Global environmental effects versus galaxy interactions
  We explore properties of close galaxy pairs and merging systems selected from
the SDSS-DR4 in different environments with the aim to assess the relative
importance of the role of interactions over global environmental processes. For
this purpose, we perform a comparative study of galaxies with and without close
companions as a function of local density and host-halo mass, carefully
removing sources of possible biases. We find that at low and high local density
environments, colours and morphologies of close galaxy pairs are very similar
to those of isolated galaxies. At intermediate densities, we detect significant
differences, indicating that close pairs could have experienced a more rapid
transition onto the red sequence than isolated galaxies. The presence of a
correlation between colours and morphologies indicates that the physical
mechanism responsible for the colour transformation also operates changing
galaxy morphologies. Regardless of dark matter halo mass, we show that the
percentage of red galaxies in close pairs and in the control sample are
comparable at low and high local density environments. However, at intermediate
local densities, the gap in the red fraction between close pairs and the
control galaxies increases from ~10% in low mass haloes up to ~50% in the most
massive ones. Our findings suggest that in intermediate density environments
galaxies are efficiently pre-processed by close encounters and mergers before
entering higher local density regions. (Abridge)
