Galaxy Clustering and Environment
Results from the DEEP2 Survey
Alison Coil UC-Berkeley
for the DEEP2 Survey Team
August 2004
The DEEP2 Collaboration
U.C. Berkeley: M. Davis (PI), A. Coil, M. Cooper, B.
Gerke, R. Yan, C. Conroy
U.C. Santa Cruz: S. Faber (Co-PI), D. Koo, P.
Guhathakurta, D. Phillips, C. Willmer, B. Weiner, R.
Schiavon, K. Noeske, A. Metevier, L. Lin, N.
Konidaris, G. Graves
Hawaii: N. Kaiser
LBNL: J. Newman
U. Pitt.: A. Connolly
JPL: P. Eisenhardt
Princeton: D. Finkbeiner
A Redshift Survey at z=1:
The DEEP2 Galaxy Redshift Survey, which uses the DEIMOS
spectrograph on the Keck II telescope, will study both galaxy
properties and the clustering of galaxies at z=1.
Comparison with local samples:
• 3.5 sq. degrees
1.00E+06
Number of Galaxies
SDSS
2dF
1.00E+05
LCRS
DEEP2
1.00E+04
CFA+
SSRS
1.00E+03
1.00E+05
z~0
z~1
PSCZ
1.00E+06
1.00E+07
Volume (h -3 Mpc3)
1.00E+08
• 4 fields (0.5o x 2o)
• primary z~0.7-1.4
• ~50,000 redshifts
• ~6·106 h-3 Mpc3
• 90 Keck nights
• One-hour exposures
• RAB=24.1
Our color cuts are highly successful!
By applying a relatively
simple BRI color cut, we
have a sample that is 13%
z<0.75, vs. >60% with no
cut.
Only 3% of objects that we
reject are at z>0.75.
In the Extended Groth Strip,
we apply no color cut,
enhancing multiwavelength
studies and also making this
test possible.
DEEP2 vs. previous surveys of distant galaxies
Galaxies found in
large numbers well
beyond z=1
note the z=0.7 color cut
Obs. R-I
Rest U-B
Color
bimodality
Slitmask spectroscopy
Using custom-milled slitmasks with DEIMOS we are obtaining
spectra of ~120 targets at a time. A total of 480 slitmasks will
be required for the survey; we can tilt slits up to 30 degrees to
obtain rotation curves.
A fully automated reduction pipeline
SDSS spectral pipeline code by
Schlegel et al. allowed us to rapidly
develop a full 2d and 1d spectral
reduction pipeline that is completely
automated. Check z’s by eye.
A few percent of one DEEP2 mask, rectified, flat-fielded, CR cleaned,
wavelength-rectified, and sky subtracted. Note the resolved [OII] doublets.
Shown is a small group of galaxies with velocity dispersion   250 km/s at z1.
Note the clean residuals of sky lines!
Status of the DEEP2 Survey
• DEIMOS commissioning began June 2002
under clear skies and was extremely successful.
• DEEP2 observing campaign began in July
2002. At the end of 3 semesters of the 6
planned, we had completed 48% of the survey
slitmasks! We are on schedule!
• Observations complete mid-2005.
Currently ~60% done!
Clustering in DEEP2: First Redshift Maps
Projected maps of two DEEP2 pointings (of 13 total). Red = early-type (from PCA).
2-point correlation function: x(r)
x(r) follows a power-law prescription locally:
x(r) = (r0/r)g with r0~5 Mpc/h and g~1.8.
r0 = scale where the probability of finding a
galaxy pair is 2x random
From the projected function wp(rp) we
can recover the real-space correlation
function x(r)= (r0/r)g
z=0.7-0.9: r0=3.53 +/-0.81
z=0.9-1.35: r0=3.12 +/-0.72
both have slope g= 1.66 +/-0.12
Errors are estimated using mock catalogs - dominated by cosmic variance
Galaxy bias: galaxy/dark matter clustering
Bias evolves with redshift:
z=3: b~4 z=0: b~1
DEEP2 sample as a whole:
b=0.96 +/-0.13 for 8=1 today
b=1.19 +/-0.16 for 8=0.8 today
could be the result of our R-band
target selection – we’re undersampling older, red stellar
populations
Coil et al. 2004 astro-ph/0305586
Galaxy formation simulation by Kauffmann et al. grey=dark matter particles colors=galaxies
Clustering as a function of Color and
Spectral Type
Red galaxies:
dashed lines
Blue galaxies:
solid lines
Redder galaxies have a larger correlation length and
larger velocity dispersion, as do absorption-line galaxies:
reside in more clustered / dense environments.
