On the methods of discovering groups and clusters of galaxies

Extract

Groups. Even if real groups did not exist, the investigation of large regions of the sky will reveal other dense clumpings of galaxies. the usual estimate of the number of optical members in these clumpings will confirm their reality. But let us remember that all these groups are false. False groupings of galaxies should emerge in large numbers because of the effects of interstellar absorption and of variability of the observating conditions. Statistical considerations of the groups detected by Turner and Gott (1976, Ap. J., Suppl. Ser., 32, 409) leads to the conclusion that the percentage of false groups members is equal to 40% or more.