On actual presence of discordant-redshift galaxies in compact groups

Abstract

Hickson's compact galaxy groups were classified using the statistical criterion which includes the radial velocities of galaxies as well as their relative positions. These groups on the whole and their components are identified as the confident and probable non-chance ones as well as probable and confident chance ones. All confident chance objects have the discordant radial velocities (the differences of radial velocitiesDV ∈ [1000; 20000] km s−1). The special class of objects ‘bright discordants’ is selected. These galaxies have the discordant radial velocities withDV ∈ [825; 8440] km s−1 and have a strong tendency to be the brightest components of their groups. The lowest difference of radial velocities for the last class of objects 〈DV〉 = (1.0±0.2) × 103 km s−1 and we accept this value of DV as the lowest value of discordant radial velocities. It is found that the biggest part of Hickson's compact groups consist of non-chance aggregations of galaxies and some of the cases of discordant-redshifts require a special study in order to explain their origin from a dynamic or some other point of view.