  We use a sample of close galaxy pairs selected from the Sloan Digital Sky
Survey Data Release 4 (SDSS DR4) to investigate in what environments galaxy
mergers occur and how the results of these mergers depend on differences in
local galaxy density. The galaxies are quantified morphologically using
two-dimensional bulge-plus-disk decompositions and compared to a control sample
matched in stellar mass, redshift and local projected density. Lower density
environments have fractionally more galaxy pairs with small projected
separations (r_p) and relative velocities (Delta v), but even high density
environments contain significant populations of pairs with parameters that
should be conducive to interactions. Metrics of asymmetry and colour are used
to identify merger activity and triggered star formation. The location of star
formation is inferred by distinguishing bulge and disk colours and calculating
bulge fractions from the SDSS images. Galaxies in the lowest density
environments show the largest changes in star formation rate, asymmetry and
bulge-total fractions at small separations, accompanied by bluer bulge colours.
At the highest local densities, the only galaxy property to show an enhancement
in the closest pairs is asymmetry. We interpret these results as evidence that
whilst interactions (leading to tidal distortions) occur at all densities,
triggered star formation is seen only in low-to-intermediate density
environments. We suggest that this is likely due to the typically higher gas
fractions of galaxies in low density environments. Finally, by
cross-correlating our sample of galaxy pairs with a cluster catalogue, we
investigate the dependence of interactions on clustercentric distance. It is
found that for close pairs the fraction of asymmetric galaxies is highest in
the cluster centres.
