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Astron. Astrophys. 356, 815-826 (2000)

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6. Discussion and conclusions

The optical analysis of the Abell 496 field has shown the existence of several structures along the line of sight. Among these, one (structure 6) is likely to be a poor, diffuse and low mass cluster, while two others (structures 4 and 9) are probably filaments more or less aligned along the line of sight, the latter presenting a smooth velocity gradient. Notice that the distances between these various structures are very large.

The cluster Abell 496 itself has quite a regular morphology. It includes 274 galaxies in the [7813,11860 km s-1] velocity range and has a velocity dispersion of 715 km s-1. Its velocity distribution implies a small amount of substructure. The analysis of the correlations between position, luminosity and velocity dispersion indicates a post-violent relaxation state. We can notice that both the distance to the cluster center and the overall velocity dispersion ranges are reduced when emission line galaxies (hereafter ELGs) are excluded. This agrees with the general scheme that ELGs are often found in the outskirts of clusters and are not as strongly tied to the cluster gravitationally (e.g. Biviano et al. 1997). There may be two samples of ELGs falling on to the main cluster, one from the back (the ELGs concentrated towards the west) and one from the front (the high velocity ELGs).

The bright luminosity function derived from our redshift catalogue shows a flattening at R[FORMULA] (M[FORMULA]), comparable to similar shapes found in other clusters. This suggests at least a bimodal distribution, one for ellipticals and one for fainter galaxies. The fact that the flattening occurs at the same absolute magnitude as for other clusters suggests that the galaxy populations in all these clusters are comparable.

At fainter magnitudes, galaxy counts derived from CCD imaging show a dip at R[FORMULA] (M[FORMULA]) which can be reproduced if we assume a magnitude cut-off similar to that observed in Coma (Adami et al. 2000). Notice that such a cut-off is observed in the very central regions of both clusters. Although this result is only based on imaging and remains to be confirmed spectroscopically, we may be evidencing a second example of a cut-off of the faint end of the luminosity function in a cluster.

We have modelled the X-ray gas and derived the X-ray gas mass and the dynamical mass, which we compare to the stellar mass. At the limiting radius (1.62 h[FORMULA] Mpc) of the image, we find a fraction of X-ray gas to total mass of 0.12-0.15 and a stellar to X-ray gas mass ratio of 0.16. We can note that Abell 496 follows exactly the two by two correlations between the X-ray luminosity (L[FORMULA] erg s-1, Wu et al. 1999), the X-ray temperature (T[FORMULA] keV) and the galaxy velocity dispersion ([FORMULA] km s-1) described in the literature (see e.g. Wu et al. 1999 and references therein). These values are typical of a richness class 1 cluster.

Abell 496 therefore appears to be a relatively quiet and simple cluster, with no strong environmental effects, although we may see an enhancement of the X-ray emission and of the number of emission line galaxies towards the north west.

While Coma has long been the archetype of a relaxed cluster and is not believed to be relaxed any more (see Biviano 1998 and the proceedings of the Coma meeting), the results presented above suggest that Abell 496 may be such a prototype, and can be used as a "template" in the future study of more complex (i.e. substructured) clusters.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000
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