Clustering of QSOs and X-ray AGN at z=1
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Clustering of QSOs and X-ray AGN at z=1. Alison Coil Hubble Fellow University of Arizona. Collaborators: Jeff Newman, Joe Hennawi, Marc Davis, Paul Nandra, Antonis Georgakakis. October 2007. Main Points.

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Clustering of QSOs and X-ray AGN at z=1

Alison Coil

Hubble Fellow

University of Arizona

Collaborators: Jeff Newman, Joe Hennawi, Marc Davis, Paul Nandra, Antonis Georgakakis

October 2007

Main points
Main Points

Different QSO/AGN formation theories predict different clustering properties - amplitude as f(z,L). Clustering constrains models (even w/ current error bars).

Clustering constrains the mass of the host DM halo and host galaxy type and timescale between evolutionary stages.

Cross-correlating with large samples of galaxies rather than using QSO/AGN samples alone gives small errors - both Poisson and cosmic variance - and allows measures on small scales. Only need selection function of galaxies, not AGN.

Results: QSOs cluster like blue/star-forming galaxies, not red galaxies, at z~1.

Results: X ray-selected AGN cluster like red galaxies at z~1 (more clustered than QSOs).

Clustering information
Clustering Information

What you can learn from clustering:

close pairs / interactions / mergers

radial profiles of galaxies/AGN w/in halos

host dark matter halo mass

halo occupation distribution function

what kinds of galaxies host AGN

trace evolving populations through different z’s

connect different samples and z’s

constrain timescales between different evolutionary stages

compare with AGN formation and evolution models

Very helpful to know clustering of galaxies at the same redshift, in the same volume, as a function of color/luminosity.

Galaxy clustering luminosity color
Galaxy Clustering: Luminosity, Color

Brighter r0 > Fainter r0 + Redder r0 > Bluer r0

At z=0 and z=1

clustering amplitude



separation - rp

Coil et al 2006

Coil et al 2008

(for ~L* galaxies:)

red:r0=5.2 Mpc/h g=2.0 b=1.6 Mmin=2 1012 Mo/h

blue:r0=3.8 Mpc/h g=1.7 b=1.3 Mmin=5 1011 Mo/h


Qso agn clustering z 1
QSO+AGN clustering - z~1

DEEP2 is a completed redshift survey using the Keck II telescope, covering multiple fields on the sky (for cosmic variance) to study galaxy evolution and LSS at z=0.7-1.5.

Sample is selected to R=24.1.

DEEP2 has a high sampling rate (60%) and precise redshifts (<30 km/s) - good for clustering and environments. Full sample has >40,000 galaxies in V~3x106 Mpc3/h3.

One field is the Extended Groth Strip (EGS), which has 8 Chandra pointings of 200 ks each over a 2 x 0.25 degree field.

(Chandra team: K. Nandra, A. Georgakakis, E. Laird)

Sdss qsos in deep2 fields
SDSS QSOs in DEEP2 fields

36 SDSS + 16 DEEP2 spectroscopic broad-line QSOs in the DEEP2 fields between z=0.7-1.4:




(near M*)


Clustering of galaxies around qsos
Clustering of Galaxies around QSOs

Clustering of DEEP2 galaxies around SDSS QSOs at z=0.7-1.4.

Errors include Poisson errors + cosmic variance (from mocks).

Similar errors as surveys with 1000s of QSOs (eg. 2dF) through use of cross-correlation with 10,000s of galaxies.

Divide by the clustering of DEEP2 galaxies around DEEP2 galaxies to get the relative bias of QSO hosts…

0.1 Mpc/h

10 Mpc/h


Coil et al. 2007 ApJ

Relative bias of qsos to deep2 galaxies
Relative bias of QSOs to DEEP2 galaxies

QSO relative bias = 0.9 (0.2)

QSO absolute bias = 1.2 (0.3)

Cluster like blue galaxies not red!

Constrains host type for QSOs (blue) and QSO host halo masses:

Min. halo mass = 5x1011 M0/h

Mean halo mass = 3x1012 M0/h

No dependence is seen on magnitude (MB=-22.8 vs -24.1) or redshift (z=0.7-1.0 vs 1.0-1.4)

group clustering: Mmin= 6 x 1012 M0/h

r0=6.2 (0.4) Mpc/h g=1.5 (0.2)

Coil et al. 2007 ApJ

Comparison w 2df sdss
Comparison w/ 2dF+SDSS:

Our QSO clustering amplitude is ~1s lower than other results (with 20% errors). Done for free by having galaxy redshifts in SDSS QSO fields.

