GROUPS OF GALAXIES WITHIN 80 MPC .1. GROUPING HIERARCHICAL METHOD AND STATISTICAL PROPERTIES
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1992
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Abstract
A hierarchial algorithm, similar to Tully's (1987) one, has been devised and applied to an all-sky sample of 4143 galaxies comprising all the objects with an apparent diameter D25 larger than 100 arcsec and having known recession velocities smaller than 6000 km s-1 (i.e. closer than 80 Mpc, with H0 = 75 km s-1 Mpc-1). This sample is at least 84% complete to these limits of diameter and redshift. The hierarchy is built on the mass density of the aggregates progressively formed by the method, corrected for the loss of faint galaxies with the distance; this correction represents the main improvement upon Tully's treatment. In the method, a group is defined as an entity having an average luminosity density higher than 8 10(9) L(B). Mpc-3, chosen as to ensure that the group is gravitationally bound and does not follow the Hubble expansion. 264 groups of at least three members have been identified in this way, among which 82 have more than five members and are located at distances lower than 40 Mpc. Our sample represents the deepest and richest collection of groups homogeneous over both hemispheres and whose global properties do not present significant biases with the distance; it can thus be used confidently for a variety of statistical studies. A first quick analysis of the sample leads to the following main conclusions: (i) almost all the crossing times are lower than H0(-1), which confirms the bound nature of our groups; (ii) the median virial mass to blue luminosity ratio of the groups is 74 M. L(B).-1, a high value, but lower than those obtained in previous studies; (iii) we confirm clearly the increasing of the M/L ratio with the group size, a result which can be taken as an indication of the presence of dark matter around galaxies to a distance of 500 kpc.
Appendix A gives detailed definitions of the various group characteristic parameters used in the study, whereas new corrections for increasing incompleteness with the distance in Huchra & Geller's (1982) method are reported in Appendix B.
Appendix A gives detailed definitions of the various group characteristic parameters used in the study, whereas new corrections for increasing incompleteness with the distance in Huchra & Geller's (1982) method are reported in Appendix B.
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CLUSTERS, OF GALAXIES, GALAXIES, GENERAL, NUMERICAL METHODS