Chandra X-ray Observations of Newly Discovered, z ~ 1 Clusters from the Red-Sequence Cluster Survey
ABSTRACT Observational studies of cluster evolution over moderate redshift ranges (to z ~ 1) are a powerful tool for constraining cosmological parameters, yet a comprehensive knowledge of the properties of these clusters has been hitherto unattained. Using a highly efficient optical selection technique, the Red-Sequence Cluster Survey (RCS) has unearthed a large sample of high redshift cluster candidates. All six of the clusters from this sample which have been observed with the Chandra X-Ray Observatory were detected in the X-ray. These Chandra follow-up observations (0.64 < z < 1.0) indicate that the clusters are systematically less luminous than their similarly rich, X-ray selected counterparts at lower redshifts, though they are consistent with standard Lx-Tx relationships. Comparisons with X-ray selected samples suggest that the discrepancy may be due in part to systematic differences in the spatial structure of the X-ray emitting gas. Our initial results from Chandra follow-up observations of six RCS clusters are presented, including beta model parameters and spectral information.
arXiv:astro-ph/0410167v1 6 Oct 2004
Chandra X-ray Observations of Newly
Discovered, z ∼ 1 Clusters from the
Red-Sequence Cluster Survey⋆
A.K. Hicksa, E. Ellingsona, M. Bautzb, H.K.C. Yeec,
M. Gladdersd, G. Garmiree
aCenter for Astrophysics and Space Astronomy, University of Colorado at
Boulder, Campus Box 389, Boulder, CO 80309, USA
bMIT Center for Space Research, 77 Massachusetts Ave., Cambridge, MA 02139,
cDepartment of Astronomy and Astrophysics, University of Toronto, 60 St. George
St., Toronto, ON, M5S 3H8, Canada
dCarnegie Observatories, 813 Santa Barbara St., Pasadena, CA, 91101, USA
eDepartment of Astronomy and Astrophysics, 525 Davey Lab, The Pennsylvania
State University, University Park, PA, 16802, USA
Observational studies of cluster evolution over moderate redshift ranges (to z ∼
1) are a powerful tool for constraining cosmological parameters, yet a comprehen-
sive knowledge of the properties of these clusters has been hitherto unattained.
Using a highly efficient optical selection technique, the Red-Sequence Cluster Sur-
vey (RCS) has unearthed a large sample of high redshift cluster candidates. All six
of the clusters from this sample which have been observed with the Chandra X-
Ray Observatory were detected in the X-ray. These Chandra follow-up observations
(0.64 < z < 1.0) indicate that the clusters are systematically less luminous than
their similarly rich, X-ray selected counterparts at lower redshifts, though they are
consistent with standard Lx− Txrelationships. Comparisons with X-ray selected
samples suggest that the discrepancy may be due in part to systematic differences
in the spatial structure of the X-ray emitting gas. Our initial results from Chan-
dra follow-up observations of six RCS clusters are presented, including β model
parameters and spectral information.
Key words: Galaxy groups, clusters, and superclusters; large scale structure of the
Universe, Galaxy clusters, X-ray sources, Observational cosmology
PACS: 98.65.-r, 98.65.Cw, 98.70.Qy, 98.80.Es
Preprint submitted to Elsevier Science2 February 2008
1The RCS Survey
The Red-Sequence Cluster Survey (Gladders and Yee, 2004) is a 90 square
degree optical survey performed at CFHT and CTIO using RCand z’ filters.
It utilizes the red sequence of elliptical galaxies to find galactic overdensities on
the sky (Gladders and Yee, 2000). The color information also guards against
projection effects, and provides photometric redshift estimates. The survey has
identified between 3500 and 4000 clusters total (in the redshift range 0.2 < z
< 1.2), over 1500 of which are at least as optically rich as Abell class 0 clusters
(Gladders and Yee, 2004). Simulations indicate that the survey is complete to
Abell Class 1 at a redshift of 1 for blue fractions less than 0.45, and that
false detection rates are less than 5%, which is significantly lower than that of
single-passband optical cluster surveys (Postman et al., 1996; Donahue et al.,
2002; Gilbank et al., 2004). Figure 1 shows an example of a z=0.773 RCS
2The Chandra Subsample
So far we have observed six high redshift (0.64 < z < 1.0) cluster candi-
dates with the Chandra X-ray Observatory. Table 1 lists our current sample
of follow-up observations. All six clusters were detected in the X-ray, using the
CIAO tool csmooth, to better than 3σ. The X-ray centroids of these clusters
were all found within 10 arcseconds of their respective optical centers.
