Issue |
A&A
Volume 514, May 2010
|
|
---|---|---|
Article Number | L3 | |
Number of page(s) | 6 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014434 | |
Published online | 18 May 2010 |
LETTER TO THE EDITOR
XMM-Newton detection of two clusters of galaxies with strong SPT Sunyaev-Zel'dovich effect signatures![[*]](/icons/foot_motif.png)
R. Suhada1 - J. Song2 - H. Böhringer1 - B. A. Benson3,5,6 - J. Mohr1,4,10 - R. Fassbender1 - A. Finoguenov1,9 - D. Pierini1 - G. W. Pratt7 - K. Andersson8 - R. Armstrong2 - S. Desai2
1 - Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany
2 -
University of Illinois, Department of Astronomy, 1002 West Green St, Urbana, IL 61801, USA
3 -
Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
4 -
Department of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 Munich, Germany
5 -
Department of Physics, University of California, Berkeley, CA 94720, USA
6 -
Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
7 - Laboratoire AIM, IRFU/Service d'Astrophysique - CEA/DSM - CNRS
- Université Paris Diderot, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette
Cedex, France
8 -
MKI, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
9 -
University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
10 -
Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
Received 15 March 2010 / Accepted 14 April 2010
Abstract
We report on the discovery of two galaxy clusters, SPT-CL J2332-5358 and SPT-CL J2342-5411, in X-rays.
These clusters were also independently detected through their Sunyaev-Zel'dovich effect
by the South Pole Telescope, and in the optical band by the Southern Cosmology Survey.
They are thus the first clusters detected under survey conditions by all major
cluster search approaches.
The X-ray detection is made within the frame
of the XMM-BCS cluster survey utilizing a novel XMM-Newton mosaic-mode of observations.
The present study makes the first scientific use of this operation mode.
We estimate the X-ray spectroscopic temperature of SPT-CL J2332-5358 (at redshift z=0.32) to be
T =9.3+3.3-1.9 keV,
implying a high mass,
.
For SPT-CL J2342-5411, at z=1.08, the available X-ray data do not allow us to directly
estimate the temperature with
good confidence. However, using our measured luminosity and scaling relations we
estimate that
keV and
.
We find a good agreement between the X-ray masses and those estimated from the
Sunyaev-Zel'dovich effect.
Key words: galaxies: clusters: individual: SPT-CL J2332-5358 - galaxies: clusters: individual: SPT-CL J2342-5411 - surveys - X-rays: galaxies: clusters
1 Introduction
Almost 40 years after the theoretical prediction of the Sunyaev-Zel'dovich effect (SZE, Sunyaev & Zeldovich 1972), i.e. the distortion
of the cosmic microwave background spectrum by the hot
gas in clusters of galaxies,
we have entered a new era where the first clusters have been discovered by
large-area SZE surveys (Staniszewski et al. 2009). Two ambitious SZE
cluster surveys are currently underway: by the South Pole Telescope (SPT)
and by the Atacama Cosmology Telescope (ACT). Recently, the SPT released a
catalog of 21 SZE-selected galaxy clusters identified in the first 200 deg2 of sky surveyed by the SPT (Vanderlinde et al. 2010, hereafter V10).
Both SPT and ACT have additionally carried out observations of
known clusters (Hincks et al. 2009; Plagge et al. 2009).
The SZE provides new prospects for precision cluster cosmology for two main reasons: (1) the SZE decrement characterized by the Comptonization parameter Y is currently considered as a robust, low-scatter proxy for cluster mass (e.g. da Silva et al. 2004; Motl et al. 2005) and (2) the SZE is not subject to the cosmological surface brightness dimming effect, resulting in a selection function closely corresponding to a selection with a fixed mass limit at all redshifts.
![]() |
Figure 1:
Left: mosaic-mode XMM-Newton image of the whole 14 deg2 survey field.
The false color image was constructed from surface brightness images in
the 0.3-0.5, 0.5-2.0 and 2.0-4.5 keV bands. Regions A, B, and C mark the
three mosaic-mode observations, region F the deeper core of the survey consisting
of 42 individual pointings.
The green circles mark the positions of SPT-CL J2332-5358 and SPT-CL J2342-5411 with a radius equal
to r
|
Open with DEXTER |
However, to be able to fully harvest the potential of the upcoming comprehensive multi-wavelength surveys, we need to have (1) a good understanding of the cluster selection function, (2) cluster redshift measurements and (3) a well-calibrated link between cluster observables and total cluster masses.
