Discovery of Galaxy Clusters Around Redshift 1
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Transcript of Discovery of Galaxy Clusters Around Redshift 1
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
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 “down-sizing” 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?
RDCS 1252.9-2927 at z=1.24Rosati et al 2004 AJ 127, 230
Contours = x-ray (Chandra+ROSAT) Image = B R K filters
Need more z>0.8 clustersNeed more x-ray luminous clusters
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 I-
band 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)
A bimodal population?
More likely these are lower mass clusters, with x-ray emission below detection limit (Donahue et al 2001 ApJ 552, L93)
142 clusters detected in optical
43 clusters detected in
X-ray
112 detected in optical but not in X-ray
ROSAT Optical X-ray Survey (ROXS)
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 (predicted for 10% of the 43 X-ray sources)
142 clusters detected in optical
43 clusters detected in
X-ray
13 detected in X-ray but not in optical
Bimodality of color-magnitude relation
I-band magnitude
Red sequence
Blue Cloud
Coma Cluster, z=0.023DeLucia et al, astro-ph/0610373
Magnitude
Color
~75% ellipticals and SOs
~75% spirals
DeLucia et al, astro-ph/0610373
Observations
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~1.0
z~0.8z~0.9
z~1.0
z=0.338, no I-band data
z~0.9, no I-band data
Evidence for redshifts around 1 I-band dropout
(redshifted enough to be faint in I-band)
J-K color consistent with models
Evidence for redshifts around 1 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
z~0.8
Color image: IJK filtersSmall circles: red sequence membersContours: ROSAT x-ray
RXJ 1605.6+2548
z~0.9 Similar color to field
galaxies
Color image: IJK filtersSmall circles: red sequence membersContours: ROSAT x-ray
RXJ 1205.2+2752
z~1.0 Two subclusters
merging?
Color image: IJK filtersSmall circles: red sequence membersContours: ROSAT x-ray
RXJ 1603.6+4316
z~1.0 Supercluster in
field at z=0.9
Color image: IJK filtersSmall circles: red sequence membersContours: ROSAT x-ray
RXJ1416.3
z~1Color image: IJK filtersSmall circles: red sequence membersContours: ROSAT x-ray
RXJ 1118.9+2117
z~1.3 High-z interaction?
separation ~20kpcColor image: IJK filtersSmall circles: red sequence membersContours: ROSAT x-ray
ROSAT Optical X-ray Survey (ROXS)
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 (predicted for 10% of the 43 X-ray sources)
142 clusters detected in optical
43 clusters detected in
X-ray
13 detected in X-ray but not in optical
XMM archive data
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
XMM Grey: K-band
Contours: ROSAT X-ray
RXJ1603: Extended ROSAT source resolved by XMM into point sources
ROSAT Optical X-ray Survey (ROXS)
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 (predicted for 10% of the 43 X-ray sources)
142 clusters detected in optical
43 clusters detected in
X-ray
13 detected in X-ray but not in optical
Conclusions 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: 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
Calibrated to Persson standard stars observed on same night
Checked with 2MASS stars observed in same field
Final correction using color of M-dwarf stars observed in same field
Cluster detection
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 x-ray
RXJ1416.3
z~1Color image: IJK filtersSmall circles: red sequence membersContours: ROSAT x-ray
Cluster detection
Check random spots in field with same algorithm, find red sequences at 18% of locations.
Detect either: concentrations of field
galaxies real clusters with faint
intracluster medium (ROSAT constellations)