Galaxy Zoo: Quantifying Morphological Indicators of Galaxy Interaction

TL;DR

This study uses Galaxy Zoo 2 classifications to identify and quantify morphological features of galaxy interactions, revealing how features like spiral arms and bars vary with pair separation and improving estimates of interaction frequency.

Contribution

It introduces a robust method to quantify morphological signatures of galaxy interactions, refining interaction frequency estimates using visual classifications and separation scales.

Findings

Spiral features are enhanced at separations < 70 h^-1 kpc.

Bars are suppressed at separations < 30 h^-1 kpc.

Interaction fraction is estimated between 0.4% and 2.7% for low-redshift galaxies.

Abstract

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.