Massive clusters of galaxies are fundamental cosmological probes
because their formation rate -- and formation history -- depends
strongly on both the value of and the type of dark matter
assumed. It is therefore of prime interest to detect and study very
massive distant clusters. However, the detailed analysis of cluster
properties (dynamics, virialization stage, galaxy content, ICM
enrichment, etc.) requires large amounts of observing time and,
beyond
, is strongly limited by the angular resolving
power of present-day instruments. As a result, the 0.2 < z < 0.3
interval offers at present the best compromise for obtaining detailed
multi-wavelength data for a statistically significant set of distant
clusters. In this context, we have selected 10 medium distant, X-ray
bright (i.e., massive) systems from a ROSAT All-Sky Survey flux
limited sample of
clusters located in the Hydra region
(Pierre et al. 1994a).
![]() ![]() |
For this sub-sample (Table 1), we undertook deep
optical (CFHT, NTT), X-ray (ROSAT, ASCA), radio (AT) and infrared
observations (ISO) (Pierre et al. 1994b). This unique
database will yield insights into the formation processes of both clusters
and galaxies, processes which are very likely to be intimately related. In
this context, a general -- and crucial -- point is the evolution of
the dynamical state of clusters. In hierarchical CDM-type scenarios,
clusters are believed to form continuously, but not necessarily at the
same rate. Indeed, using a power ratio technique applied to clusters
extracted from classical CDM simulations, Tsai & Buote (1996)
find that there is a continuous competition between relaxation and
formation rates throughout the whole cluster history. They do not
observe a significant change from high redshifts until , where
both effects come into balance; at more recent times the formation rate
levels off. Focusing on the very inner cluster regions (a few tens of
kpc), the influence of the central galaxy appears to be responsible
for the disturbed X-ray structure often observed, this region being
more or less isolated from the rest of the cluster. Also,
isophote morphologies derived from deep ROSAT HRI pointings suggest
that below
the outer parts of clusters are less disturbed
than in the 0.1-0.3 redshift range (Pierre & Starck 1998).
As time goes on, cluster masses increase through matter accretion
along filaments and, consequently, potential wells deepen. The
merging process, however, takes time to produce relaxed systems. It
is therefore of major interest to study cluster X-ray temperatures,
together with optical data and X-ray morphologies, since the ICM
temperature may not only reflect the depth of the gravitational
potential but also contain the signature of recent mergers. This has
been investigated at low redshift with detailed temperature maps
(e.g., Briel & Henry 1996). For distant clusters where
spatially resolved X-ray spectroscopy is not possible, deviations from the
well known correlations (,
, metallicity) can give an
indication of the degree of cluster relaxation.
![]() ![]() ![]() |
We present here the first two ASCA observations of our sample clusters, obtained under the ESA/ISAS time allocation; ASCA observations of the remaining eight clusters, performed under ISAS time, will be presented in a forthcoming paper (Matsumoto et al. 1999, in preparation). General information about the two clusters is given in Table 2. A 1300 is very luminous in the X-ray band. Both optical and ROSAT PSPC observations indicate that the cluster is in a recent post-merging phase (Lémonon et al. 1997). This is corroborated by the presence of a radio halo at the cluster center (Reid et al. 1999), a rare phenomenon at this redshift, but one increasingly associated with mergers.
ISOCAM observations of A 1732 revealed that the IR emitting galaxies
tend to avoid the cluster center and are most likely due to galaxy
interactions (Pierre et al. 1996). For A 1300, we find the ISO
galaxies to be some 50% more luminous at 15 m than in our other
sample clusters; this may be interpreted in terms of enhanced star
formation associated with the merger (Lémonon et al. 1999).
Throughout the paper we assume km s
Mpc
and q0= 1/2.
Copyright The European Southern Observatory (ESO)