Colours and the Holmberg effect for galaxy pairs


Melnyk (2006) showed that the fraction of galaxies with UV excess in the sample of interacting galaxies in groups is 6%, single peculiar galaxies is 8%, and isolated galaxies is 4%. It was also found  that the number of active galaxies decreases with increasing population of the group of galaxies, while the frequency of occurrence of early-type (E/S0) galaxies increases.

A detailed study of the morphology of galaxy pairs is a necessary step in understanding the evolution of galaxies, since it is assumed that the two galaxies have evolved in parallel, under the influence of a common gravitational interaction and mutual gas dynamic processes over several billion years. This assumption was supported by the so-called Holmberg effect, in which the color indices of the galaxies in pairs are correlated. In his historic paper, Holmberg (1958) was the first to find a B-V dependence for 32 pairs of galaxies with a correlation coefficient of R  = +0.80 ±0.6. Similar strong correlations were noted later in a number of papers.

Colour-colour diagram for the pair components.
Fig. 7. (holg_ur) Colour-colour diagram for the pair components. The x-axis corresponds to the color index of the brighter component of a pair. EE— pairs of early types (-2-0), SS— pairs of spirals (1-6), LL— pairs of late spirals and irregular galaxies (7-10), and, correspondingly, mixed pairs: EL, LE, ES, SE, LS, SL, here the first type in the pair is the brighter component. (Fig. 5 from Melnyk, Dobrycheva & Vavoliva (2012).

Hovewer the authors of the latest papers, see for example Franco-Balderas et al. (2003, 2004), Allam et al. (2004) and Deng et al. (2010), noticed the weak not significant tendencies of the Holmberg effect or reported about the absence of any correlations.

In our paper Melnyk, Dobrycheva & Vavoliva (2012), we studied the Holberg effect on the basis of the pairs defined by the second order Voronoi tesselation method (Elyiv et al. 2009). We also did not find any significant correllations between the observed colours of pair members (fig. 7). The weak correlations in the colour indices could be fully explained by a dependence of the morphological type of a galaxy on its color index.