Galaxy Clustering: color, type, luminosity
Color
B-R>0.7: r0= 4.32 (0.73) g=1.84 (0.07)
B-R<0.7: r0= 2.81 (0.48) g=1.52 (0.06)
Spectral Type
Absorption: r0= 6.61 (1.12) g=1.48 (0.06)
Emission: r0= 3.17 (0.54) g=1.68 (0.07)
Luminosity
Brighter MB<-19.75: r0= 3.70 (0.65) g=1.60 (0.06)
Fainter MB>-19.75: r0= 2.80 (0.48) g=1.54 (0.06)
Redder, passively-evolving and/or more luminous
galaxies cluster more strongly than bluer, starforming, less luminous galaxies - similar as z~0 results
Projected Angular 2-pt corr. fnct: w(q)
Have photometry for many more galaxies than spectra:
~350,000 galaxies over 5 deg2, incl. z<0.7
Projected angular 2-point correlation function: w(q)
Constrain the 3d galaxy clustering x(r) * if the z dist. of the sources is known
slope = -0.8:
7”
3’
Smooth decrease in clustering with magnitude. Errors are variance across 15 pointings.
Evolution of Galaxy Clustering
Angular correlation function is an integral of the
3-d clustering along the line of sight.
Use DEEP2 spectroscopic sample to measure
the redshift distribution of sources in various
magnitude ranges. Data is from the Groth Strip
where we have no photo-z cut – 3320 galaxies.
dn/dz=A z2 e(-z/z0)/z03
x(r,z)= x(r,z=0) (1+z) –(3+e-g)
e=-1.2 fixed in comoving coords.
e=0 fixed in proper coords.
e>0 clustering grows in proper coords.
We find no single value of e fits our data – e must evolve with z.
We see significant growth in the clustering amplitude from z>1 to 0.
Angular Clustering as fnct. of R-I color
blue
red
In addition to a trend of redder
galaxies being more clustered, the
bluest galaxies (R-I <0.2) are also
highly clustered – unexpected!
blue
Redshift distribution of color samples:
Reddest galaxies are at z~0.85, narrow
Bluer samples are at lower z, wider
Bluest samples have significant
components at z<0.5 and z~1.7
red
Clustering as a function of R-I color
Corrected for
z>1.4 galaxies
Bluest galaxies are a mix of brightest
objects at z>1.4, local faint blue dwarfs,
and AGN between z~0-2
Reddest galaxies are likely z>0.5
progenitors of local ellipticals
Coil et al. 2004 astro-ph/0403423
Galaxy properties and environment
environment
Measure galaxy environment using projected Nth-nearest neighbor
distance. See strong trends of restframe color and OII equivalent
width with environment. No residual trend in OII EW once the
correlation of environment with color is removed.
blue
color
red
OII equivalent width
Cooper et al. in prep
Color-magnitude vs environment
Color vs mag. w/ density contours
darker regions = more dense
Redder galaxies reside in
denser environments, with
the brightest red galaxies
in the most dense
environments. Within the
blue galaxy population,
the brightest also lie in the
most dense environments
- progenitors of central
cluster galaxies at z~0?
Bright blue galaxies in densest environments
Galaxy Groups and Clusters in DEEP2
Voronoi-based methods
can also be used to
identify clusters and
groups of galaxies
(Marinoni et al. 2002).
red=absorption-dominated
DEEP2 group catalogs
in two of our pointings
will be published
shortly.
This will allow both the
study of group property
distributions and of
group vs. field galaxies.
red=pairs; blue=N>2; sizelog ()  log (halo mass)
Gerke et al. in prep
Now looking at group correlations and
void statistics…
Group-group
correlation function is
larger than the galaxygalaxy correlation
function. Field
galaxies are less
clustered than the full
galaxy sample, which
is less clustered than
galaxies in groups.
Coil et al. in prep
Conclusions
Galaxy Clustering:
1. r0~3.5 Mpc/h at z~1, b~1.0-1.2 for DEEP2 galaxies.
2. Red, passively-evolving galaxies have larger fingers of God and r0
than blue, star-forming galaxies at z~1.
3. See some luminosity-dependence in the clustering strength.
4. Strong dependence of angular clustering on observed color:
red galaxies at z~0.8 have r0~6.5 Mpc/h
blue galaxies at z>1.4 have r0>~5 Mpc/h
Environment:
1. Color and OII EW correlate strongly with local environment at z~1.
2. Find a population of bright blue galaxies in the densest environments
at z~1 which do not exist at z~0 - central cluster galaxies?