Published on November 28, 2007
Black Holes: Black Holes Black Holes and Galaxy Centers : 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 mc2/2 with e: efficiency for black holes Accretion onto black holes could be the most efficient energy production mechanism, IF black holes are abundant! Do Black Holes Exist? A brief History: 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 MBH x 107 - 109 Msun “Super-massive black holes” (SMBH) How to detect a black hole?: 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 explanations e.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 Slide5: 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 B. Methods Two separate issues: qualitative proof existence and estimate/measurement of masses (and spin) Observed Ka Lines: Observed Ka Lines X-ray observations of galaxy centers: we see gas move at relativistic speeds close to last stable orbit Relativistic Jets in AGNs Require Black Holes: Relativistic Jets in AGNs Require Black Holes Broad Optical/UV Emission Lines in Active Galactic Nuclei: Broad Optical/UV Emission Lines in Active Galactic Nuclei Measuring Black Hole Masses: 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 NGC4258 (cntd.): NGC4258 (cntd.) Example 2: the Galactic Center: 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 Stellar Motions in the G.C.: Stellar Motions in the G.C. Stellar distribution in the G.C. Black Holes in Nearby Nuclei: Black Holes in Nearby Nuclei Example 3: M32 (van der Marel, de Zeeuw & Rix, 1997) Compare predicted stellar kinematics with HST data MBH~3x106MSun Census of Nearby Black Holes: 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? MBH and Host Galaxy Properties: 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. MBH vs Mstars and Concentration: 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) 5105 M / Mpc3 Measuring Black Holes in High-Redshift Galaxies: 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 Reverberation Mapping of QSOs: Reverberation Mapping of QSOs QSO Spectra Reverberation Mapping of QSOs: Reverberation Mapping of QSOs How Early Did Massive Black Holes Exist?: How Early Did Massive Black Holes Exist? Pentericci et al 2001 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 Questions: How did massive Black Holes get to the to galaxy centers in the first place? How did they grow in mass? Black Hole Growth: Black Hole Growth Increase in total mass (integrated over all black holes): Accretion onto the black hole Individual mass increase: Accretion or Merging QSO Luminosity Function and BH Growth: 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? Black Hole Merging: 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 ?? Black Hole Merging in a Cosmological Context : 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? Initial Black Holes: 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) Overall Census: 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!