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Massive black holes in galactic centers

Massive black holes in galactic centers. Qingjuan Yu December 13, 2002. Outline. Introduction Evolution of massive binary black holes Observational constraints on growth of massive black holes Summary. Galactic center. (Tremaine et al. 2002). NGC 4258. Quasar PKS 2349 (HST).

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Massive black holes in galactic centers

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  1. Massive black holes in galactic centers Qingjuan Yu December 13, 2002

  2. Outline • Introduction • Evolution of massive binary black holes • Observational constraints on growth of massive black holes • Summary

  3. Galactic center (Tremaine et al. 2002) NGC 4258 Quasar PKS 2349 (HST) M87 (HST) Introduction: Most galaxies house massive black holes (BHs) at their centers • suggested by QSO energetics and demography (e.g. Soltan 1982, Rees 1984) • observations: massive dark objects in nearby galactic centers (e.g. Kormendy & Richstone 1995, Magorrian et al. 1998)

  4. Questions • Is it possible that the massive BHs in some galactic centers are binary black holes (BBHs) (e.g. by galaxy mergers, Begelman, Blandford & Rees 1980) • How do BBHs evolve? (separation as a function of time) • Do BBHs merge or how long can they survive? (e.g. comparison with a Hubble time) • Orbital properties of surviving BBHs? Possible observational characteristics of surviving BBHs? (appropriate methods to probe BBHs?) • How do BHs become massive? Accretion during bright QSO phases? ADAF? Obscured accretion? … • Relations between local BH mass function and QSO luminosity function? Consistent with observations?

  5. Why? • Study of BH growth and the evolution of BBHs • Studying/testing BH physics and gravitation theory • BBH mergers  gravitational waves (detected by LISA? BBH merger rates?) • Physical processes in the vicinity of surviving BBHs  dynamics in strong gravitation fields (BBH surviving rates?) • Understanding galaxy formation • The M• –and M• –L correlations  a close link between the formation and evolution of galaxies and their central BHs. • Probe of the hierarchical model

  6. Evolution of massive BBHs

  7. Dynamical friction stage Dynamical friction 1010yr increasing decreasing a 10kpc 10-5pc Evolution of massive BBHs

  8. 2. Non-hard binary stage 3. Hard binary stage Dynamical friction 1010yr three-body interactions with low-J stars; -1 (E: BBH energy) increasing (Heggie 1975) decreasing a 10kpc 10-5pc (Quinlan 1996) Evolution of massive BBHs bound dynamical friction (two-body interactions) and three-body interactions with stars passing in their vicinity

  9. 4. Gravitational radiation stage Gravitational radiation Dynamical friction 1010yr increasing (Peters 1964) decreasing a 10kpc 10-5pc Evolution of massive BBHs

  10. Gravitational radiation Dynamical friction 1010yr increasing 10kpc 10-5pc decreasing a Evolution of massive BBHs • Main uncertainty is in the non-hard binary stage and the hard binary stage.. Are low-J stars depleted before the gravitational radiation stage? • Analogy: stellar tidal disruption rates around massive BHs (e.g. Magorrian & Tremaine 1999) bottleneck

  11. Loss Region • Loss cone: • tidal disruption: • Hard BBH system: • BBH energy loss rate: determined by the rate of removal of stars from the loss cone. • Depletion of the initial population of stars in the loss cone; • New stars are scattered into the loss cone by two-body relaxation; • Steady state: controlled by the balance between the loss rate and the rate at which stars refill the loss cone, • Rate of refilling the loss cone caused by two-body relaxation: solving the steady-state Fokker-Planck equation.

