Discovery of Galaxy
Clusters Around Redshift 1
Deborah Haarsma, Calvin College
@ GLCW, June 1, 2007
Collaborating with
Calvin College: students Andrew Butler, Katie Shomsky
Michigan State University: Megan Donahue, Mark Voit, Ming Sun,
student Seth Bruch
National Optical Astronomy Observatory: Mark Dickinson
Space Telescope Science Institute: Marc Postman
Lawrence Berkeley National Laboratory: Greg Aldering
European Southern Observatory: Piero Rosati
University of Colorado: John Stocke
Insights into galaxy evolution from clusters
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When did stars form? Single burst or
long timescale? How did star formation
stop? (gas stripping? winds? mergers?
AGN feedback?)
How and when did galaxies assemble
into clusters? Reconciling “downsizing” with hierarchical structure
formation.
How are galaxy populations in clusters
still changing? Dry mergers, evolution
of luminosity function.
How do the answers to these questions
depend on the mass of the cluster? The
density of the intracluster medium?
Field vs. cluster populations?
Need more z>0.8 clusters
Need more x-ray luminous clusters
RDCS 1252.9-2927 at z=1.24
Rosati et al 2004 AJ 127, 230
Contours = x-ray (Chandra+ROSAT)
Image = B R K filters
ROSAT Optical X-ray Survey (ROXS)
Goal is not just finding clusters, but
understanding cluster detection
methods and selection functions
 4.8 sq deg
 ROSAT X-ray data and Kitt Peak Iband data of whole field
 “wavelet” method (Rosati) to detect
extended X-ray sources
 “matched filter” method (Postman) to
detect concentrations of optical
galaxies of appropriate brightness
ROSAT Optical X-ray Survey (ROXS)
142 clusters
detected in
optical
43 clusters
detected in
X-ray
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112 detected
in optical but
not in X-ray
A bimodal
population?
More likely these are
lower mass clusters,
with x-ray emission
below detection limit
(Donahue et al
2001 ApJ 552, L93)
ROSAT Optical X-ray Survey (ROXS)
142 clusters
detected in
optical
43 clusters
detected in
X-ray
Three options:
 A real cluster at z>1,
failed to detect in
optical due to I-band
drop outs
 A real cluster at
moderate z, failed to
detect in optical due to
poorness or extent
(predicted for none
with z<0.4)
 A constellation of X-ray
point sources
13 detected in (predicted for 10% of
X-ray but not the 43 X-ray sources)
in optical
Bimodality of color-magnitude relation
Color
~75%
ellipticals
and SOs
Red
sequence
Blue Cloud
~75%
spirals
Magnitude
Coma Cluster, z=0.023
DeLucia et al, astro-ph/0610373
I-band magnitude
DeLucia et al, astro-ph/0610373
Observations
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April 2005, KPNO 4-meter
FLAMINGOS camera, 10' field of view
10 fields observed, containing 11 x-ray
sources
2.5 hour/field in J filter (1.2 μm),
1 hour/field in Ks filter (2.2μm)
Observations by Megan Donahue,
Mark Dickinson, and Greg Aldering
Data reduction by Andrew Butler
(Calvin undergrad) and Seth Bruch
(then a UWisc undergrad)
Red sequence detections
z~1.4
z~1.0
z=0.338, no I-band data
z~1.0
z~1.0
z~0.9
z~0.9, no I-band data
z~0.8
Evidence for redshifts around 1
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I-band dropout
(redshifted enough to
be faint in I-band)
J-K color consistent with
models
Evidence for redshifts around 1
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I-band dropout
(redshifted enough to
be faint in I-band)
J-K color consistent with
models
J-K color consistent with
other clusters 0.8<z<1.2
Brightness of BCG
consistent with other
clusters 0.8<z<1.2
Colors redder than field
galaxy population
+ Our clusters
o Clusters in literature with
spectroscopic redshifts 0.8<z<1.2
RXJ 1606.1+2558
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z~0.8
Color image: IJK filters
Small circles: red sequence members
Contours: ROSAT x-ray
RXJ 1605.6+2548
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z~0.9
Similar color to
field galaxies
Color image: IJK filters
Small circles: red sequence members
Contours: ROSAT x-ray
RXJ 1205.2+2752
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z~1.0
Two subclusters
merging?
