Morphology and color indices of galaxies in Pairs: Criteria for the classification of galaxies

Abstract

We have examined the relationship between the morphological types and the u-r, g-i, and r-z color indices of 365 pairs of galaxies with radial velocities 3000<V<9500 km/s selected from the SDSS DR5 survey using the second order Voronoi tesselation method. It is shown that the paired galaxies include a smaller fraction of blue compact galaxies than isolated galaxies. Using the relationship between the color index and a parameter such as the inverse concentration index, absolute magnitude, and the de Vaucouleurs or scale radius, we propose a “color indices criterion” for the morphological classification of galaxies into three classes: (1) elliptical and lenticular E-S0, (2) spiral Sa-Scd, and (3) late spiral Sd-Sdm and irregular Im/BCG. We did not detect the Holmberg effect in our sample; the correlation found in the color indices of the components of galactic pairs is fully explained by the dependence of the morphological type of a galaxy on its color index.