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)