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The ESO nearby Abell cluster survey
Received 10 February 1998 / Accepted 14 April 1998
We have analyzed the projected galaxy distributions in a subset of the ENACS cluster sample, viz. in those 77 clusters that have z 0.1 and and for which ENACS and COSMOS data are available. For 20 % of these, the distribution of galaxies in the COSMOS catalogue does not allow a reliable centre position to be determined. For the other 62 clusters, we first determined the centre and elongation of the galaxy distribution. Subsequently, we made Maximum-Likelihood fits to the distribution of COSMOS galaxies for 4 theoretical profiles, two with `cores' (generalized King- and Hubble-profiles) and two with `cusps' (generalized Navarro, Frenk and White, or NFW, and de Vaucouleurs profiles).
We obtain average core radii (or characteristic radii for the profiles without core) of 128, 189, 292 and 1582 kpc for fits with King, Hubble, NFW and de Vaucouleurs profiles respectively, with dispersions around these average values of 88, 116, 191 and 771 kpc. The surface density of background galaxies is about 4 10-5 gals arcsec-2 (with a spread of about 2 10-5), and there is very good agreement between the values found for the 4 profiles. There is also very good agreement on the outer logarithmic slope of the projected galaxy distribution, which is that for the non-generalized King- and Hubble-profile (i.e. = = 1, with the corresponding values for the two other model-profiles).
We use the Likelihood ratio to investigate whether the observations are significantly better described by profiles with cusps or by profiles with cores. Taking the King and NFW profiles as `model' of either class, we find that about 75% of the clusters are better fit by the King profile than by the NFW profile. However, for the individual clusters the preference for the King profile is rarely significant at a confidence level of more than 90%. When we limit ourselves to the central regions it appears that the signifance increases drastically, with 65% of the clusters showing a strong preference for a King over an NFW profile. At the same time, about 10% of the clusters are clearly better fitted by an NFW profile than by a King profile in their centres.
We constructed composite clusters from the COSMOS and ENACS data, taking special care to avoid the creation of artificial cusps (due to ellipticity), and the destruction of real cusps (due to non-perfect centering). When adding the galaxy distributions to produce a composite cluster, we either applied no scaling of the projected distances, scaling with the core radii of the individual clusters or scaling with , which is designed to take differences in mass into account. In all three cases we find that the King profile is clearly preferred (at more than 95% confidence) over the NFW profile (over the entire aperture of 5 core-radii). However, this `preference' is not shared by the brightest galaxies. We conclude that the brighter galaxies are represented almost equally well by King and NFW profiles, but that the distribution of the fainter galaxies clearly shows a core rather than a cusp.
Finally, we compared the outer slope of the galaxy distributions in our clusters with results for model calculations for various choices of fluctuation spectrum and cosmological parameters. We conclude that the observed profile slope indicates a low value for . This is consistent with the direct estimate of based on the -ratios of the individual clusters.
Key words: galaxies: clusters: general galaxies: kinematics and dynamics cosmology: observations cosmology: large-scale structure of the Universe
* Based on observations collected at the European Southern Observatory (La Silla, Chile)
Send offprint requests to: C. Adami
Online publication: July 7, 1998