Interacting galaxies in the nearby Universe: only moderate increase of
  star formation
  We investigate the influence of interactions on the star formation by
studying a sample of almost 1500 of the nearest galaxies, all within a distance
of ~45 Mpc. We define the massive star formation rate (SFR), as measured from
far-IR emission, and the specific star formation rate (SSFR), which is the
former quantity normalised by the stellar mass of the galaxy, and explore their
distribution with morphological type and with stellar mass. We then calculate
the relative enhancement of these quantities for each galaxy by normalising
them by the median SFR and SSFR values of individual control populations of
similar non-interacting galaxies. We find that both SFR and SSFR are enhanced
in interacting galaxies, and more so as the degree of interaction is higher.
The increase is, however, moderate, reaching a maximum of a factor of 1.9 for
the highest degree of interaction (mergers). The SFR and SSFR are enhanced
statistically in the population, but in many individual interacting galaxies
they are not enhanced at all. We discuss how those galaxies with the largest
SFR and/or SSFR enhancement can be defined as starbursts. This study is based
on a representative sample of nearby galaxies, including many low-mass and
dwarf/irregular galaxies, and we argue that it should be used to place
constraints on studies based on samples of galaxies at larger distances, beyond
the local Universe.
