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
1. Different QSO/AGN formation theories predict different
clustering properties - amplitude as f(z,L). Clustering
constrains models (even w/ current error bars).
2. Clustering constrains the mass of the host DM halo and host
galaxy type and timescale between evolutionary stages.
3. 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.
4. Results: QSOs cluster like blue/star-forming galaxies, not red
galaxies, at z~1.
5. Results: X ray-selected AGN cluster like red galaxies at z~1
(more clustered than QSOs).
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
clustering amplitude
Brighter r0 > Fainter r0 + Redder r0 > Bluer r0
At z=0 and z=1
luminosity
separation - rp
color
Coil et al 2006
Coil et al 2008
(for ~L* galaxies:)
z=0.9: 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
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
36 SDSS + 16 DEEP2 spectroscopic broad-line
QSOs in the DEEP2 fields between z=0.7-1.4:
DEEP2
MB
SDSS
(near M*)
redshift
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.
0.1 Mpc/h
separation
10 Mpc/h
Divide by the clustering of
DEEP2 galaxies around DEEP2
galaxies to get the relative bias
of QSO hosts…
Coil et al. 2007 ApJ
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
group clustering: Mmin= 6 x 1012 M0/h
r0=6.2 (0.4) Mpc/h g=1.5 (0.2)
No dependence is seen on
magnitude (MB=-22.8 vs -24.1)
or redshift (z=0.7-1.0 vs 1.0-1.4)
Coil et al. 2007 ApJ
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
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)
red
color
blue
quasars
z=0.7-1.4
-16
MB
Nandra et al. 2006 ApJL
-24
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
ACS Imaging of X-ray AGN
~60% of hosts have earlytype 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 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)
Jacknife errors for now - will update w/ simulations
Coil et al. in prep
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 AGN
Results:
- 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
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 luminositydependence 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 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 crosscorrelation of QSO and matches our z=1 cross-correlation very well!
Have their QSOs in the right halos and galaxies at z=1.
Conclusions
1. QSOs are less clustered than X-ray selected AGN at z~1.
2. QSOs cluster like blue star-forming galaxies while AGN cluster like
red quiescent galaxies (overdense regions).
3. Red galaxies w/ AGN are more clustered than blue galaxies w/ AGN.
4. These results favor galaxies undergoing a bright QSO phase before
settling on the red sequence with a lower luminosity AGN.
5. 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).