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Black Holes

Black Holes. Black Holes and Galaxy Centers. Two most important energy production mechanisms in astrophysics: nuclear fusion (e.g. stars) E fusion ~ 0.007 mc² gravitational accretion onto “deep potential wells” such as white dwarfs, neutron stars & black holes requires conversion of:

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Black Holes

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  1. Black Holes Vatican 2003 Lecture 30 HWR

  2. Black Holes and Galaxy Centers Two most important energy production mechanisms in astrophysics: • nuclear fusion (e.g. stars)Efusion ~ 0.007 mc² • gravitational accretion onto “deep potential wells” • such as white dwarfs, neutron stars & black holes • requires conversion of: potential energy  thermal energy  radiation Eaccretion ~e mc2/2with e: efficiency for black holes • Accretion onto black holes could be the most efficient energy production mechanism, IF black holes are abundant! Vatican 2003 Lecture 30 HWR

  3. Do Black Holes Exist? A brief History • Late 60‘s: Zeldovich, Lynden-Bell, Salpeter and others considered that • QSOs (“Quasi-stellar Objects”) have “sustained” luminosities of ~1046 ergs/sec • flux variability (at short wavelengths)at tvar~ hours  rflux~ tvar · c luminosity equal to a whole galaxy from a region the size of the solar system accretion onto BH only sustainable and viable energy production mechanism • To sustain accretion, Fradiation < -Fgrav on electrons (“Eddington limit”):  LQSO MBHx 107 - 109Msun  “Super-massive black holes” (SMBH) Vatican 2003 Lecture 30 HWR

  4. How to detect a black hole? A. Criteria • demonstrate the existence of an event horizon • “relativistically deep” potential well • gravitational redshift of light • Doppler boosting, transverse Doppler effect, etc. • show relativistic (v  0.1 c) material motion • exclude alternative explanationse.g. lower limit on dynamical mass +upper limit on emitted radiation  astrophysical “mass-to-light-ratio” limit from considering alternatives: • clusters of neutron stars • clusters of planets, etc. • must be stable for t ~ tuniverse ~ 1010 years Vatican 2003 Lecture 30 HWR

  5. Predicted Emission Line Profiles for  Schwarzschild and Kerr Black Holes B. Methods Two separate issues: qualitative proof existence and estimate/measurement of masses (and spin) 1. Fe - K lines (X-ray spectroscopy) for few objects (~ 6.5 keV) • emission from very hot plasma • line widths and line shapes match expectations for material orbiting v  0.3 c • in a few cases last stable orbit implies SMBH of high spin Vatican 2003 Lecture 30 HWR

  6. Observed Ka Lines X-ray observations of galaxy centers: we see gas move at relativistic speeds  close to last stable orbit Vatican 2003 Lecture 30 HWR

  7. Relativistic Jets in AGNs Require Black Holes Vatican 2003 Lecture 30 HWR

  8. Ha several 1000km/s wide!! Broad Optical/UV Emission Lines in Active Galactic Nuclei Vatican 2003 Lecture 30 HWR

  9. Measuring Black Hole Masses Example 1: NGC4258 • the nucleus of the nearby spiral galaxy shows several spots of H2O maser emission (Myashi etal 1995) • Velocities and positions fit a Keplerian disk perfectly  r > 5x1012Msun/pc3 Vatican 2003 Lecture 30 HWR

  10. NGC4258 (cntd.) Vatican 2003 Lecture 30 HWR

  11. G.C. seen with the VLT: Schoedel et al Example 2: the Galactic Center • Genzel, Eckardt,Schoedel et al. 1990- • Ghez et al 1995 – • At the geometric center of the Milky Ways inner stellar mass distribution lies the radio source SgrA* • Is it a black hole? • Method:at only 10kpc distance, we can watch stars move Vatican 2003 Lecture 30 HWR

  12. Orbit of one star over the last 10 years Stellar Motions in the G.C. Stellar distribution in the G.C. Vatican 2003 Lecture 30 HWR

  13. Stellar density  Fstars, then add FBH Black Holes in Nearby Nuclei • Example 3: M32 (van der Marel, de Zeeuw & Rix, 1997) Compare predicted stellar kinematics with HST data MBH~3x106MSun Vatican 2003 Lecture 30 HWR

