Colours of isolated galaxies selected from the Two-Micron All-Sky Survey

The properties of isolated galaxies are driven mainly by intrinsic evolution and not by the external influence of their environments. Therefore, the sample of isolated galaxies can be considered as a reference sample to study different environmental effects. We have made detailed comparisons between the near-infrared (Two-Micron All-Sky Survey, 2MASS) and optical (Sloan Digital Sky Survey, SDSS) colours of the 2MASS Isolated Galaxies (2MIG) sample selected from the 2MASS Extended Source Catalogue and with other objects from the wide range of denser environments (field, groups/clusters, triplets and pairs). We have found that early-type galaxies show similar (J - H)(rest) and (g - r)(rest) colours in practically all types of environments, except for the massive early-type galaxies located in compact pairs with high velocity difference (dV similar to 180 km s(-1)), which are significantly redder and brighter than isolated galaxies. We assume that these pairs are located in the centres of more populated groups and clusters. In general, galaxies in groups and pairs of spiral and late morphological types have redder near-infrared colours (J - H)(rest) than 2MIG isolated galaxies. The (g - r)(rest) colours of galaxies in groups and pairs with high velocity difference are also significantly redder than the corresponding colours of 2MIG isolated galaxies. In contrast, the members of most compact pairs (dV similar to 50 km s(-1), R similar to 30 kpc) show the same (g - r)(rest) colour and even tend to be bluer than 2MIG isolated galaxies. In summary, our results show that the colours of galaxies depend very much on external factors.