On the pairwise velocity dispersion of galaxies

Previous statistical results have shown that the pairwise velocity dispersion | ₆ (r ₏) | of galaxies at small separations | (r ₏ 1h^-1Mpc) | is about 300 km s–1. This value is significantly lower than the predictions of currently favoured models of structure formation, unless a large pairwise velocity bias exists between galaxies and the underlying mass. Using various samples of galaxies (CfA, SSRS, IRAS, CfA slice, samples containing ellipticals or spirals), we show that the results of | ₆ (r ₏) | are sensitive to sampling effects, such as the inclusion of dominating clusters, and the correction of the Virgocentric infall. The value of | ₆ (r ₏=1h^-1Mpc) | is as high as | 1000 km\, s^-1| for the CfA slice including Coma, and as low as | 300 km\, s^-1| for the spiral sample and for the CfA sample with a correction of the Virgocentric infall. The values of σg that we obtain are consistent with the high values predicted by currently favoured models of structure formation, which suggests that a high pairwise velocity bias between galaxies and mass is not required by present data.