4 Conclusion

  We have constructed a fast method, based on the MST, to characterize the aggregation level of sets of galaxies. By using the first 3 momenta of the MST edge length histogram of a given set of galaxies, we are able to discriminate efficiently between different galaxy distributions compared to Poisson ones. The method is not very sensitive to a uniform background contamination. Even with a ratio equal to 5 between field galaxies and cluster members, we discriminate the structure. The implications of first results could be important, in particular to detect possible distant clusters (or structures) in deep photometric survey. The method also shows that the bright galaxies clusters are more aggregated than the faint ones in 75% of our selected sample. This result is very coherent with Adami et al. (1998) who show that the bright galaxies in clusters are more aggregated than the faint ones.

Acknowledgements

C.A. thanks V. Buat and H. Moulinec for useful discussions and B. Nichol for reading an earlier draft of this paper. C.A. and A.M. thanks J.D. Barrow for useful discussions.