In order to address these issues and to best understand the results of the different survey techniques, we are conducting a coordinated multi-wavelength survey in a test region (which will be covered by both SZE surveys) in the optical by the Blanco Cosmology Survey (BCS, 100 deg2), in the mid-infrared with SPITZER (14 deg2) and in X-rays with XMM-Newton.
Here we present the X-ray detections of two clusters, SPT-CL J2332-5358 and SPT-CL J2342-5411. These clusters were independently detected by SPT (V10) and confirmed to be coincident with overdensities of red galaxies (High et al. 2010). In addition, SPT-CL J2332-5358 has been recently detected in the optical (SCSO J233227-535827, Menanteau et al. 2010). This source is also coincident with the X-ray source 1RXS J233224.3-535840 in the ROSAT Bright Source Catalog (Voges et al. 1999). The present XMM-Newton observations enable us to confirm both objects as X-ray luminous clusters of galaxies.
Throughout the article we adopt a CDM cosmology with
km s-1 Mpc-1).
2 XMM-Newton data reduction
The sources SPT-CL J2332-5358 and SPT-CL J2342-5411 were discovered as high-significance extended sources in XMM-Newton observations carried out in the framework of the XMM-BCS cluster survey (Suhada et al., in prep.).
The X-ray survey currently extends over 14
deg2 (Fig. 1).
The core of the XMM-Newton field consists of a deeper region covering 6 deg2 with
42 partially overlapping 12 ks long individual pointings and three
large scale (
2.7 deg2 each) mosaic-mode observations.
Each of the three mosaics consists of 19
stable pointings (3.5 ks exposures) and the slews between them, with a total time
90 ks per mosaic.
2.1 XMM-Newton mosaic-mode observations
The mosaic-mode observation is a new observation mode of XMM-Newton and this is the first instance of its scientific use. Mosaic-mode observations were designed to significantly increase the efficiency of observations covering areas larger than the field of view of the telescope. Before the implementation of this mode such observations could only be achieved by consecutive independent single pointings. Each of these individual pointings then required its own instrumental overhead, which particularly for the EPIC pn camera can be a significant part of the total observing time, especially if the required exposure times for the pointings themselves are short.
The mosaic-mode observation starts as a standard observation with operational
overhead (telescope pointing and guide star acquisition) followed by
instrumental overhead, when a charge zero level (i.e. offset table) is
calculated for the pn camera, which typically amounts to 3-4 ks (MOS cameras
are operated with fixed offset tables and their setup is negligible). After the
setups are finished, the observation itself starts. In our mosaics,
each stable pointing has an exposure of 3.5 ks, followed by a slew to the next
field offset by
.
Science data are also collected during the
slew and different from the standard operating mode the observation is not interrupted
by a new instrumental setup sequence, but the same offset table is used during the
whole mosaic.
Without the mosaic-mode, surveys of this kind would practically be
unfeasible, with observing efficiency (i.e. the ratio of integration time to
total time) around only 50%, compared to 80% efficiency achieved
with the present setup. More information on the mosaic-mode observations can be
found in the XMM-Newton User Handbook
.
2.2 X-ray data analysis
Both SPT-CL J2342-5411 and SPT-CL J2332-5358 were detected in the mosaic observations carried out in December 2009 (mosaic A, OBSID: 0604870301 and mosaic B, OBSID: 0604873401 respectively). We defer a more detailed description of the survey data reduction to a forthcoming publication of the X-ray cluster catalog. Here we summarize the main steps and highlight the differences of treating mosaic eventlists with respect to standard observations.
The EPIC data were processed with the current XMM-Newton Standard Analysis
System (SAS) version 9.0.0. We calibrated the raw observational data files in a standard
way. Events in bad pixels, bad columns and close to the chip
gaps were excluded from further analysis.
The eventlists were screened for high-background periods caused by soft proton
flares following the two-step cleaning method of Pratt & Arnaud (2003), but setting a
limit in both energy bands.
The clean exposure times are 71.9/72.6 ks for pn, and 85.0/88.7 ks for the MOS
cameras for the entire mosaic A/B respectively. The
beginning of the mosaic sequence B in the pn camera was strongly affected
by soft proton flaring, therefore the effective exposure at the SPT-CL J2332-5358 location is only 0.1 ks
in pn, while it is 3.1 ks in each MOS camera.
