive Stars P At zero metallicity: P IMF may have been top-heavy P Little main-sequence mass loss P Fate of star depends on mass: P < 140 M  : SN  BH or IMBH P M  : e - e + instability  explosion, no remnant P M  : Main Seq  no SN  IMBH P > 10 5 M  : post-Newtonian instability, no Main Seq  IMBH

Roeland van der Marel - Space Telescope Science Institute 9 Dynamical Evolution of Star Clusters P Many physical processes in a dense stellar environment can in principle give runaway BH growth P Negative heat capacity of gravity  core collapse P Binary heating normally halts core collapse in systems with N * < Rees (1984)

Roeland van der Marel - Space Telescope Science Institute 10 A Scenario for IMBH Formation in Star Clusters P When core collapse sets in, energy equipartition is not maintained  the most massive stars sink to the center first P Calculations show that an IMBH can form due to runaway collisions (Portegies Zwart & McMillan) P Requires initial T relax < 25 Myr or present T relax < 100 Myr GRAPE 6

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

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

Roeland van der Marel - Space Telescope Science Institute 13 Where Could IMBHs be Hiding? P Galaxies Disks/Spheroids/Halos? P Galactic nuclei ? P Centers of Star Clusters ?

Roeland van der Marel - Space Telescope Science Institute 14 What processes might reveal IMBHs? P Gravitational lensing  brightening / distortion of background objects P Dynamics  influence on other objects P Progenitors  metals, light, … P Accretion  X-rays P Space-time distortion  Gravitational Waves (LIGO/LISA?)

Roeland van der Marel - Space Telescope Science Institute 15 Finding Black Holes Through Microlensing P Halo BHs produce microlensing:

Roeland van der Marel - Space Telescope Science Institute 16 Galactic Halo Black Holes: LMC Microlensing P Microlensing timescale ~ 140 (M BH /M  ) 1/2 days P Observations: P efficiency small for timescales of a few years P ~1 long-duration event expected for a halo made of 100 M  IMBHs P None detected P Conclusion (MACHO team): P Galactic Halo not fully composed of BHs with M BH  M 

Roeland van der Marel - Space Telescope Science Institute 17 Dynamical Constraints on IMBHs in Dark Halos P Are dark halos made entirely of IMBHs? P dynamical interactions  observational consequences P Limits on viable BH masses: P BH accumulation in the galaxy center by dynamical friction  M BH ≲ 10 6 M  (stringent) P disk heating  M BH ≲ 10 6 M  (stringent) P heating of small dark-matter dominated systems  M BH ≲ M  (?) P globular cluster disruption  M BH ≲ M  (?)

Roeland van der Marel - Space Telescope Science Institute 18 Limits on IMBHs from Population III stars P Background Light Limits: P All Pop III stars (below 10 5 M  ) shine bright during their main-sequence life P Contribution to extragalactic background light (IR) uncertain (dust reprocessing) P Barely consistent with  = 0.02 P Metal Enrichment Limits: P Pop III stars with M BH < 260 M  shed most metals at the end of their life  cannot contribute more than  = P Pop III stars with M BH > 260 M  do not go supernova  no  limit

Roeland van der Marel - Space Telescope Science Institute 19 How many Pop III IMBH remnants could there be? P Madau & Rees (2001): P Assume: one IMBH formed in each minihalo that was collapsing at z=20 from a 3  peak P Then:  IMBH similar to  SMBH = P IMBHs would reside in galaxies and be sinking towards their centers

Roeland van der Marel - Space Telescope Science Institute 20 Finding Individual IMBHs P Is there evidence for individual IMBHs? P Bulge-star microlensing P Galaxy centers P Globular clusters P Ultra-Luminous X-ray sources

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

Roeland van der Marel - Space Telescope Science Institute 22 An IMBH from Bulge-Star Microlensing? P 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. P Caveat: phase-space distribution function of lenses assumed known. Bennett et al. (2002)

Roeland van der Marel - Space Telescope Science Institute 23 BHs in Galaxy Centers P BHs in galaxy centers can be found and weighed using dynamics of stars or gas Brown et al. (1999)

Roeland van der Marel - Space Telescope Science Institute 24 Measuring Stellar Motions in External Galaxies Without BH With BH

Roeland van der Marel - Space Telescope Science Institute 25 Other Examples of Known Super-massive BHs NGC 7052NGC 6240

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

Roeland van der Marel - Space Telescope Science Institute 27 Black Hole constraints in Low Dispersion Systems P AGN activity: P Some very late-type galaxies are active, e.g., NGC 4395 (Sm), POX52 (dE) P BH mass estimated at M BH ~ 10 5 M  P Stellar kinematics: only M BH upper limits P Irregulars ?? Dwarf Spheroidals ?? P Dwarf Ellipticals (Geha, Guhathakurta & vdM) P  = km/s; M BH < 10 7 M  P Late-Type spirals (IC 342 Boeker, vdM & Vacca) P  = 33 km/s; M BH < M 

Roeland van der Marel - Space Telescope Science Institute 28 Case Study: IC 342 P  = 33 km/s  M BH < M  (upper limit).

