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

Roeland van der Marel

Intermediate-Mass Intermediate-Mass Black Holes:Black Holes:Formation Formation Mechanisms and Mechanisms and Observational Observational ConstraintsConstraints

Roeland van der Marel - Space Telescope Science [email protected] http://www.stsci.edu/~marel

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Known Black Holes Known Black Holes (BHs)(BHs)in the Universein the Universe

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 of

active galaxies and quasars ~1 in every galaxy


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Intermediate-MassIntermediate-MassBlack Holes (IMBHs)Black Holes (IMBHs)

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


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Possible Mechanisms Possible Mechanisms for IMBH Formation for IMBH Formation

Primordial From Population III stars In Dense Star Clusters As part of Supermassive BH

formation


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Primordial Black Hole Primordial Black Hole FormationFormation

BHs may form primordially Requires unusual pressure

conditions (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


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Primordial Black Holes:Primordial Black Holes:Hawking RadiationHawking Radiation

Primordial BHs withM < 1015 g would have evaporated by now

Hawking radiation is unimportant for BHs of astronomical interest


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Present-Day Evolution Present-Day Evolution of Massive Starsof 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


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Population III evolution Population III evolution of Massive Starsof 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 - 105 M: Main Seq no SN IMBH > 105 M: post-Newtonian instability, no Main Seq

IMBH


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Dynamical Evolution of Dynamical Evolution of Star ClustersStar 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)


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A Scenario for IMBH A Scenario for IMBH Formation in Star Formation in Star ClustersClusters

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 Myr

or present Trelax < 100 MyrGRAPE 6


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IMBHs and IMBHs and Supermassive Black Supermassive Black Hole FormationHole 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


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How much mass could How much mass could there be in IMBHs?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


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Where Could IMBHs be Where Could IMBHs be Hiding?Hiding?

Galaxies Disks/Spheroids/Halos? Galactic nuclei ? Centers of Star Clusters ?


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What processes might What processes might reveal IMBHs?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?)


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Finding Black HolesFinding Black HolesThrough MicrolensingThrough Microlensing

Halo BHs produce microlensing:


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Galactic Halo Black Galactic Halo Black Holes:Holes:LMC MicrolensingLMC Microlensing

Microlensing timescale ~ 140 (MBH /M)1/2 days

Observations: efficiency small for timescales of a few years ~1 long-duration event expected for a halo made

of 100 M IMBHs None detected

Conclusion (MACHO team): Galactic Halo not

fully composed of BHs withMBH 1 - 30 M


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Dynamical Constraints Dynamical Constraints on IMBHs in Dark Haloson 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 (?)


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Limits on IMBHs from Limits on IMBHs from Population III starsPopulation III stars

Background Light Limits: All Pop III stars (below 105 M ) 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


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How many Pop III IMBH How many Pop III IMBH remnants could there remnants could there be? 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


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Finding Individual Finding Individual IMBHsIMBHs

Is there evidence for individual IMBHs? Bulge-star microlensing Galaxy centers Globular clusters Ultra-Luminous X-ray sources


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Individual Black Holes From Individual Black Holes From Bulge-Star MicrolensingBulge-Star Microlensing

Seven long-timescale events were detected that show parallax: Allows mass estimate Three lenses have

M > 3 M and L < 1 L Possible BHs

First such BHs detected outside binaries! Bennett et al. (2000)


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An IMBH from Bulge-An IMBH from Bulge-Star Microlensing?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)


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BHs in Galaxy CentersBHs in Galaxy Centers

BHs in galaxy centers can be found and weighed using dynamics of stars or gas

Brown et al. (1999)


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Measuring Stellar Measuring Stellar Motions in External Motions in External GalaxiesGalaxies

Without BH

With BH


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Other Examples of Other Examples of KnownKnownSuper-massive BHsSuper-massive BHs

NGC 7052 NGC 6240


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IMBHs in Galaxy IMBHs in Galaxy Centers?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?


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Black Hole constraints Black Hole constraints in Low Dispersion in Low Dispersion Systems 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


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Case Study: Case Study: IC 342IC 342

= 33 km/s MBH < 105.7 M (upper limit).


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Central Star Clusters in Central Star Clusters in Late Type GalaxiesLate Type Galaxies

Late-type galaxies generally have nuclear star clusters M ~ 106 M Barely resolved (<0.1”)

BH measurement: Requires spatial

resolution of cluster restricted to HST data

for Local Group galaxies

Boeker et al. (2002)


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M33M33

Nucleus/star cluster dominates central few arcsec

HST/STIS: Gebhardt et al.,

Merritt et al. = 24 km/s MBH < 1500-3000 M

(upper limit)


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Globular Clusters:Globular Clusters:G1 (Andromeda)G1 (Andromeda)

Gebhardt, Rich, Ho (2002): HST/STIS data

Unusually Massive Cluster

Nucleus Disrupted Satellite Galaxy?


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G1:G1: Models 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


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G1:G1: Results Results

Gebhardt et al.:MBH = 2.0 (+1.4,-0.8) x 104

M

Baumgardt et al.:no black hole


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G1:G1: Interpretation 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 …….


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Globular Clusters: Globular Clusters: M15M15

High central density 1800 stars with known ground-based

velocities Guhathakurta et al. (1996)Sosin & King (1997)


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M15:M15: HST/STIS Project HST/STIS Project

V=13.7

V=18.1

vdM et al., Gerssen et al. (2002)


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M15: M15: Observations & Observations & ReductionReduction

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


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M15:M15: Results 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


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M15: M15: Evidence for Evidence for Central Dark MassCentral Dark Mass

Jeans Models with constant (M/L)* MBH = 3.2 (+2.2,-2.2) x 103

M

The inferred central (M/L) increase could be due to an IMBH or to mass segregation


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M15:M15: Models with Core Models with Core Collapse & Mass Collapse & Mass SegregationSegregation

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 103

M

N-body models (Baumgardt et al.): similar results


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M15:M15: Interpretation 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 rotation unexplained … But: no X-ray counterpart (Ho et al. 2003)


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Importance of Importance of (possible) IMBHs in (possible) IMBHs in Globular ClustersGlobular 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?


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IMBHs in Globular IMBHs in Globular Clusters:Clusters:What’s Next?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


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Ultra-LuminousUltra-LuminousX-ray SourcesX-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)


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Generic Properties of Generic Properties of ULXsULXs

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


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Accretion Models:Accretion Models:Isotropic Emission?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 a

unique correspondencewith star clusters

Companion star consumedin 106-7 years


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Frequency of Frequency of OccurrenceOccurrence

Average ~1 ULX per 4 galaxies Strong correlation with

star 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)


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Accretion Models:Accretion Models:Anisotropic emission?Anisotropic emission?

Normal binary in unusual accretion mode: Thin accretion disk with radiation-driven inhomogeneities? Short-lived anisotropic

super-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


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ULXs: Spectral ULXs: Spectral InformationInformation

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


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ULX: What’s next?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


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Conclusions:Conclusions:

The existence of IMBHs is not merely a remote possibility Predicted theoretically as the

result of realistic scenarios Might explain a number of

observational findings Much more work needed to

prove their existence unequivocally

Could be important forgravitational wave detection

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

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