The source in addition lies
partially on the missing MOS1 CCD#6, yielding a total combined MOS effective
exposure of only 4 ks.
Local exposure times for SPT-CL J2342-5411 were
2.8 ks in pn,
2.1 ks in MOS1
and
2.3 ks in MOS2.
As the main source-detection algorithm we utilized the sliding box technique and a maximum likelihood source fitting in their current, improved implementation in the SAS tasks eboxdetect and emldetect.
Mosaic data of this extent is too extensive to fit into the memory storage during the detection process. Therefore we segmented the mosaic into several overlapping parts, which could be handled by the SAS tasks. Segmenting the mosaic into sky-chunks for source detection is preferable to splitting it into individual stable pointings, because we also wished to include counts gathered during the slews between the pointings and utilize the greater depth in the regions where two neighboring pointings overlap.
The mosaic segments have a typical size of 1 deg2 and overlap by
along all borders. This way the
input images, exposure, and background maps could be accommodated by the
ebox- and emldetect tasks ran with increased memory buffer
(imagebuffersize = 2000 flag).
In order to get a reliable measurement of the flux and trace the emission of
the clusters as far out as possible, we implemented a refined version of the
growth curve method (Böhringer et al. 2000; Suhada et al., in prep.). The cumulative source flux
as a function of radius (i.e. the growth curves) for the two systems are
displayed in Figs. A.1 and A.2 (in the online Appendix).
The total source flux was determined iteratively by fitting a line to the flat
part of the background-subtracted growth curve.
We define the plateau radius (
)
as the aperture where
the growth curve reaches the total flux.
For SPT-CL J2332-5358 we detected source emission out to
,
with
total source flux of
keV) =
erg s-1 cm-2,
corresponding to a total luminosity
keV) =
erg s-1.
Errors of the flux and luminosity include the Poisson errors and a
5% systematic error in the background estimation.
The X-ray morphology of this cluster agrees well with the SZE signal on the largest scales (Fig. 1)
and its peak is close to the position of the brightest cluster galaxy (BCG, Fig. A.5).
We detect a significant X-ray extension up to
SE from the BCG.
A detailed characterization
of the galaxy distribution and the correlation between X-ray and optical
morphology will be given in a forthcoming paper.
For SPT-CL J2342-5411 we found
,
2.0 keV) =
erg s-1 cm-2
and a total luminosity
keV) =
erg s-1 (Fig. A.2).
2.2.1 X-ray spectroscopy
Table 1: Basic X-ray parameters of SPT-CL J2332-5358 and SPT-CL J2342-5411.
The available survey data, although modest in exposure, allow us to get a first
temperature estimate for SPT-CL J2332-5358.
In order to determine a suitable aperture
for spectroscopic measurements, we created a wavelet reconstruction
(Vikhlinin et al. 1998) of the combined 0.5-2.0 keV band image.
We found that a circular aperture with 70'' radius well encloses the region where
the cluster emission is registered at
significance.
A background spectrum was extracted
from an annulus concentric with the source and spanning the radial distance from
to
.
The inner radius was selected based on the growth-curve analysis as the radius
where cluster emission is no longer observable (Fig. A.1). The outer
radius is constrained by the field of view. We excised all detected point sources
from each extracted spectrum after a visual check.
We fitted the spectrum with a single temperature MEKAL model, fixing the column
density to the galactic value
cm-2 (Dickey & Lockman 1990),
metal abundance to
and redshift to z=0.32 (photometric, High et al. 2010; consitent with Menanteau et al. 2010).
To avoid biases stemming from
analyzing low-count spectra, we used a minimally binned spectrum (
1 cts/bin)
and C-statistics.
The fitted temperature is
T =9.3+3.3-1.9 keV
(
errors) for the joint fit from all three cameras
(Fig. A.3).
In order to check for possible systematics in the background subtraction, we also fitted
the spectrum using background spectra extracted from a
completely independent circular region (
on different chips than the source but roughly at the same
off-axis angle). The test background gives a consistent result,
T =9.4+3.5-1.9 keV.
Based on our temperature measurement we estimated several important physical
parameters (Table 1), including mass in r500 and r200 apertures
from the M-T scaling relation,
assuming self-similar evolution. For parameters obtained from scaling relations we included
the measurement errors of the luminosity and temperature and the intrinsic scatter
of the scaling relations. We used a beta model to extrapolate
the observed flux and luminosity out to r500 (because the estimated r500 value is higher than the measured
).
This extrapolation is negligible (
1.5%).