  14. Census of Nearby Black Holes Employing “broad brush” methods:Whenever a “non-luminous”, compact mass excess (“MDO”) in galaxy centers could be studied, an SMBH is the only viable explanation. attempt census of MDO‘s and identify with MBH HST with its superior resolution has been pivotal in such studies e.g. Gebhardt et al 2002 Questions: Do all galaxies have black holes at their centers? Do big (i.e. massive) black holes live in big galaxies? Vatican 2003 Lecture 30 HWR

  15. MBH and Host Galaxy Properties • Magorrian et al. 1998: claimed that more luminous galaxies have more massive black hole measurements. [all their BH measurements were spurious, though] • Ferrarese & Merrit, 2000; Gebhardt et al 2000: MBH ~ s*n • MBH can be predicted from the velocity dispersion at 1kpc • No good physical explanation, yet. Vatican 2003 Lecture 30 HWR

  16. MBH vs Mstars and Concentration • The black hole mass seems to correlate well with many global properties of the galaxy’s bulge(not disk!!) • E.g. Haering & Rix 2003: MBH vs Mstars • M-s relation is an easy way to measure MBH from s • Mean density of BH today r(BH)  5105 M / Mpc3 Vatican 2003 Lecture 30 HWR

  17. Measuring Black Holes in High-Redshift Galaxies • Reverberation mapping: Measure light-travel time between • accretion disk at the very center (continuum) • Broad line region Measure velocity width of broad lines MBH~s2BLRxR/G Netzer, Peterson, Maoz, Kaspi and others Vatican 2003 Lecture 30 HWR

  18. ..and their time variation Reverberation Mapping of QSOs QSO Spectra Vatican 2003 Lecture 30 HWR

  19.  Most luminous QSO‘s have   0.1 Eddington limit Line – cont. time-lag Reverberation Mapping of QSOs Vatican 2003 Lecture 30 HWR

  20. How Early Did Massive Black Holes Exist? Very Luminous QSOs exist at z>6! “Eddington Limit”: Gravitational force inward > Radiation Force Outward  MBH > 109 MSun 0.6 Gyrs after the Big Bang Pentericci et al 2001 Questions: How did massive Black Holes get to the to galaxy centers in the first place? How did they grow in mass? Vatican 2003 Lecture 30 HWR

  21. Black Hole Growth • Increase in total mass (integrated over all black holes):Accretion onto the black hole • Individual mass increase: Accretion or Merging Vatican 2003 Lecture 30 HWR

  22. Yu & Tremaine 2002  rBH, QSO  3-5105 M  Mpc (0.1/) QSO Luminosity Function and BH Growth QSO luminosity function varies widely as a function of z Are present day black holes the result of luminous accretion in QSO phases? Vatican 2003 Lecture 30 HWR

  23. Black Hole Merging • BH’s sit at the centers of every (most) (proto-)galaxy • Early galaxies (and their halos) merge • What happens to the black holes at their centers? • Step 1: • Black holes in central cusps sink to the • center via dynamical friction, until i.e. the BH’s dominate the mass in the center • Step 3: • Black holes spiral in, emitting gravitational radiation • Step 2: ahang-up > 100 agrav.rad !! How to get from 1 to 3 ?? Vatican 2003 Lecture 30 HWR

  24. Black Hole Merging in a Cosmological Context Volonteri et al. 2003: • Start with MBH~200 MSun in every sub-halo • Follow merging tree • can get 108MSun by now • But: it is hard to get 109 by z~6 Question: what was the initial black hole mass? Vatican 2003 Lecture 30 HWR

  25. Initial Black Holes • Possible Key: Very first generation of stars (Pop III) • No metals no cooling  gas clouds stay hotter  Jeans mass is higher (where fragmentation sets in) first stars very massive • Possible masses 200 MSun 106 MSun(?) • Bromm etal 1999,2003, Abell et al 2000 Formation of 105 MSun star at z~30 (Bromm and Loeb 2003) Vatican 2003 Lecture 30 HWR

  26. Overall Census • All galaxies with bulges seem to have black holes • The black hole mass correlates tightly with the overall mass and dynamics of the host bulge • Averaging over the galaxy population in the present day universe, the mean black hole density is <rBH>~5x105MSun/Mpc3 (Yu & Tremaine 2003) •  BH growth and bulge formation are connected E.g. both happen during mergers with gas inflow • Black holes grow through (luminous) accretion • were all galaxies QSOs sometime in the past? • How important is the merging of BH’s during the merging of their galaxies? Not clear yet! Vatican 2003 Lecture 30 HWR

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