Introduction Active Galactic Nuclei

41  Download (0)

Full text


Introduction Active Galactic Nuclei

Lecture -10- Quasar Surveys and Evolution


Lecture at 9.00-10.30 on Tue June 17




This Lecture

Give a general overview of quasar surveys and their LF evolution.

Read Chapt.10-11 of Peterson

(somewhat out of date, but read anyway)


Techniques for finding quasars:

Broad-band colors (nowadays)

Slitless spectrosopy

Some major surveys:

e.g. SDSS, 2dF, Combo-17

Introduction to the evolution of quasars (e.g. LFs)



Why do Quasar Surveys?

Uniformly selected quasar samples form the input for studies of:

Cosmology (less important nowadays)

Quasar Structure & Evolution

Co-evolution of AGNs, Galaxies & SMBHs

Measured Function: dN(F,z, {P})/(dF dz dΩ d{P})

(Number of AGN a.f.o. flux, redshift and area + rest)

-> From this a Luminosity Function can be derived a.f.o. Redshift -> Evolution?


Biases in Quasar Surveys

Flux-limit samples can be biased (mostly near the flux-limit) because of noise: Eddington Bias

Because of noise (eg Gaussian), the observed

A(s) = dN(S)/dS is convolved with the noise PDF.

i.e. A'(S) ∝ A(s) ⊗ PDF.

If dN(S)/dS steepens near the flux-limit, MORE faint sources will go into the sample, than brighter sources out of the sample -> #-count is biased!

Am≈ A'  m−m2 2

d2 A' m


In magnitudes (see Peterson for derivation)


Biases in Quasar Surveys

More possible selection issues/biases:


Sources “fluctuate” in/out of the sample. In case of Gaussian distributed variability the effect is like the Eddington Bias.

EW of emissions line vary:

Spectroscopic Surveys can be biased

Absorption Lines:

Absorption by intervening IGM can alter spectra (in particular shortward of Lya)

Internal Absorption:

Dust (e.g. torus) in AGN can obscure them (Type-II quasars?)


The problem: AGNs are rare objects!

How to find them among the much more numerous stars and galaxies on images of the sky?

The solution: Use the observed ways that quasars/AGNs are “not stars.”

-> Make use of SEDs, emission-line spectra, variability, or lack of proper motions to distinguish AGNs from stars & galaxies

Finding Quasars and AGN


Crowed fields make AGN surveys difficult

Compact AGN and stars are

hard to separate using only a single color


Consider the radio, optical/UV, and X-ray emission from AGNs

Normal stars and galaxies emit very weakly in radio and X-ray regions.

Normal stars and galaxies in general have “red”

colors at optical wavelengths

How to use these properties in practice to find quasars/AGNs?

Use Spectral Energy Distribution


Radio observations

– Led to the discovery of the first quasar

– Isolate a very important component of the quasar/AGN population

Requirements for success

– Radio surveys to faint flux limits – Wide sky coverage

– Excellent positions

– Corresponding optical imaging &


Begin with radio surveys

(previous lecture)

But not all AGN (only ~10%) are radio loud -> subsample


The various surveys of all types need follow-up observations to confirm which objects are quasars/AGNs

– Generally, this means optical spectroscopy to establish that candidates are indeed AGNs and to determine their redshifts

Follow-up Observations


Surveys should now be quantifiable in their

selection parameters (statistical completeness)

– selection efficiency as f(brightness, wavelength)

Surveys also need to be effective

– efficient (high succes rate)

– but not overwhelmed with false positives, i.e. objects which meet criteria but are not AGNs (e.g. stars)

This is all now possible with digitally based surveys

(old surveys were based on e.g. photographic plates)

Additional Survey Requirements


Quasars/AGNshave strong, broad emission lines, different from emission regions ionized by stars

– This led Smith (1975) and Osmer & Smith

(1976) to develop the slitless spectrum technique:

– Very effective for finding quasars at z > 2 via direct detection of Lya emission

– Schmidt, Schneider, Gunn 1986; deep digital survey with slitless spectra

Optical Surveys:

Spectroscopic Techniques


Direct image (HST)

Slitless spectra

Example of Slitless Spectroscopy



– Quasars are more bright in the ultraviolet than most stars

– This allowed quasars to be found by their “UV excess”

– Very effective for redshifts z =2.3, but not higher, as Lya moves into the UV passband – At higher redshifts, quasars differ from

stars/galaxies at red and near-IR wavelengths

Optical Surveys:

Multi-color Techniques


Optical Surveys:

Multi-color Techniques

SDSS (Fan et al .1999) Quasar selection

Simulated Quasar color track

Multi-color observation allow AGN and stars to

be separated.


Optical Surveys:

Multi-color Techniques

At very high redshifts, contamination with

L dwarfs (very low-mass stars) might become a problem (Fan et al. 2001)


SDSS Quasar Spectra


SDSS Highest-z Quasar Spectra

Finding high-z quasars means observing at longer wavelenths!

