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The Build-up of Quasars. Gordon Richards Drexel University. With thanks to Michael Strauss, Yue Shen (Princeton), Don Schneider, Nic Ross (Penn State), Adam Myers (Illinois ), Phil Hopkins ( Berkeley ), and a host of other people from the SDSS Collaboration. Caveats. - PowerPoint PPT Presentation
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The Build-up of Quasars
Gordon RichardsDrexel University
With thanks to Michael Strauss, Yue Shen (Princeton), Don Schneider, Nic Ross (Penn State), Adam Myers (Illinois), Phil Hopkins (Berkeley), and a host of other people from the SDSS Collaboration
CaveatsI tend to be biased towards:
• High redshift (z>1)• High luminosity• Optical Selection• “Quasar” mode accretion• Unobscured (i.e., type 1)• Quasar=QSO=AGN=any
actively accreting supermassive black hole
The Redshift Desert• Redshift desert for galaxies due to lack of spectral
features in the optical octave at z~2.• No redshift desert for quasars (in the galaxy sense),
but there is in reality. And just as frustrating.
Galaxy
Quasar
Franx 2003
The z~2.7 Quasar Desert
Schneider et al. 2007
Observed
Corrected
Z~2.7 Quasar ColorsAt 2.5<z<3.0 quasars cross through the locus of stars, making those quasars harder to identify (efficiently).
X-ray and IR Selection• X-ray and IR selection don’t suffer from the same problem (and they allow selection of obscured quasars).• But they do have their own problems.• Area surveyed by X-ray is tiny.• Mid-IR has its own 3.5<z<5 desert.• Not clear that optical/radio/MIR/X-ray selecting same objects (at least at lower luminosity), see Hickox et al. 2008.
Quasar Luminosity FunctionAs with star formation rate, quasars peaked at redshift 2-3.
Richards et al. 2006
The rise and fall is even more dramatic in time than redshift.
The Rise of Quasars at z~6Mere existence z~6 quasars constrains formation models
Eddington argument: If the luminosity of a quasar is high enough, then radiation pressure from electron scattering will prevent further gravitational infall.
LE = 1.38x1038 M/Msun erg/sME = 8x107L46Msun
Sets an upper limit to the luminosity for a given mass, or equivalently a minimum mass for a given luminosity.
Making SMBHs at z~6The luminosities of the z~6 quasars imply BH masses in excess of 109 MSun.
But z~6 is <1Gyr after the Big Bang.
Assembling that much mass in so little time is difficult (but not impossible).
Tanaka & Haiman 2009
Quasar Luminosity FunctionSDSS is relatively shallow.
It probes only the tip of the iceberg.
Need fainter surveys to get full picture.
e.g., Richards et al. 2006
Cosmic Downsizing
Ueda et al. 2003
Hasinger et al. 2005
X-ray surveys probe much deeper. Here we see that peak depends on the luminosity.
Cosmic Downsizing
Hasinger et al. 2005
X-ray surveys probe muchlarger dynamic range. SDSS+2SLAQ
Croom et al. 2009
How does the quasar luminosity function relate to the physics of BH accretion and galaxy evolution?
Quasar Luminosity Function
Croom et al. 2004
Space density of quasars as a function of redshift and luminosity
Typically fit by double power-law
Density Evolution
Number of quasars is changing as a function of time.
Luminosity EvolutionSpace density of quasars is constant.
Brightness of individual (long-lived) quasars is changing.
Luminosity vs. Redshift
Usually we split into L or z instead of making a 3-D plot, but the information is the same.
0.5
1.5
2.5
3.54.5
Luminosity Evolution
• Pure density or pure luminosity evolution don’t lead to cosmic downsizing.• The slopes must evolve with redshift.
Cosmic Downsizing
Luminosity Dependent Density Evolution
To get cosmic downsizing, the number of quasar must change as a function of time, as a function of luminosity. i.e., the slopes must evolve.
Bolometric QLF
Hopkins, Richards, & Hernquist 2007
Hopkins et al. 2005
Hopkins et al. 2006
Most QLF models assume they are either “on” or “off” and that there is a mass/luminosity hierarchy.Hopkins et al.: quasar phase is episodic with a much smaller range of mass than previously thought.QLF is the convolution of the formation rate and the lifetime.
QSO QLF != Galaxy QLF
Benson et al. 2003
Hopkins et al. 2005
Hopkins et al. 2006
Most QLF models assume they are either “on” or “off” and that there is a mass/luminosity hierarchy.Hopkins et al.: quasar phase is episodic and “all quasars are created equal” (with regard to mass/peak luminosity).QLF is the convolution of the formation rate and the lifetime.
