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Astron. Astrophys. 349, 97-107 (1999)

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1. Introduction

It is presently believed that large scale structures (in particular clusters of galaxies) are formed through hierarchical clustering. From this point of view, clusters are formed by merging of smaller clusters or by groups falling onto a larger cluster. However, the effect of merging is not the same if a cluster undergoes a major merger or accretes small groups or even single galaxies (Salvador-Solé et al. 1998).

Notice that merging does not occur isotropically around a cluster. Groups tend to fall onto clusters along filaments, thus explaining the preferential orientations often observed (see e.g. West et al. 1995, Durret et al. 1998). Therefore the true orientation and the true ellipticity are interesting quantities to derive. The imprints of such merging events can be seen directly on the X-ray images, but also in substructures which can be detected at optical (density maps, velocity structure), X-ray and radio wavelengths; this was first noticed by Baier (1977) from optical data.

Recent studies have revealed that about 50% of clusters show multi-component structure both in their galaxy distribution (Bird 1994, Escalera et al. 1994, West 1994, Maccagni et al. 1995) and in X-rays (Mohr et al. 1993, Grebenev et al. 1995, West et al. 1995, Zabludoff & Zaritsky 1995). However one may notice that, when the amount of data grows and techniques of analysis are improved, this percentage appears to grow too (Girardi et al. 1997). This leads one to question the dynamical meaning of these substructures.

The properties of relatively poor and cool clusters are somewhat more difficult to analyze because of their limited signal, but prove to be interesting as a link between rich clusters and groups. Their relaxation time scales are larger and one can wonder if they are sufficiently virialized (e.g. Mahdavi et al. 1999 and references therein). The properties of the X-ray emitting gas can be different from those of rich/hot clusters (see e.g. David et al. 1996). In particular, various relations between optical and X-ray properties (as the [FORMULA] - [FORMULA] relation which seems linked to global properties), X-ray/X-ray relations (as the [FORMULA]-[FORMULA] relation) or X-ray to dynamical properties (as the X-ray gas fraction) may differ from those of rich clusters (Mahdavi et al. 1997, Markevitch 1998, Arnaud & Evrard 1999): they are probes to test cluster formation and evolution models, particularly the fine modeling of the gas heating (Cavaliere et al. 1998). Very few low temperature clusters are included in these relations, so adding even a single cluster at the border-line is important.

We present a multi-wavelength analysis of the cluster ABCG 194 based on optical data taken from the literature and on X-ray data from the ROSAT archive, coupled with different techniques of analysis.

ABCG 194 is a linear cluster of richness 0 and Bautz-Morgan type III (Struble & Rood 1982) with central coordinates [FORMULA] and [FORMULA] (Chapman et al. 1988, hereafter CGH). Its average redshift is 0.018, corresponding to a heliocentric velocity of 5340 km s-1, and to a spatial scaling of 1.87 Mpc/degree (H0 = 50 km/s/Mpc, q0= 0). The velocity interval corresponding to the cluster is 4000-6600 km s-1 according to CGH, but we will show in Sect. 4.2 that it is in fact 4300-6200 km s-1.

We have somewhat arbitrarily divided our analysis into two scales: the global aspect at large scale, and the smaller scale features. The first description is related to the cluster as an astrophysical object by itself, while for the second description, which takes into account as much as possible the kinematics, dynamical processes are important.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999