  12. `Diffusion’ regime: • at small radii, the loss cone is nearly empty. • `Pinhole’ regime: • at large radii, the loss cone is full. Lightman & Shapiro (1977)

  13. Effects of galaxy shapes • Spherical system (loss cone J<Jlc); • Axisymmetric (flattened) system (loss wedge |Jz|<Jlc): • Js: characteristic angular momentum marking the transition from centrophilic to centrophobic orbits • J<~Js: centrophilic orbits • J>~Js: centrophobic orbits • Stars on centrophilic orbits with |Jz|<Jlc can precess into the loss cone • Triaxiality (loss region J<Js).

  14. (Magorrian & Tremaine 1999) Jz=0

  15. Gravitational radiation Gravitational radiation Dynamical friction 1010yr 1010yr increasing 10kpc 10-5pc decreasing a Dynamical friction increasing 10kpc 10-5pc decreasing a Evolution of massive BBHs • Main uncertainty is in the non-hard binary stage and the hard binary stage.. • Are low-J stars depleted before the gravitational radiation stage? • Analogy: stellar tidal disruption rates around massive BHs (e.g. Magorrian & Tremaine 1999) bottleneck

  16. Role of the BBH orbital eccentricity (Artymowicz 2000)

  17. Sample: nearby early-type galaxies observed by HST (Faber et al. 1997) Surface brightness profiles: I(R)R- (R0) Core galaxies (00.3): high luminosity and dispersion, large BHs, low central density Power-law galaxies ( 0.5): low luminosity and dispersion, small BHs, high central density BBH evolution in realistic galaxy models

  18. BBH evolution in realistic galaxy models Sample: nearby early-type galaxies observed by HST (Faber et al. 1997) (Yu 2002)

  19. surviving BBHs merged BBHs increasing velocity dispersion increasing flattening surviving BBHs increasing triaxiality merged BBHs BBH evolution in realistic galaxy models (Yu 2002): • Depends on BH masses, and velocity dispersions and shapes of host galaxies • small BHs (m2/m1<10-3) do not decay into galactic centers; • BBHs are more likely to have merged in low-dispersion galaxies and survive in high-dispersion galaxies; • BBHs are more likely to have merged in highly flattened or triaxial galaxies and survive in spherical and nearly spherical galaxies • Estimated orbital properties of surviving BBHs: • separation: 10-3 –10 pc

  20. Estimated orbital properties of surviving BBHs Semimajor axes Orbital velocities Orbital periods (Yu 2002)

  21. Estimated orbital properties of surviving BBHs Semimajor axes Orbital velocities Orbital periods (Yu 2002)

  22. Gas dynamics around BBHs • Begelman, Blandford & Rees (1980) • flung out of the system • accrete onto the larger BH (causing orbital contraction as the product of Mr is adiabatically invariant). • Ivanov, Papaloizou & Polnarev 1999; Gould & Rix 2000; Armitage & Natarajan 2002 • Planet-like migration

  23. double nuclei (upper limit ~ HST resolution) bending or wiggling of jets (e.g. Blandford, Begelman, Rees 1980) double-peaked emission lines from broad line regions associated with BBHs in active galactic nuclei (AGNs) (Gaskell 1996) periodic behavior in the radio, optical, X-ray or -ray light curves (e.g. Valtaoja et al. 2000, Rieger & Mannheim 2000) broad asymmetric Iron K emission line shape from a two-accretion-disc system associated with a BBH (Yu & Lu 2001) Possible observational characteristics of surviving BBHs

  24. Relations between QSOs and BHs in nearby galaxies • Soltan’s argument: QSO luminosity function (L,t)traces the accretion history of local remnant BHs (Soltan 1982; Chokshi & Turner 1992; Small & Blandford 1992).

