Demographics of SDSS Quasars in Two-Dimension

Yue ShenCarnegie ObservatoriesIn collaboration with Brandon Kelly (UCSB)

Motivation

The abundance (and clustering) of quasars are key to understand the evolution of quasars/SMBHs in the hierarchical structure formation paradigm

Abundance measurements form a basis for any cosmological quasar models

Likely tied to formation and evolution of galaxies Key science goal in many current and upcoming

extragalactic survey programs

Quasar Luminosity Function Evolves strongly with redshift

Richards et al. (2006, SDSS DR3)

The space density of bright quasars peaks around z~2-3

For bright quasars, the abundance follows a “pure luminosity evolution” (PLE) a fading, long-lived quasar population?

Such a simple picture doesn’t fit other observations.

PLE

SDSS DR7 LF (Shen & Kelly 2012)

Z=

Quasar clustering measurements suggest low-z quasars are not simply the descendants of high-z quasars

Dashed lines: predicted evolution of the linear bias for a passive population

z~2 quasars should end up in cluster-sized environment at z~0.5

YS, McBride, White, Zheng, et al. (2013)

Quasars evolve in the mass-luminosity plane

Virial masses

A better representation of the evolution of the quasar population

Contains richer information about the growth of SMBHs

Estimating quasar BH masses

The broad-line region (BLR) is assumed to be virialized

BLR size (reverberation

mapping)Virial

velocity

Reverberation mapping is time consuming, and we only have BLR size measured this way for ~40 AGNs

Bentz et al. (2009)

b~0.5, Consistent with naive predictions of photoionization models

Reverberation mapping R~Lb (Kaspi et al. 2000)

Single-epoch virial BH mass estimators

Vestergaard & Peterson(2006), McLure & Dunlop (2004), Greene et al. (2005), and many more …

Currently the only practical method to estimate BH mass for large spectroscopic quasar samples

Many physical and practical concerns that need to be addressed (Shen 2013): we are extrapolating from ~40 local AGNs with reverberation mapping data to high-z, high-luminosity quasars

Large uncertainties for individual BH mass estimates: ~ a factor of 3 (~0.5 dex)

Continuum luminosity

Broad line width

Quasar abundance in the mass-luminosity plane

Two major problems: 1. Flux limit of the

sample2. Scatter in BH mass

due to errors in mass estimates

Virial masses

The sample used: ~58,000 uniformly selected DR7 SDSS quasars, with good spectra to estimate BH mass

Forward modeling in the mass-luminosity plane (Shen & Kelly 2012, Kelly & Shen 2013)

True BH masses

Flux limit and mass errors are more easily accounted for

More information is preserved

Caveats: SDSS only probes the tip of the quasar population, poorly constraining the faint-

end of the LF and the low-mass end of the BHMF – need deeper data Systematic uncertainties of quasar BH mass estimates have dramatic effects on

the BHMF estimates – need to improve the BH weighing method

LF BHMF

Downsizing in terms of quasar luminosity and BH mass

Properly accounting for the selection effect of flux-limited samples and errors in the virial BH mass estimates

Flux limit

Red: true massBlack: virial mass estimates

Eddington limit Interpret the

“observed distributions” with caution

Summary Instead of measuring LF and BHMF separately,

measure 2D density in the M-L plane (if you have a spectroscopic sample); this gives you more information on the evolution of the quasar population

Forward modeling in the M-L plane makes it easier to account for the sample flux-limit and errors in mass estimates

The “observed” distribution in the M-L plane is biased; don’t interpret it directly!

There is urgent need to improve quasar BH mass weighing methods (~0.5 dex error is inconvenient)