Galaxy pairs in the Sloan Digital Sky Survey - XI. A new method for measuring the influence of the closest companion out to wide separations

TL;DR

This paper introduces a new statistical method to measure how a galaxy's closest companion influences its properties across wide separations, revealing that nearby companions significantly increase galaxy asymmetry even at large distances.

Contribution

The authors develop a novel approach for assessing galaxy interactions over wide separations, effectively controlling for environment and galaxy properties, which improves upon traditional close pair studies.

Findings

Galaxy asymmetry is significantly increased by close companions.

Asymmetry enhancement peaks at <10 kpc separation.

Influence of companions persists out to at least 50 kpc.

Abstract

We describe a statistical approach for measuring the influence that a galaxy's closest companion has on the galaxy's properties out to arbitrarily wide separations. We begin by identifying the closest companion for every galaxy in a large spectroscopic sample of Sloan Digital Sky Survey galaxies. We then characterize the local environment of each galaxy by using the number of galaxies within 2 Mpc and by determining the isolation of the galaxy pair from other neighbouring galaxies. We introduce a sophisticated algorithm for creating a statistical control sample for each galaxy, matching on stellar mass, redshift, local density and isolation. Unlike traditional studies of close galaxy pairs, this approach is effective in a wide range of environments, regardless of how far away the closest companion is (although a very distant closest companion is unlikely to have a measurable influence on the galaxy in question). We apply this methodology to measurements of galaxy asymmetry, and find that the presence of nearby companions drives a clear enhancement in galaxy asymmetries. The asymmetry excess peaks at the smallest projected separations (< 10 kpc), where the mean asymmetry is enhanced by a factor of 2.0 $\pm$ 0.2. Enhancements in mean asymmetry decline as pair separation increases, but remain statistically significant (1-2$\sigma$) out to projected separations of at least 50 kpc.