  25. Accreted BH mass density: • •QSO=2.2105(0.1/) M/Mpc3 (Chokshi & Turner 1992) • •X=(6-9)105(0.1/) M/Mpc3 (Fabian & Iwasawa 1999), •X=(7.5-16.8)105(0.1/) M/Mpc3 (Elvis, Risaliti & Zamorani 2002) • •IR=7.5105(0.1/) M/Mpc3 (Haehnelt & Kauffmann 2001) • •HX+OPT+FIR=9105(0.1/) M/Mpc3 (Barger et al. 2001) • … … • Local BH mass density (h=0.65): • •,local=3.5105 M/Mpc3 (Salucci et al. 1999) • •,local=3.7105 M/Mpc3 (Merritt & Ferrarese 2001) • •,local=(42)105 M/Mpc3 (Marconi & Salvati 2001) • •,local=(2.00.7)105 M/Mpc3 (Aller & Richstone 2002) • … …

  26. New estimates of BH mass densities • Total local BH mass density: • local BH mass function nM(M,t0): • SDSS early-type galaxy sample n(,t0) (Bernardi et al. 2001) • the tight M• –relation (Tremaine et al. 2002) • •,local=(2.50.4)105 M/Mpc3 (h=0.65) (Yu & Tremaine 2002) • BH mass density accreted due to optically bright QSO phases: • (L,t): 2dF QSO Redshift survey (Boyle et al. 2000) • •,acc=2.1105[0.1(1- ) /] M/Mpc3 (Yu & Tremaine 2002)

  27. Update of Soltan’s argument: relations in distributions and including effects of BH mergers • Total mass density: • Partial mass density: • no BH mergers • including BH mergers  increase total mass in high-mass BHs • Normalized mass density distribution: m2 m1 m2+m1

  28. Partial mass density (distributions): >108M QSO (m,,h)= (0.3,0.7,0.65) Local Comparison with observations: Discrepancy in distributions Expected inequality for partial mass density:

  29. Other accretion mechanisms? Sub-Eddington luminosity? Uncertainties of the QSOLF at the bright end? Bolometric correction? Gravitational radiation during BH mergers? BHs left in galactic halos after a galaxy merger? Incomplete merging of binary BHs? Ejection of BHs from galactic centers? (three-body interactions with other massive BHs or gravitational radiation reaction during BH coalescence) Comparison with observations: Discrepancy in distributions

  30. Partial mass density (distributions): Possible solutions: Luminous QSOs (Lbol>1046erg/s) have a high-efficiency (e.g., ~0.2). If true, growth of high-mass BHs (M•>108M) comes mainly from accretion during optically bright QSO phases (scarcity of Type II QSOs). less luminous QSOs have a low efficiency (<0.1) have a high efficiency, but a significant fraction should be obscured (obscured growth for low-mass BHs, Fabian 1999). Super-Eddington luminosity (Begelman 2001, 2002). >108M QSO (m,,h)= (0.3,0.7,0.65) Local Comparison with observations: Discrepancy in distributions Expected inequality for partial mass density: Maximum efficiency allowed in thin-disk accretion models: ~0.31(Thorne 1974).

  31. X-ray background spectrum (Fabian & Iwasawa 1999)

  32. ~(3-13)107 yr QSO mean lifetime • The mean lifetime of QSOs is comparable to the Salpeter time (the time for a BH accreting with the Eddington luminosity to e-fold in mass).

  33. Summary • The orbital evolution of BBHs depends on the velocity dispersion and shape of the host galaxy, and the masses of BHs. • BBHs are most likely to survive in spherical or nearly spherical and high-velocity dispersion galaxies. • The upper limit of the separations of surviving BBHs is close to the HST resolution for the typical nearby galaxies (at Virgo). • The absence of double nuclei in the centers of nearby galaxies does not necessarily mean that they have no BBHs. • If all galaxies are highly triaxial, there will be no surviving BBHs.

  34. Summary • Expected relations between the local BH mass function and the QSO luminosity function are inconsistent with observations unless luminous QSOs (Lbol>1046erg/s) have a super-Eddington luminosity or have a high efficiency (e.g., ~0.2). • Growth of high-mass BHs (>108M) comes mainly from accretion during optically bright QSO phases, rather than obscured accretion (suggested by the X-ray background spectrum), accretion by ADAF, accretion of non-baryonic dark matter etc.

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