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.