The determination of the galaxy distribution in clusters provides information on the status and the history of these structures through the study of their dynamics. Detailed studies spanning the entire range of morphologies of rich clusters of galaxies are important for understanding the formation and evolution of these systems. In a class of current cosmological models (e.g. cold dark matter dominated), rich clusters are formed hierarchically, by accretion of smaller subunits.
Several clusters are indeed known to present very lumpy morphologies (see e.g. Kriessler & Beers 1997 and references therein) revealing that these systems are in a merging process. The best studied examples are A2256 (Briel, Henry & Böhringer 1992), where a small group is detected in the X-ray band nearby the cluster center, and Coma, where a number of substructures are revealed (Biviano et al. 1996). Among the most spectacular cases are the encounters between clusters of similar richness, as for the A 3558 complex (Bardelli et al. 1994, 1996, 1998a, 1998b), where the dynamical processes reach unusual intensities, or the cluster A 3528, which is actually split into two merging X-ray emitting regions of similar properties (Schindler 1996).
The study of merging clusters is important because this process is thought to be responsible for a wide number of properties of the cluster galaxy population. Radio halos and relicts of radiosources are found in clusters that visually present some degree of disturbance (Feretti & Giovannini 1996) and Burns et al. (1994) explained as a consequence of a merging event the presence of post-starburst galaxies in the large scale X-ray emitting filament connecting Coma with the NGC 4839 group.
A good starting point to individuate merging cluster candidates is that to extract close pairs from supercluster catalogues, as f.i. the list of Zucca et al. (1993), which reports groups of ACO clusters (Abell, Corwin & Olowin 1989) as a function of the density contrast. In this catalogue, the cluster pairs individuated by a density excess are very close systems, where often the nuclei are separated by less than one Abell radius ( h-1 Mpc, hereafter h=Ho/100): one of these pairs is formed by A 548 and A 3367.
The centers of the two clusters in the ACO catalog are separated on the plane of the sky by 77 arcmin, corresponding to h-1 Mpc at the distance of A 548. The separation in velocity was less clear: in fact the cluster A 548 is reported to have a mean velocity of km/s (determined on 133 redshifts, Davis et al. 1995), while A 3367 had reported a value of km/s (based on 6 velocities, Postman, Huchra & Geller 1992). However, Postman & Lauer (1995) reported a velocity of 13461 km/s for the brightest member of A 3367, clearly inconsistent with the above mean value.
The cluster A 548 [; ] is a cluster of richness class 1 and Bautz-Morgan type III. This cluster has been extensively studied both in the optical and X-ray wavelength. Davis et al. (1995) reported a global velocity dispersion of 903 km/s, but this cluster appears dynamically very complex. From the analysis of a mosaic of ROSAT PSPC observations, Davis et al. (1995) found the presence of three extended sources (dubbed S1, S2 and S3) with luminosities in the range erg/s in the keV band. Performing a substructure analysis of the optical sample, they detected three subcondensations: two of these groups (labelled as a and b in their Table 4b) correspond to the extended X-ray emissions S1 and S2 respectively (see their Table 2). These optical subclumps were already found by Escalera et al. (1994) with the use of a wavelet decomposition analysis. They described A 548 as a binary cluster, when large spatial scales are considered. Moreover, they detected at smaller scales a central subgroup. These three components can be identified with the a, c and b substructures of Davis et al (1995), respectively. Another indication of the complex dynamical situation of this cluster appears from the different behaviour of galaxies with and without emission lines in their spectra: Biviano et al. (1997) in their analysis of the ENACS survey (Katgert et al. 1996, 1998) found a significant offset between the mean velocities of these two types of objects.
On the contrary, the cluster A 3367 was little studied so far. It has coordinates ; , is of richness class 0 and is classified as a Bautz-Morgan type I-II. No other relevant date was found in the literature apart the mean velocity reported above.
For these reasons we decided to concentrate our redshift survey on A 3367 and on the region between A 3367 and A 548, and in this paper we present a sample of 180 new radial velocities.
The paper is organized as follows: in Sect. 2 we present the sample and the data reduction, in Sect. 3 we discuss the dynamical properties of the three peaks found in the sample and finally in Sect. 4 we summarize our results.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998