Redshift Asymmetry and Color-VelocityCorrelation in Nearby Galaxy Groups: Evidence of Dust?

An excess of higher redshift galaxies, relative to the brightest member, in loose groups, was discovered by Arp in 1970, and was later confirmed by several authors. This effect is found to be present also in nearby small groups, identified by Tully in the Nearby Galaxy Catalogue. The asymmetry found is not likely to be ascribed to contamination by interlopers. A significant anticorrelation has also been found between the color excess E(B-V)(0)T and the velocity difference, with an associated E(B-V)(0)T variation on the order of 0.1-0.2. Such anticorrelation is not induced by obvious selection effects, such as the color-magnitude relation. A possible explanation is that groups are still collapsing and contain diffuse dust in the intragroup medium, with a mean optical depth, in the blue bland, on the order of 0.1-0.2. The mean dust density inside groups would be about 4 x 10 exp -31 g/cu cm; if the presence of dust is not only a local effect, the deduced cosmological density Omega(dust) would be in the range 10 exp -4 to 10 exp -5.