Merger Scenario w/ Feedback
• merge gas-rich galaxies
• form buried quasars
• feedback expels the gas
• revealing the quasar
• shutting down accretion and star formation
Granato et al. 2004, DiMatteo et al. 2005, Springel et al. 2005, Hopkins et al. 2005/6a-ze.g., Kauffmann & Haehnelt 2000
How Can We Test This?
• The Quasar Luminosity Function• active lifetime (e.g., Martini 2004)• accretion rate (e.g., Kollmeier et al. 2006)• MBH distribution (e.g., Vestergaard & Osmer 2009)• Quasar Clustering • L, z dependence (e.g., Lidz et al. 2006 ; Shen et al. 2009)• small scales (e.g., Hennawi et al. 2006; Myers et al. 2008)
In addition to the evolution of the QLF slopes, we can probe:
Clustering• Red Points are, on
average, randomly distributed, black points are clustered
• Red points: ()=0• Black points: ()>0• Can vary as a function of,
e.g., angular distance, (blue circles)
• Red: ()=0 on all scales• Black: () is larger on
smaller scalesA. Myers
Quasar Clustering• Quasars are more
clustered on small scales than large scales.
• Comparing with models of dark matter clustering gives the “bias” (overdensity of galaxies to DM)
• Linear bias (bQ=1) ruled out at high significance.
Myers et al. 2007
Galaxy ClusteringThe comoving clustering
length of luminous galaxies is roughly independent of z at least to z ~ 5.
Therefore, the distribution of galaxies must be increasingly biased relative to the dark matter at high redshift, galaxies=b dark
matterOuchi et al. 2004
How about quasars? Quasars are powered by the ubiquitous
super-massive black holes in the cores of ordinary massive galaxies
Therefore, we’d expect that the clustering of quasars should be similar to that of luminous galaxies, at the same redshift.
Bahcall, Kirhakos et al.
Comoving Correlation Length
Ross et al. 2009
SDSS Quasars
Quasar Bias Evolution
Ross et al. 2009
As with galaxies, constant clustering length means strongly evolving bias.
What happens at higher redshift?• If very massive BHs are associated with very
massive DM halos, then high-redshift quasars should sit in very rare, many peaks in the density field.
• So we expect high-redshift quasars to be more strongly clustered.
Shen et al. 2007
For 2.9 < z < 3.5: r0=16.9±1.7 Mpc/h; b~10For z > 3.5:r0=24.3±2.4b~15
• Use ellipsoidal collapse model (Sheth, Mo & Tormen, 2001) to
turn estimates of bQ into mass of halos
hosting UVX quasars.• Find very little
evolution in halo mass with redshift.
• Our mean halo mass of ~5x1012h-1MSolar is
halfway between characteristic masses
from Croom et al. (2005) and Porciani et
al. (2004).• This is comparable to the
mass of galaxy groups, supporting the idea that quasars are triggered by
mergers.
Hierarchical Halo Merging
• Lacey & Cole (1993)• Typical quasar hosts
double in mass every Gyr or so
• Constancy of quasar host halo mass thus limits quasar lifetime to around 106.5 to 107.5 yrsTime Mass
Time for 2x Mass
CDM theory tells us the expected space density of halos. Comparing with the observed quasar density allows us to determine the fraction of time a quasar is shining.
Clustering’s Luminosity Dependence
• Quasars accreting over a wide range of luminosity are driven by a narrow range of black hole masses
• M- relation mean a wide range of quasar luminosities will then occupy a narrow range of MDMH
Lidz et al. 2006
old model
new model
No L Dependence for Quasars
Zehavi et al. 2005
galaxies
Shen et al. 2007
quasars
What Next?
Hopkins et al. 2007
Measuring bias of faint high-z quasars will break degeneracies between feedback models.
bright quasars (e.g., SDSS) faint quasars (e.g., LSST)
Richards et al. 2006
What We (Used To) Expect
1. Galaxies (and their DM halos) grow through hierarchical mergers
2. Quasars inhabit rarer high-density peaks3. If quasars long lived, their BHs grow with cosmic time4. MBH-σ relation implies that the most luminous quasars are
in the most massive halos.5. More luminous quasars should be more strongly clustered
(b/c sample higher mass peaks).6. QLF from wide range of BH masses (DMH masses) and
narrow range of accretion rates.
What We Get1. Galaxies (and their DM halos) grow through hierarchical
mergers2. Something causes the growth of galaxies and their BHs to
terminate even as DM halos continue to grow3. Quasars always turn on in potential wells of a certain size (at
earlier times these correspond to relatively higher density peaks).4. Quasars turn off on timescales shorter than hierarchical merger
times, are always seen in similar mass halos (on average).5. MBH-σ relation then implies that quasars trace similar mass black
holes (on average)6. Thus little luminosity dependence to quasar clustering (L
depends on accretion rate more than mass). 7. Need a wide range of accretion rates for a narrow range of MBH
to be consistent with QLF.
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