Colours of galaxies in different environments


In paper Melnyk, Mitronova & Karachentseva (2013) we showed that the colours of galaxies are strongly depend on the environmental influence. In this paper we made detailed comparisons between the near-infrared (2MASS; J-H) and optical (SDSS; g-r) colours of 2MIG galaxies with other objects from the wide range of denser environments (field, groups/clusters, triplets and pairs). We found that early type galaxies (E) show similar J-H and g-r colours practically in all types of environments (fig. 1). Exception is for the massive E galaxies located in compact pairs with high velocity difference (dV~180 km/s; CP1), 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 (M) and late (L) morphological types have redder near-infrared colours J-H than 2MIG isolated galaxies. The g-r colours of galaxies in groups (HDCG) and pairs (CP1, CP2) with high velocity difference are also significantly redder than corresponding colours of 2MIG galaxies (figs. 2 and 3). On the contrary, the members of most compact pairs (dV~50 km/s, R~30 kpc; KP+KT and CP3 samples) show the same g-r colour and even tend to be bluer than 2MIG galaxies. Our main results are illustrated in figures 1,2,3.