Roeland van der Marel - Space Telescope Science Institute 29 Central Star Clusters in Late Type Galaxies P Late-type galaxies generally have nuclear star clusters P M ~ 10 6 M  P Barely resolved (<0.1”) P BH measurement: P Requires spatial resolution of cluster P restricted to HST data for Local Group galaxies Boeker et al. (2002)

Roeland van der Marel - Space Telescope Science Institute 30 M33 P Nucleus/star cluster dominates central few arcsec P HST/STIS: P Gebhardt et al., Merritt et al. P  = 24 km/s P M BH < M  (upper limit)

Roeland van der Marel - Space Telescope Science Institute 31 Globular Clusters: G1 (Andromeda) P Gebhardt, Rich, Ho (2002): HST/STIS data Unusually Massive Cluster Nucleus Disrupted Satellite Galaxy?

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

Roeland van der Marel - Space Telescope Science Institute 33 G1: Results P Gebhardt et al.: M BH = 2.0 (+1.4,-0.8) x 10 4 M  P Baumgardt et al.: no black hole

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

Roeland van der Marel - Space Telescope Science Institute 35 Globular Clusters: M15 P High central density P 1800 stars with known ground-based velocities Guhathakurta et al. (1996) Sosin & King (1997)

Roeland van der Marel - Space Telescope Science Institute 36 M15: HST/STIS Project V=13.7 V=18.1 P vdM et al., Gerssen et al. (2002)

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

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

Roeland van der Marel - Space Telescope Science Institute 39 M15: Evidence for Central Dark Mass P Jeans Models with constant (M/L) *  M BH = 3.2 (+2.2,-2.2) x 10 3 M  P The inferred central (M/L) increase could be due to an IMBH or to mass segregation

Roeland van der Marel - Space Telescope Science Institute 40 M15: Models with Core Collapse & Mass Segregation P Fokker Planck Models (Dull et al. 1997,2003) P Results: P No BH: statistically consistent with data P BH does improve fit: M BH = 1.7 (+2.7,-1.7) x 10 3 M  P N-body models (Baumgardt et al.): similar results

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

Roeland van der Marel - Space Telescope Science Institute 42 Importance of (possible) IMBHs in Globular Clusters P New link between formation and evolution of galaxies, globular clusters and central BHs? P Do the seeds in supermassive BHs come from globular cluster IMBHs?

Roeland van der Marel - Space Telescope Science Institute 43 IMBHs in Globular Clusters: What’s Next? P Study nearby clusters with (non-collapsed) cores P Understand rotation P Study proper motions with HST P Study more M31 globular clusters with HST P Improve models and data-model comparison

Roeland van der Marel - Space Telescope Science Institute 44 Ultra-Luminous X-ray Sources P Many nearby galaxies have `Ultra-Luminous’ X-ray sources (ULX) P L X > ergs/sec (if assumed isotropic) P Brighter than the Eddington limit for a normal X-ray binary P Fainter than Seyfert nuclei P Point sources M82 Kaaret et al. (2001)

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

Roeland van der Marel - Space Telescope Science Institute 46 Accretion Models: Isotropic Emission? P Isotropic emission requires that the accreting objects is an IMBH (  M  ) P Problems: P How does an IMBH-star binary form? P Late-stage acquisition of the binary companion  Dense stellar environment P Observations: not a unique correspondence with star clusters P Companion star consumed in years

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

Roeland van der Marel - Space Telescope Science Institute 48 Accretion Models: Anisotropic emission? P Normal binary in unusual accretion mode: P Thin accretion disk with radiation-driven inhomogeneities? P Short-lived anisotropic super-Eddington stage; [think SS433 and Galactic micro-quasars] P Relativistic Beaming? P Difficult to explain most luminous ULXs P L X = ergs/sec P 1 per 100 galaxies

Roeland van der Marel - Space Telescope Science Institute 49 ULXs: Spectral Information P ULX spectra well fit by multi-color disk black body model (or sometimes a single power-law) P Inner-disk T ~ 1-2 keV  similar to XRBs P 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 50 ULX: What’s next? P Optical counterparts P few reported P Systematic study underway (Colbert, Ptak, Roye, vdM) P ULX Catalog P HST Archive P Timing P Spectra density breaks, QPOs P Associated with inner stable orbit?  f  M -1

Roeland van der Marel - Space Telescope Science Institute 51 Conclusions: P The existence of IMBHs is not merely a remote possibility P Predicted theoretically as the result of realistic scenarios P Might explain a number of observational findings P Much more work needed to prove their existence unequivocally P Could be important for gravitational wave detection