The available photon statistics for SPT-CL J2342-5411 is much lower and allows us to carry
out a tentative analysis only. Following the previously described procedure,
we extracted the source spectrum from a
region and the background spectrum
from a concentric annulus with
inner and
outer radius. Fixing the column
density to the galactic value
cm-2,
metal abundance to
and redshift to z=1.08 (photometric, High et al. 2010),
we found that the spectrum is consistent with a single temperature MEKAL model (Fig. A.4).
The temperature is only weakly constrained,
T =6.7+5.2-2.4 keV,
and therefore we opted for the use of luminosity-based scaling relations (L-T and
)
to estimate the physical parameters of the system (Table 1).
The evolution of the scaling relations and their intrinsic scatter is currently not firmly established out to
.
We assumed self-similar evolution of the scaling relations and no evolution of their intrinsic scatters.
The error from these assumptions for SPT-CL J2342-5411 is expected to be smaller than the quoted measurement errors.
3 Discussion and conclusions
We have presented first results from the XMM-BCS cluster survey, providing X-ray detections of two SZE-selected systems, SPT-CL J2332-5358 and SPT-CL J2342-5411. The X-ray analysis is based on mosaic-mode XMM-Newton observations - the first time observations of this kind have been carried out.
The system SPT-CL J2332-5358 ranks among the hottest known clusters (T =9.3 keV) and is exceptionally massive
(
). Our mass estimate is in excellent agreement
with the ROSAT-based result of Menanteau et al. (2010).
The SZE-inferred mass reported by V10 is
,
where the error bars represent the statistical and systematic uncertainties, both at
68% confidence. They note that this mass is biased low by a bright
dusty point source identified in the 220 GHz SPT
data. A preliminary analysis indicates that this point source decreases the
SZE mass estimate by a factor of
1.5. This would imply a corrected SZE mass estimate of
,
which agrees well
with our X-ray estimate.
This is an initial study of the system SPT-CL J2332-5358. Other aspects of the system are investigated in forthcoming papers: 1) using a deeper XMM observation Andersson et al. (in prep.) performing a detailed comparison of the X-ray and SZE properties of this cluster; 2) and a characterization of the galaxy population and morphology will be addressed in Song et al. (in prep.).
The source SPT-CL J2342-5411 belongs to one of the most distant known clusters (z=1.08)
with X-ray and SZE detections. The discovery of such a distant system in
both SZE and X-ray surveys demonstrates the great potential of the two
observational approaches for cosmological and cluster evolution studies.
The estimated mass for this system,
,
is consistent
with the SZE mass M500 = 2.66
0.50
(V10).
SPT-CL J2332-5358 and SPT-CL J2342-5411 are the first galaxy clusters discovered independently in X-ray, SZE and optical surveys. These clusters exemplify the promise of multi-wavelength cluster surveys and give a glimpse of the possible synergies of current and future large-scale survey experiments, including SPT, Planck, eRosita, and the Dark Energy Survey.
AcknowledgementsWe thank the XMM-Newton SOC for implementing the mosaic-mode. Particularly, we thank Pedro Rodriguez for useful discussions about the data analysis. We also thank the Blanco Cosmology Survey team for executing and processing the observations used in this paper. We thank the SPT team for SZE mass estimates and maps of this cluster. The South Pole Telescope is supported by the National Science Foundation through grants ANT-0638937 and ANT-0130612. The SPT also thanks the National Science Foundation (NSF) Office of Polar Programs, the United States Antarctic Program and the Raytheon Polar Services Company for their support of the project. R.S. acknowledges support by the DfG in the program SPP1177. H.B. acknowledges support for the research group through The Cluster of Excellence ``Origin and Structure of the Universe'', funded by the Excellence Initiative of the Federal Government of Germany, EXC project number 153. B.B. acknowledges additional support from a KICP Fellowship.
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Online Material
Appendix A: Supplementary electronic material
![]() |
Figure A.1:
The growth curve of SPT-CL J2332-5358: red curve shows the integrated
flux as a function of outer integration radius for the MOS2
camera in the 0.5-2.0 keV band. Error bars on the growth curve
indicate the Poisson error of the flux measurement including a 5%
systematic error in the background estimation. Source flux is detected
out to
|
Open with DEXTER |
![]() |
Figure A.2:
The growth curve of SPT-CL J2342-5411: red curve shows the integrated
flux as a function of
outer integration radius for the joint MOS1 and MOS2
cameras (0.5-2.0 keV band), blue curve for the pn camera. Error
bars on the growth curve indicate the Poisson error of the flux
measurement including a 5% systematic error in the background
estimation. Source flux is detected out to
|
Open with DEXTER |
The Appendix provides additional information concerning the X-ray analysis of SPT-CL J2332-5358 and SPT-CL J2342-5411.
