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

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5. Conclusions

We have presented a morphological and structural analysis of ABCG 194 both at optical and X-ray wavelengths. From dynamical and velocity dispersion studies, we are left with a sample of 91 galaxies really belonging to the cluster (4200-6100 km s-1), the mean cluster velocity being 5326 km s-1 with a standard deviation of 360 km s-1. This value coupled with the temperature of 2.6 keV determined for this cluster from ASCA data falls on the [FORMULA]-[FORMULA] relation (Wu et al. 1998). The main underlying structure (53 galaxies) has the same mean velocity but a standard deviation of 300 km s-1. The X-ray map shows an ellipticity similar to that observed in the optical.

The X-ray luminosity of ABCG 194 is [FORMULA] erg s-1, and falls notably below the [FORMULA]-[FORMULA] relation for clusters (Markevitch et al. 1998, Arnaud & Evrard 1999), i.e. this cluster is underluminous for its temperature. On the other hand, the cluster is within the dispersion of the [FORMULA]-[FORMULA] relation (Mahdavi et al. 1997). The [FORMULA]-model fit to the X-ray gas gives [FORMULA], which appears larger than expected from the low value of the X-ray gas temperature (Arnaud & Evrard 1999). A [FORMULA]-model fit to the galaxy density distribution also gives values of [FORMULA] consistent with 1, as in the ENACS sample (Adami et al. 1998).

ABCG 194 appears therefore to be a very poor cluster, comparable to those recently studied by Mahdavi et al. (1999), as confirmed by the stellar, X-ray gas and dynamical masses which we have calculated.

The number of counts in angular sectors from the X-ray [FORMULA]-model center (Fig. 7) shows privileged directions along the major axis PA found in the X-ray and optical analysis, but also along an axis perpendicular to this one. Except for individual sources, no X-ray substructures are found with a wavelet analysis. Only 9 X-ray sources are found with our method, while 38 are detected with the Snowden software.

The various analyses described above show that at large scale ABCG 194 is overall a relaxed cluster, with a few superimposed groups: the velocity distribution of the "Main" relaxed underlying cluster is not far from gaussian. However, both at optical and X-ray wavelengths, a bright linear structure is observed in the central part, and emission is much stronger south east of this line than north west. This central structure is well defined dynamically and is also rich in radio and X-ray galaxies.

We can try to interpret these results in the framework of hierarchical structure formation. As suggested by numerical simulations such as those by Tormen et al. (1997), the time interval between two possible major mergings is large compared to the timescale of violent relaxation. The cluster therefore has time to relax between two mergers. This is also the interpretation proposed for detailed features observed for example in the Coma cluster (Biviano et al. 1996). We suggest that in ABCG 194 a group around the bright elliptical galaxy NGC 547 may have merged into the cluster; due to dynamical friction, this group has stopped near the cluster center and was disrupted (González- Casado et al. 1994). The galaxy excess towards the south east could then be a remnant of this group.

ABCG 194 therefore appears as one of the very few poor clusters which are well studied at various wavelengths. Such objects are intermediate cases between rich clusters and groups, and as such are interesting to compare the physical processes taking place in these objects, which may be influenced by the very different relaxation timescales of these various systems.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999