Gunn-Peterson trough


SDSS Quasar Luminosity Function

Strong evolution with time!


2dF, UVX survey: >25,000 quasars,

SDSS, Multicolor survey: 77,429 quasars (2007, DR5; en route to 100,000)

COMBO-17: 300 quasars to R < 24,

Some Current Optical Surveys


Optical Surveys:

Multi-color Techniques

Combo-17 Survey: 12 narrow-band + 4 broad-band filters

Excellent photo-z determinations!


Optical Surveys:

Multi-color Techniques


Narrow-band filters are sensitive to strong

emission lines of AGN

Optical Surveys:

Multi-color Techniques


Optical Surveys:

Multi-color Techniques

COMBO-17: Resulting LF, note the strong evolution with redshift (peaking around z~2)


X-Ray emission is perhaps the most common and distinctive feature of AGNs & quasars

– Historically, the first satellites did not have the sensitivity and spatial resolution needed for

AGN surveys.

– Now, thanks to Chandra and XMM, deep X-Ray surveys yield the highest surface densities of all techniques.

X-ray Surveys

See review by Brandt & Hasinger, 2005, Annual Reviews for a good summary Examples of important surveys: –ROSAT –ASCA –BeppoSAX –Chandra Deep Fields –XMM-Newton


Chandra Deep Field South

X-ray surveys allow detection of distant

X-ray quasars, also those shrouded in

gas and dust (e.g.

type-II quasars).


Chandra Deep Field North

HDF-North CDF-North

Identification in comparison to deep optical images


X-ray variability of AGN

At least 50% of the sources are variable, and that the fraction of variable AGNs may be be as large as 95%


(also some statistical fluctuations)


XMM Lockman Hole/Deep-Field North

Less resolution, but deeper and

X-ray spectra.



Chandra Number Counts

X-ray surveys provide excellent, deep point-source #counts of AGN (which dominate surveys out of the Galactic plane)


A bit more on AGN evolution


Evolution of quasar densities

Exponential decline of quasar density at high redshift,

different from normal galaxies, mostly luminosity dependent

Richards et al. 2005, Fan et al. 2005

SFR of galaxies Density of quasars

Bouwens et al.


Quasar Density at z~6

From SDSS i-dropout survey Density declines by a

factor of ~40 from between z~2.5 and z~6

Cosmological implication MBH~109-10 Msun

Mhalo ~ 1012-13 Msun

rare, 5-6 sigma peaks at z~6 (density of 1 per Gpc3)

Assembly of massive dark matter halo environment?

Assembly of supermassive

BHs? Fan et al. 2004


Evolution of the Shape of Quasar LF

Richards et al. 2005


High-z quasar LF different from low-z

Bright-end slope of QLF is a strong function of redshift

Transition at z~2-3 (where quasar density peaks in the universe)

Different formation mechanism at low and high-z?

Richards, et al.; Fan et al. 2005

Evolution of the Shape of

Quasar LF


Simulating z~6 Quasars

The largest halo in Millennium simulation (500 Mpc cube) at z=6.2

Virial mass 5x1012 M_sun Stellar mass 5x1010 M_sun SFR: 300 M_sun/year

Resembles properties of SDSS quasars

Even the largest N-body simulation not big enough to produce one SDSS z~6 quasar…

Today: 1.5 x 1015 M_sun cluster

Much massive halos existed at z~6, but..

How to assemble such mass BHs and their host galaxies in less than 1 Gyr??

The universe was ~20 tedd old

Initial assembly from seed BH at z>>10

Little or no feedback to stop BH/galaxy growth



Dark matter galaxy

Springel et al. 2005




Ly a

Ly a forest

Rapid chemical enrichment in quasar vicinity

Quasar env has supersolar metallicity -- metal lines, CO, dust etc.

High-z quasars and their environments mature early on

The Lack of Evolution in

Quasar Intrinsic Spectral Properties


Early Growth of Supermassive Black Holes

Vestergaard 2004 Dietrich and Hamann 2004

Billion solar mass BH at z~6 indicates very early growth of BHs in the Universe

Formation timescale (assuming Eddington)

Lack of spectral evolution in high-redshift quasars -> quasar BH estimate valid at high-z BH mass estimate: using emission line width to approximate gravitational velocity,

accurate to a factor of 3 – 5 locally



AGN surveys are now dominated by a few large surveys like SDSS, 2dF (only ~10% of AGN are radio-loud)

X-ray surveys are now an important new way of finding (also “hidden”) AGN

The QSO LF evolves rapidly from z=0, peaks at z~2 and declines at z>2. But QSO properties (e.g. Spectra) and metalicity remain similar from z=0 to z~6.

High-z AGN implies rapid SMBH growth in the early Universe


Reminder: Think about & write essay!

Deadline: Same as exam-day -> June 25





Related subjects :