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1 Introduction

Massive clusters of galaxies are fundamental cosmological probes because their formation rate -- and formation history -- depends strongly on both the value of $\Omega$ 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 $z \sim 0.5$, 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 $\sim40$ clusters located in the Hydra region (Pierre et al. 1994a).

Table 1: The cluster sample

{\vert\vert c\vert c\vert c\vert\vert}
\hline \hline
ROSAT ID & ...
 ... 0.105 \\  RX J1325.1$-$2013 & Abell 1732 & 0.192 \\  \hline \hline\end{tabular}
Note that, according to the RASS, these objects all have a luminosity $\geq 5\ 10^{44}$ ergs-1. However, our subsequent deep HRI pointings have revealed that some are contaminated by a point-like source -- i.e., an AGN -- which is supported by the presence of a strong radio source at the cluster center. This will be discussed in detail by Lémonon et al. (1998).

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 $z \sim 0.6$, 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 $z \sim 0.1$ 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 ($L_{\rm X}$, $T_{\rm X}$, metallicity) can give an indication of the degree of cluster relaxation.

Table 2: Cluster data

{\vert\vert c\vert c\vert c\vert c\vert c\vert\vert}
\hline \hli...
 ... & 4.5 \\ A 1732 & 0.193 & 1100 & 9.2 (HRI) & 7.8 \\  \hline
Velocity dispersions and luminosities are from Lémonon et al. (1999). $N_{\rm H}$ is a $\lambda$21cm value from Dickey & Lockman (1990).

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 $\mu$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 $H_{0}\!=\!50$ km s$^{\!-1}$ Mpc$^{\!-1}$and q0= 1/2.

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