![]() |
Figure A.3:
XMM-Newton X-ray spectrum of SPT-CL J2332-5358
fitted with a single temperature MEKAL model which gives
T =9.3+3.3-1.9 keV.
The column density is fixed to the galactic value
|
Open with DEXTER |
![]() |
Figure A.4:
XMM-Newton X-ray spectrum of SPT-CL J2342-5411. The available
low-count spectrum (displayed for completeness) allows us to draw only
tentative conclusions.
The spectrum is consistent with a single temperature MEKAL model, with
the temperature only weakly constrained to
T =6.7+5.2-2.4 keV.
The column density is fixed to the galactic value
|
Open with DEXTER |
![]() |
Figure A.5:
Left: color image of SPT-CL J2332-5358 (photo-z = 0.32).
Right: color image of SPT-CL J2342-5411 (photo-z=1.08).
Both images were obtained from the Blanco Cosmology Survey imaging in the gri bands.
X-ray contours are overlaid in white. Green circles show the estimated r
|
Open with DEXTER |
Footnotes
- ... signatures
- Appendix A is only available in electronic form at http://www.aanda.org
- ... Handbook
- xmm.esac.esa.int/external/xmm_user_support/documentation/uhb/XMM_UHB.pdf
- ... data
- Due to current limitations of the backscale task, used to calculate the area scaling factors of the spectra, we omitted the slew part of the survey for spectroscopical purposes and filtered from the mosaic eventlist only events detected during the relevant stable pointing period (the slew part would contribute only a few tens of counts in this case).
All Tables
Table 1: Basic X-ray parameters of SPT-CL J2332-5358 and SPT-CL J2342-5411.
All Figures
![]() |
Figure 1:
Left: mosaic-mode XMM-Newton image of the whole 14 deg2 survey field.
The false color image was constructed from surface brightness images in
the 0.3-0.5, 0.5-2.0 and 2.0-4.5 keV bands. Regions A, B, and C mark the
three mosaic-mode observations, region F the deeper core of the survey consisting
of 42 individual pointings.
The green circles mark the positions of SPT-CL J2332-5358 and SPT-CL J2342-5411 with a radius equal
to r
|
Open with DEXTER | |
In the text |
![]() |
Figure A.1:
The growth curve of SPT-CL J2332-5358: red curve shows the integrated
flux as a function of outer integration radius for the MOS2
camera in the 0.5-2.0 keV band. Error bars on the growth curve
indicate the Poisson error of the flux measurement including a 5%
systematic error in the background estimation. Source flux is detected
out to
|
Open with DEXTER | |
In the text |
![]() |
Figure A.2:
The growth curve of SPT-CL J2342-5411: red curve shows the integrated
flux as a function of
outer integration radius for the joint MOS1 and MOS2
cameras (0.5-2.0 keV band), blue curve for the pn camera. Error
bars on the growth curve indicate the Poisson error of the flux
measurement including a 5% systematic error in the background
estimation. Source flux is detected out to
|
Open with DEXTER | |
In the text |
![]() |
Figure A.3:
XMM-Newton X-ray spectrum of SPT-CL J2332-5358
fitted with a single temperature MEKAL model which gives
T =9.3+3.3-1.9 keV.
The column density is fixed to the galactic value
|
Open with DEXTER | |
In the text |
![]() |
Figure A.4:
XMM-Newton X-ray spectrum of SPT-CL J2342-5411. The available
low-count spectrum (displayed for completeness) allows us to draw only
tentative conclusions.
The spectrum is consistent with a single temperature MEKAL model, with
the temperature only weakly constrained to
T =6.7+5.2-2.4 keV.
The column density is fixed to the galactic value
|
Open with DEXTER | |
In the text |
![]() |
Figure A.5:
Left: color image of SPT-CL J2332-5358 (photo-z = 0.32).
Right: color image of SPT-CL J2342-5411 (photo-z=1.08).
Both images were obtained from the Blanco Cosmology Survey imaging in the gri bands.
X-ray contours are overlaid in white. Green circles show the estimated r
|
Open with DEXTER | |
In the text |
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