Galaxy Zoo: Quantifying Morphological Indicators of Galaxy Interaction
  We use Galaxy Zoo 2 visual classifications to study the morphological
signatures of interaction between similar-mass galaxy pairs in the Sloan
Digital Sky Survey. We find that many observable features correlate with
projected pair separation; not only obvious indicators of merging, disturbance
and tidal tails, but also more regular features, such as spiral arms and bars.
These trends are robustly quantified, using a control sample to account for
observational biases, producing measurements of the strength and separation
scale of various morphological responses to pair interaction. For example, we
find that the presence of spiral features is enhanced at scales < 70 h^-1 kpc,
probably due to both increased star formation and the formation of tidal tails.
On the other hand, the likelihood of identifying a bar decreases significantly
in pairs with separations < 30 h^-1 kpc, suggesting that bars are suppressed by
close interactions between galaxies of similar mass. We go on to show how
morphological indicators of physical interactions provide a way of
significantly refining standard estimates for the frequency of close pair
interactions, based on velocity offset and projected separation. The presence
of loosely wound spiral arms is found to be a particularly reliable signal of
an interaction, for projected pair separations up to ~100 h^-1 kpc. We use this
indicator to demonstrate our method, constraining the fraction of low-redshift
galaxies in truly interacting pairs, with M_* > 10^9.5 M_Sun and mass ratio <
4, to be between 0.4 - 2.7 per cent.
