the evolution of quasars and massive black holes “quasar hosts and the black hole-spheroid...
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The Evolution of Quasars andMassive Black Holes
“Quasar Hosts and the Black Hole-Spheroid Connection”: Dunlop 2004
“The Evolution of Quasars”: Osmer 2004
David R. LawMay 23, 2005 Image: A. Siemiginowska, M. Weiss (APOD 031128)
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Quasars: Introduction and Low Redshifts
• Quasars are high-luminosity AGN (MB < -23), LX >1044 erg/s• AGN distinguished by broad SED and strong emission lines.
Permitted lines (Ly,H,C IV, Mg II, etc) 5000 km/s wide, prominent forbidden lines ([O I], [O II], [O III], [S II]). Broad Fe II emission.
• Above a certain luminosity, AGN only hosted by massive elliptical galaxies, disk components occur only at low luminosities
• HST imaging indicates hosts of low redshift quasars almost entirely massive ellipticals with Mv < -24, L > L*
• Host galaxies appear to be similar for both active and quiescent quasars
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We can observe local structure well, consistent with de Vaucoleurs r1/4 profile.
(Dunlop 2004)
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The Black Hole - Spheroid Connection
• Detected systems probably not result of mergers: although disk galaxy mergers can produce r1/4 profiles the light is inconsistent with a nuclear starburst, and Scoville et al. (2003) detected no molecular gas in most luminous quasars (though some gas in disky hosts)
• All spheroids appear to contain black holes with masses given by a fairly precise mass ratio (Magorrian et al. 1998, et al)
• Mass relation in nearby quasars is the same as that in quiescent galaxies: MBH = 0.0012 MSph
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Observed correlation between nuclear luminosityand bulge/total light fraction
(Dunlop 2004)
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(Dunlop 2004)
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Finding Quasars at High Redshift
• In local universe, exploit spectral differences between quasars and stars.
• Search methods: UV excess, slitless spectra, color selection criteria, spectroscopic surveys (e.g. Steidel survey of z = 3 Lyman Break Galaxies).
• Large digital surveys like 2dF and SDSS revolutionized quasar searches, using population synthesis showed weak AGN emission in almost all early and mid-type galaxies.
• Seems that quasars are still too few to account for ionization of IGM at high redshift- more out there?
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Estimating Black Hole Mass
• Standard method involves virial theorem applied to broad line emission regions: Mbh = R v2/G
• Empirical correlation between radius of the broad line region and the 5100 continuum flux.
• In local universe, H the line of choice• Beyond z = 1 this gets shifted into NIR, so use Mg II (2799)
, another low ionization line.• C IV also used, but correlation with H less well understood.• Reverberation mapping. What is this?
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(Dunlop 2004)
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Estimating Host Mass
• Tricky- search methods at high redshift preferentially select bright quasars, which outshine hosts
• Some use of [O III] to trace velocity dispersion in central regions of host galaxy
• Predict most luminous high z quasars require elliptical hosts (or their progenitors) of L ~ 10 L*
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Dunlop (2004) disparages the usefulness of this relation?
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Population Evolution
• Most luminous quasars are emitting at near Eddington limit• Space densities increase dramatically with redshift-
population density increased by more than 2 dex from z=0 to 2. (This is seen in even smallest original samples of 20 objects- Schmidt 1968).
• Either space density increasing by factor of over 100, or characteristic luminosity increasing by factor of 30. These models give very different results for quasar lifetime- 107 vs. 109 years.
• Schmidt- power law and exponential forms could both fit data trend with redshift.
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(Dunlop 2004)
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(Osmer 2004)
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(Dunlop 2004)
Assuming passive evolution, hosts of luminous quasars almost unchanged in mass between redshift z = 2 and today
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Population Evolution: Conclusions
• At least 10% of present massive ellipticals were active quasars at z = 2.
• Almost every spheroidal system had an AGN phase• No evident chemical evolution in quasars from z = 3 to today• Quasars appear to have built up their mass early: over 109
solar masses at redshifts z > 3.6 (Fan et al. 2001)• Therefore low redshift Seyfert galaxies and AGN have
achieved a large fraction of their final black hole growth.• What shuts off the accretion?
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Future Work
• Want to match observations of AGN over a range of redshifts to the local mass function of black holes in galaxies.
• High angular resolution studies are necessary so the morphology of host galaxies can be analyzed
• Adaptive optics with integral field spectroscopy could allow such high resolution studies and suppress the central source flux to probe faint host galaxy.