Spectra were extracted from 300 kpc (ΛCDM with H0= 70, Ωm= 0.3, and
ΩΛ= 0.7) radius regions around the X-ray centroids of these clusters, with
backgrounds taken from each respective aimpoint chip. The spectra were fit
in XSPEC using absorbed single temperature models with galactic column
densities and abundances fixed at 0.3 solar. Five of the six observations yielded
enough counts to constrain a temperature. Results are listed in Table 2.
⋆COSPAR conference attendance was partially supported by an International
Travel Grant from the American Astronomical Society and the National Science
Email address: email@example.com (A.K. Hicks).
Fig. 1. Top: HST image of RCS 0224-0002 spectroscopically confirmed at z=0.773,
with overlayed X-ray contours. The outer gravitational lensing arc has been spectro-
scopically confirmed at a redshift of z=4.87 (Gladders, Yee, and Ellingson, 2002),
with the others expected to fall within the range 1.8< z <3.6 The five linearly
spaced contours indicate values between 9.4×10−6and 2.1×10−5counts/pix2/cm2
and were created using a Gaussian smoothed (5 pixel FWHM) 0.29-7.0 keV Chan-
dra flux image. Bottom: Adaptively smoothed flux image of RCS 0224-0002 created
with the CIAO tool csmooth. The boxed area indicates the region shown in the
3Optical and X-ray Properties
Bgc, explained in detail in Yee and Lopez-Cruz (1999), is a parameter which
describes the optical richness of a cluster of galaxies. Technically it is the
galaxy-cluster spatial covariance amplitude, but in essence it is simply a mea-
sure of galaxy overdensity within a given aperture, normalized for the expected
spatial distribution of galaxies in the cluster and the evolving galaxy luminos-
ity function. All of the RCS clusters that have been observed with Chandra
have richnesses that imply Abell richness classes of at least 1.
There exist a few challenges in the calculation of Bgcat high redshift. One is
the lack of a complete knowledge of the galaxy luminosity function at these
redshifts. Another is uncertainty due to cluster galaxy evolution. The latter
uncertainty can be minimized by employing the parameter Bgc,red, which is
essentially the same as Bgc, but is calculated using only galaxies in the red-
sequence. Throughout this paper we will use only the parameter Bgc,red
It is expected that Bgcshould correlate strongly with X-ray temperature for
relaxed clusters, and a trend has been seen when optical richness is plotted
versus temperature for moderate redshift (z ∼ 0.3), X-ray selected clusters
(Yee and Ellingson, 2003). However, the same correlation is not apparent when
RCS clusters are added to the plot (Figure 3.1).
In comparison with lower redshift clusters, high redshift optically selected
clusters with similar Bgcvalues appear systematically cooler and less lumi-
nous than their X-ray selected counterparts, possibly suggesting that a smaller
fraction of the intra-cluster gas in these objects has collapsed and become viri-
alized. This interpretation may seem intuitive, given that X-ray cluster surveys
preferentially select more relaxed clusters with deep potential wells and often-
times cooling cores. Our findings reinforce the need to question what defines
a cluster, and whether X-ray selected clusters primarily represent a highly
virialized, high X-ray luminosity tail of the cluster distribution.