Clustering of x ray agn
Clustering of X-ray AGN

  • Chandra survey in the EGS: 200 ks depth

  • Have ~10,000 galaxies and ~250 (so far) X-ray sources w/ z=0.2-1.4

  • 1/2 from DEEP2 (R=24.1)

  • 1/2 from MMT (R=23)

  • X-ray AGN hosts are bright and on red sequence or massive end of blue cloud.

  • (even the faint ones: Lx~1042-44 erg/s)

  • Log Eddington ratio=

  • -2.5 to -4.0









Nandra et al. 2006 ApJL

Acs imaging of x ray agn
ACS Imaging of X-ray AGN

~60% of hosts have early-type morphologies

(Pierce et al. 2007)

Not dominated by major mergers but early-types do show more minor merger asymmetry than normal ellipticals

(Georgakakis et al. 2007, submitted)

(to my untrained eye, ~50% spheroids

~33% spirals

~10% mergers/Irr

9% blue point sources)

Clustering of x ray agn1
Clustering of X-ray AGN

Split into 2 redshift bins: z=0.2-0.7 and z=0.7-1.4

X-ray AGN cluster like red galaxies, not blue galaxies, at both z~0.5 and z~0.9

Relative bias of X-ray AGN to galaxies:

z=0.7-1.4 red gals: 1.1 (0.1) blue gals: 1.7 (0.1)

z=0.2-0.7 red gals: 1.1 (0.1) blue gals: 1.4 (0.1)

Coil et al. in prep

Jacknife errors for now - will update w/ simulations

Clustering of x ray agn2
Clustering of X-ray AGN

AGN on red sequence are more clustered than in blue cloud

Relative bias of red to blue AGN:

z=0.7-1.4 red/blue AGN=1.5

z=0.2-0.7 red/blue AGN=1.3

Similar to galaxy red/blue relative bias=1.4

Relative bias of brighter to fainter AGN:

z=0.7-1.4 MB= -21.3/-20.5 bias=1.2

z=0.2-0.7 MB= -20.7/-19.8 bias=1.2

Similar to galaxy bright/faint bias for same magnitude range - 1.2

(errors may be too large to make this statistically significant)

Coil et al. in prep

Clustering of x ray agn3
Clustering of X-ray AGN


  • split into 2 z bins - same results at z~0.5 and z~0.9

  • X-ray AGN cluster like red galaxies, not blue galaxies

  • X-ray AGN are more clustered than QSOs!

  • red AGN are more clustered than blue AGN

    Consistent w/ galaxies undergoing a QSO phase before settling on the red sequence w/ an X-ray AGN

Coil et al. in prep

Qso agn evolution models
QSO/AGN Evolution Models

  • Competing QSO/AGN formation and evolution models predict different clustering properties.

  • All assume major mergers trigger QSO formation.

  • Kauffmann and Haenelt 2001 predict a strong luminosity-dependence to QSO clustering:

    • assume an exponentially declining light curve, time=t

    • M_B ~ gas mass accreted / t

    • gas mass accreted ~ host halo mass

    • luminosity~halo mass

brighter QSOs cluster much more

ruled out by clustering observations

Qso agn evolution models1
QSO/AGN Evolution Models

Lidz, Hopkins predict less luminosity-dependence (still some)

- bright and faint QSOs have similar clustering

- bias predicted at z~1 is a bit too high (b~2)

- would predict X-ray AGN cluster like quasars (?)

Croton et al. 2006 include a ‘radio’ mode for low-L AGN

- if halo M > Mthreshold then no cold gas accretion, shuts off SF - no

QSO - but there is a low-L AGN fed by hot gas

- predict blue galaxies have QSOs and red galaxies have low-L

AGN - good qualitative agreement with our results

Scannapieco + Thacker 2006 model predicts both auto- and cross-correlation of QSO and matches our z=1 cross-correlation very well! Have their QSOs in the right halos and galaxies at z=1.


QSOs are less clustered than X-ray selected AGN at z~1.

QSOs cluster like blue star-forming galaxies while AGN cluster like red quiescent galaxies (overdense regions).

Red galaxies w/ AGN are more clustered than blue galaxies w/ AGN.

These results favor galaxies undergoing a bright QSO phase before settling on the red sequence with a lower luminosity AGN.

Possibly QSO=early merger phase, X-ray AGN=later merger phase.

Measuring QSO/AGN clustering in fields with galaxy redshifts

allows cross-correlation (small scales with small errors) and

environment measures. Compare with red and blue galaxies at the

same z and in the same volume (cosmic variance ~cancels).