CL0048-2942: An Optically Detected Cluster At z ' 0.64 ∗
Catarina Lobo (lobo@astro.up.pt)
CAUP & DMA Porto, Portugal
Margarida Serote Roos
CAAUL Lisbon, Portugal
Florence Durret
IAP Paris, France
Angela Iovino
OAB Milano, Italy
Abstract. We present preliminary spectroscopic analysis of a recently confirmed distant
galaxy cluster. This system belongs to a catalog of cluster candidates detected on the ESO
Imaging Survey (Renzini & da Costa, 1997) data by running a detection algorithm (Lobo et
al., 2000). Deeper BVRI follow-up photometry allowed us to select objects for targeted VLT
spectroscopy thus producing redshifts of member and adjacent field galaxies.
Keywords: clusters, galaxies, redshifts
1. Introduction
Cluster candidate CL0048-2942 was identified in the ESO Imaging Survey
dataset (I band) by a matched filter -like detection algorithm (Lobo et al.,
2000) as well as by another independent method (Olsen et al., 1999). It was
then followed-up in BVRI photometry with the ESO 3.6-m telescope - see
Figure 1. A photometric catalog was thus obtained gathering all objects with
I < 22.0 in the 3.6-m field of view and some of its objects were selected
for targeted spectroscopy following a color and surface brightness criteria (to
avoid contamination from stars and to choose the most promising galaxies expected at redshifts z >
∼ 0.4). Spectroscopy was performed at the VLT, yielding
redshifts that confirmed the existence of a gravitationally bound system.
2. Spectroscopic Observations and Reduction
Multi-object spectroscopy was performed at the VLT: 2 nights (September
1999) with MOS@FORS1/UT1 provided partial coverage of selected objects
in the cluster candidate field; further data were gathered in service mode (first
semester 2000) with MOS@FORS2/UT4. Both runs used grism 300V+10
with order separation filter GG435. This setup provided a useful field of view
∗ Based on observations made at the European Southern Observatory, Paranal, Chile.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.
CL0048-2942: a z ' 0.64 cluster
55
Figure 1. BVI color-combined image of the CL0048-2942 field, produced according to the
technique of Szalay et al. (1999). The reddish galaxies denounced already th possible presence
of a real cluster. Field of view is approximately 5 × 5 arcmin2 .
of 4.7 × 6.8 arcmin2 , with each spectrum covering the spectral range 4450–
8650 Å. The dispersion of 112 Å/mm (2.69 Å/pixel) gives a resolution of
500 or about 12 Å for a slit width of 1.2 arcsec. The MOS system allowed
positioning 19 movable slit blade pairs on pre-selected objects. Seven different masks were constructed and used to observe repeatedly a sample of
I < 22.0 objects. Due to the rigidity of the MOS system and to the obvious
clustering of targets, 2 to 3 slits/mask had to be positioned on objects that
weren’t present in the photometric catalog (chosen by brightness and shape
parameters). Each mask was exposed for 30 min. and used more than once
during both runs to later remove cosmic rays in the same spectra or to serve
as control of the redshift measure.
All data were reduced according to standard procedures using subroutines
in the IRAF package. Following bias subtraction and flat-fielding, the spectra
were extracted using the apall task that produces a 1D spectrum from the 2D
image and does the sky subtraction at the same time. Wavelength calibration
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Figure 2. Spectrum of an observed galaxy where several absorption lines have been identified.
was carried out using Helium-Argon-HgCd lamps taken on the nights of the
cluster observations; the rms error in the wavelength solution was always less
than 0.1 with a maximum deviation of 0.15 Å for all lines used.
3. Redshift measurements
For all galaxies with clear absorption lines, redshifts were measured with
rvsao.xcsao, cross-correlating the galaxy spectrum with spectra of velocity
standard stars obtained during the same run - Figure 2 shows an example.
Whenever more than one spectra was available for the same object, absorption
line redshifts were measured from individual spectra; these values were then
averaged to give a final galaxy redshift with an error bar corresponding to the
dispersion of the various measurements.
For objects with a spectrum showing several emission lines, the line positions were measured with a Gaussian fit, using splot.onedspec, and an average velocity was computed. In many cases, only one emission line was
detected and assumed to be [OII], an hypothesis supported by the presence of
a continuum strongly increasing bluewards of [OII].
CL0048-2942: a z ' 0.64 cluster
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Figure 3. Velocity histogram for the observed galaxies.
4. Preliminary Results
In a final balance, a total of 60 different objects in the field were observed
(some more than once). A reliable redshift was computed for 53 of these,
yielding a success rate of 88%. Among these 53 galaxies, 43 have I ≤ 22.0.
This translates into a completeness of ∼ 60% in the I = 22.0 limited photometric sample, a relatively low number that is due to the rigidity of the
MOS system (we could not observe the full I ≤ 22.0 sample with 7 masks
only, due to geometry constraints). MXU observations with FORS2 had been
prepared and scheduled for service mode in 2001 but were not carried out,
unfortunately. The histogram of velocities - Figure 3 - confirms the existence
of a cluster. A rough estimate for the limits of the velocity range defining
cluster members is 135 000 km/s < v < 140 000 km/s, with 22 galaxies belonging to the cluster (centered around z = 0.64), all with I ≤ 22.0.
The use of a more sophisticated algorithm (Serna & Gerbal, 1996) that detects structure by working on velocities and magnitudes confirms this simple
velocity histogram analysis. The cluster thus unveiled has a line-of-sight velocity dispersion of ∼ 720 km/s, typical of clusters at low and intermediate
redshifts.
A ROSAT PSPC image, that serendipitously contained our cluster, shows
an excess of counts (a factor 2.3) in the cluster area with respect to the
background, suggesting the presence of X-ray emission from our (spectroscopically confirmed) cluster.
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These data will also be used to perform stellar population synthesis, providing insight on the role of the environment in the evolution of galaxies.
Acknowledgements
The authors acknowledge financial support from ESO/PRO/15130/1999 (FCT
/ MCT, Portugal). MSR acknowledges financial support from FCT/Portugal
through grant ref. SFRH/BPD/5684/2001.
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