3.2 X-ray Surface Brightness
A radial surface brightness profile was computed over the range 0.29-7.0 keV
in circular annuli for each cluster. We were able to constrain β models for four
clusters. Best fit parameters are expressed in Table 2 and an example is shown
in Figure 3.2. The results of these fits are interesting in that β values seem
systematically low for these clusters, with the implication that, on average,
Fig. 2. Txvs. Bgc,red. Each square represents one of 13 moderate redshift (0.1 < z
< 0.6) CNOC clusters (Yee, Ellingson and Carlberg, 1996) taken from the Chandra
archive, and triangles denote RCS clusters. Temperatures were calculated using
spectra extracted within a 300 kpc (ΛCDM) radius region and fit over the 0.6-7.0
keV energy range. A single temperature model was used, with galactic absorption.
The abundance of RCS clusters was fixed at 0.3. Temperature error bars show 90%
confidence intervals, and Bgc,rederror bars are shown at 1σ.
Fig. 3. Surface brightness was calculated for the 0.29-7.0 keV band in 2 arcsecond
radial bins. The solid line represents the best fitting β model, and the dotted line
indicates the fit-determined background value.
high redshift optically selected clusters are less centrally condensed than X-ray
3.3The Lx− TxRelationship
Though many of our results indicate that RCS clusters differ significantly from
X-ray selected clusters, their X-ray properties are consistent with a standard
Lx− Txrelationship. Figure 3.3 is a plot of Lx vs. Txfor 13 CNOC and 5
RCS clusters. The consistency of RCS clusters with the Lx− Txrelationship
Fig. 4. Lxvs. Tx. Squares indicate 13 CNOC clusters and triangles represent the 5
RCS clusters. Unabsorbed luminosities were calculated with XSPEC and converted
to bolometric X-ray luminosities with PIMMS. Temperatures were obtained with
the same method as in Figure 3.1. The dashed line indicates a standard power law
with slope 2.88 (Arnaud and Evrard, 1999)
of moderate redshift X-ray selected clusters implies that though RCS clusters
may possess relatively low mass virialized cores of gas, the gas is in a similar
physical state to that found in X-ray selected samples.
4Summary and Discussion
The Red-Sequence Cluster Survey has uncovered a large sample of high red-
shift cluster candidates. All of the RCS clusters observed with Chandra were
detected at greater than 3σ significance in the X-ray. This is evidence that
the RCS method is a reliable way to detect high redshift clusters. The X-ray
properties of RCS clusters are consistent with a standard Lx−Txrelationship.
This leads us to believe that the X-ray gas in these clusters is in a physical
state very similar to that found in X-ray selected clusters.
The results of detailed X-ray analysis imply that though these clusters have
extended structures of galaxies, they possess relatively small virialized cores.
These findings are similar to those from a number of previously conducted
optical surveys (Donahue et al., 2002; Lubin, Mulchaey and Postman, 2004;
Gilbank et al., 2004). Optical, high redshift cluster surveys regularly find can-
didates with lower X-ray luminosities than those of X-ray selected clusters.
Our study suggests that the RCS survey is detecting a different population
of clusters from that found in X-ray selected samples, possibly including ulti-
mately very rich clusters which are currently in the early stages of virialization.
Similar objects have recently been described by Ford et al. (2004). Such sys-
tems may be expected to be more common at high redshifts in a low matter
density universe. Optical surveys are much more sensitive to such systems than
are X-ray surveys, especially if any related filamentary structures lie along the
line of sight (though it is unlikely that all of the RCS cluster candidates that
were followed-up with Chandra are associated with such structures).
Upcoming Chandra observations of five new and two previously observed RCS
clusters in the next AO, along with ongoing velocity dispersion measurements
and weak lensing analysis should help to provide more definitive constraints
on the masses, dynamical states, and gas content of high redshift optically
selected samples of galaxy clusters.
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Chandra Observed RCS Cluster Sample
RCS 0224-00020.77ACIS-S 12620gravitational lens
RCS 0439-2904 0.95ACIS-S77905 photometric redshift
RCS 1326+2903 0.95ACIS-S63590 photometric redshift
RCS 1417+53051.0ACIS-I62820 photometric redshift
RCS 1419+53260.64ACIS-S 9910 gravitational lens
RCS 1620+2929 0.87ACIS-S36640gravitational lens?
RCS 0224-00020.77 838+186
RCS 1620+29290.87 957+222