Galaxy interactions I: Major and minor mergers
  We study galaxy pair samples selected from the Sloan Digital Sky Survey
(SDSS-DR7) and we perform an analysis of minor and major mergers with the aim
of investigating the dependence of galaxy properties on interactions. We build
a galaxy pair catalog requiring rp < 25 kpc h-1 and Delta V < 350 km s-1 within
redshift z<0.1. By visual inspection of SDSS images we removed false
identifications and we classify the interactions into three categories: pairs
undergoing merging, M; pairs with evident tidal features, T; and non disturbed,
N. We also divide the pair sample into minor and major interactions according
to the luminosity ratio of the galaxy members. We study star formation activity
through colors and star formation rates. We find that 10% of the pairs are
classified as M. These systems show an excess of young stellar populations as
inferred from the Dn(4000) spectral index, colors, and star formation rates of
the member galaxies, an effect which we argue, is directly related to the
ongoing merging process. We find 30% of the pairs exhibiting tidal features (T
pairs) with member galaxies showing evidence of old stellar populations.
Regardless of the color distribution, we find a prominent blue peak in the
strongest mergers, while pairs with tidal signs under a minor merger show a
strong red peak. Therefore, our results show that galaxy interactions are
important in driving the evolution of galaxy bimodality. By adding stellar
masses and star formation rates of the two members of the pairs, we explore the
global efficiency of star formation of the pairs as a whole. We find that, at a
given total stellar mass, major mergers are significantly more efficient (a
factor 2) in forming new stars, with respect to both minor mergers or a control
sample of non-interacting galaxies.
