Quasar Luminosity Functions at High Redshifts

Gordon RichardsDrexel University

With thanks to Michael Strauss, Xiaohui Fan, Don Schneider, and Linhua Jiang

Quasar Luminosity Function

Croom et al. 2004

Space density of quasars as a function of redshift and luminosity

QLF: 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

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.

old model

Lidz et al. 2006

new model

Quasar Luminosity Function

Croom et al. 2004

Space density of quasars as a function of redshift and luminosity

Typically fit by double power-law

Parameterization of the QLF

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.

Cosmic Downsizing

Ueda et al. 2003

Hasinger et al. 2005

X-ray surveys probe much deeper than optical and reveal that the peak depends on the luminosity.

Cosmic Downsizing

Hasinger et al. 2005

X-ray surveys probe muchlarger dynamic range. SDSS+2SLAQ

Croom, Richards et al. 2009 See also Bongiorno et al. 2007 (VVDS)

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.

Luminosity vs. Redshift

PLE vs. Luminosity and vs. Redshift

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

Richards et al. 2006

Bright end slope flattens with redshift at high-z. Similarly in Fan et al. 2001

Fontanot et al. 2007 argue (with 11 objects) that this is a selection effect.

Bolometric QLF

Hopkins, Richards, & Hernquist 2007

Jiang et al. 2009

At z~6, slope is flatter than for z<2.But not as flat at the z~4 SDSS measurement.

Willott et al. 2010

CFHTLS probes faint enough to see evidence for a break at z~6.

Bright-ned slope flatter than high-z.

Photo-ionization Rate

Volume emissivity

Photo-ionization rate (per hydrogen atom)

Siana et al. 2008

Photoionization Rate at z~6

Willot et al. 2010

“… the quasar population … is insufficient to get even close to the required photon emission rate density. … the photon rate density is between20 and 100 times lower than the required rate.”

Conclusions

• We need better measurements of both the bright and faint end slopes of the z>4 QLF

• Current measurements of the QLF allow one to get whatever answer you want (or don’t want) for the number of faint high-z quasars and the resulting re-ionization rate.

QSO QLF != Galaxy QLF

Benson et al. 2003

Clustering’s Luminosity Dependence

• Quasars accreting over a wide range of luminosity are driven by a narrow range of black hole masses

• M- relation means a wide range of quasar luminosities will then occupy a narrow range of MDMH

old model

Lidz et al. 2006

new model

Constraints from Lensing (or Lack Thereof)

At z~5

Richards et al. 2006