Color image: IJK filters
Small circles: red sequence members
Contours: ROSAT x-ray
RXJ 1603.6+4316
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z~1.0
Supercluster in
field at z=0.9
Color image: IJK filters
Small circles: red sequence members
Contours: ROSAT x-ray
RXJ1416.3
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z~1
Color image: IJK filters
Small circles: red sequence members
Contours: ROSAT x-ray
RXJ 1118.9+2117
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z~1.3
High-z interaction? Color image: IJK filters
separation ~20kpc Small circles: red sequence members
Contours: ROSAT x-ray
ROSAT Optical X-ray Survey (ROXS)
142 clusters
detected in
optical
43 clusters
detected in
X-ray
Three options:
 A real cluster at z>1,
failed to detect in
optical due to I-band
drop outs
 A real cluster at
moderate z, failed to
detect in optical due to
poorness or extent
(predicted for none
with z<0.4)
 A constellation of X-ray
point sources
13 detected in (predicted for 10% of
X-ray but not the 43 X-ray sources)
in optical
XMM archive data
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5 of 6 fields had observations in archive, 10-40ks
Analyzed by Ming Sun
Not sufficient depth to detect extended emission (cluster
x-ray luminosities 1043-1044erg/s at z=0.8-1.0 would be
below 5 sigma detection limit)
In all 5 cases, one or more point sources detected
RXJ1416.3: Extended ROSAT source resolved by
XMM into two point sources
XMM
Grey: K-band
Contours: ROSAT X-ray
RXJ1603: Extended ROSAT source resolved by
XMM into point sources
XMM
Grey: K-band
Contours: ROSAT X-ray
ROSAT Optical X-ray Survey (ROXS)
142 clusters
detected in
optical
43 clusters
detected in
X-ray
Three options:
 A real cluster at z>1,
failed to detect in
optical due to I-band
drop outs
 A real cluster at
moderate z, failed to
detect in optical due to
poorness or extent
(none predicted for
z<0.4)
 A constellation of X-ray
point sources
13 detected in (predicted for 10% of
X-ray but not the 43 X-ray sources)
in optical
Conclusions
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Matched filter method reliably detects low and moderate redshift
optical clusters using single filter
Near infrared colors are effective for detecting z~1 clusters
Clusters can have multiple X-ray AGN. Good X-ray resolution
essential for separating intracluster medium emission from AGN
emission.
Find 4 to 6 new clusters, estimated redshift 0.8-1.3 (but without
luminous intracluster medium)
Follow-up plans:
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Gemini spectroscopy to get firm cluster redshift – IN PROGRES
Spitzer imaging for photometric redshifts, identify cluster members
Measure blue fraction (Butcher-Oemler effect), K-band luminosity
function, slope & scatter of red sequence
HST imaging to classify galaxy morphologies, morphology-density
relation
VLA imaging to study AGN
If time …
Flux and color
calibration
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Calibrated to
Persson standard
stars observed on
same night
Checked with
2MASS stars
observed in same
field
Final correction
using color of Mdwarf stars
observed in same
field
Cluster detection
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Optical could be 1' or
more from X-ray
center due to wide
ROSAT PSF and
registration
Yet cluster radius is
only ~20" at z=1
(0.25h50-1 Mpc)
So, require at least 4
objects of similar color,
in circle 25" radius,
located within 1.5' of xray
RXJ1416.3
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z~1
Color image: IJK filters
Small circles: red sequence members
Contours: ROSAT x-ray
Cluster detection
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Check random spots in
field with same
algorithm, find red
sequences at 18% of
locations.
Detect either:
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concentrations of field
galaxies
real clusters with faint
intracluster medium
(ROSAT constellations)