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Astron. Astrophys. 331, 493-505 (1998)

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6. Conclusions

We have re-examined the evidence for the existence of a Fundamental Plane (FP) for galaxy clusters, using the new data from the ESO Nearby Abell Cluster Survey (for velocity dispersions and correction for field contamination), and the Cosmos Galaxy Catalogue (for the characteristic radii of the galaxy distributions and the luminosities). We have derived the luminosities, radii and velocity dispersions for our dataset of 29 rich clusters in a homogeneous manner, and we have studied the correlations between these parameters. Our clusters are found to define a quite narrow FP, which confirms the result obtained earlier by S93.

Our result is qualitatively similar to that of S93 although there are quantitative differences that are partly due to the use of different profiles to describe the projected galaxy distribution. On the other hand, our result appears to indicate that the FP for galaxy clusters is significantly different from that for elliptical galaxies. However, more work is needed to establish accurately the qualitative difference between the two, which is a prerequisite for a more detailed understanding of the physical implications of the difference.

The FP that we find for our galaxy clusters differs from the virial prediction, primarily in the sense that the cluster luminosity appears to vary linearly, rather than quadratically, with velocity dispersion. There are several possible effects that could be responsible for that. E.g., cluster cores may have a variety of dynamical structures, or M /L may not be constant, or they may not all be fully virialized, (or combinations of these). Assuming that non-constant M /L is the only reason for the difference between the observed FP and the virial prediction, we estimate that the M /L ratio of rich clusters varies by at most a factor of 2 to 3.

We conclude that a significant part of the observed scatter around the FP is likely to be intrinsic. However, there may also be a contribution from distance errors that are induced by peculiar velocities of clusters with respect to the Hubble flow. Assuming that all the intrinsic scatter in the FP is due to deviations from a pure Hubble flow, we can set an upper limit to the typical cluster peculiar velocities of about 1000 km s-1 , in agreement with other, independent results.

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998