Building a control sample for galaxy pairs

TL;DR

This study uses mock galaxy catalogues to evaluate how different methods of constructing control samples for galaxy pairs introduce biases, affecting the interpretation of interaction effects.

Contribution

It demonstrates how selection biases influence galaxy properties in control samples and proposes criteria to build unbiased control samples for studying galaxy interactions.

Findings

Galaxies in pairs tend to be older and more bulge-dominated.

Selection biases cause paired galaxies to appear in denser environments.

Imposing constraints reduces biases by approximately 70%.

Abstract

Several observational works have attempted to isolate the effects of galaxy interactions by comparing galaxies in pairs with isolated galaxies. However, different authors have proposed different ways to build these so-called control samples (CS). By using mock galaxy catalogues of the SDSS-DR4 built up from the Millennium Simulation, we explore how the way of building a CS might introduce biases which could affect the interpretation of results. We make use of the fact that the physics of interactions is not included in the semianalytic model, to infer that any difference between the mock control and pair samples can be ascribed to selection biases. Thus, we find that galaxies in pairs artificially tend to be older and more bulge-dominated, and to have less cold gas and different metallicities than their isolated counterparts. Also because of a biased selection, galaxies in pairs tend to live in higher density environments, and in haloes of larger masses. We find that imposing constraints on redshift, stellar masses and local densities diminishes the selection biases by ~70%. Based on these findings, we suggest observers how to build an unique and unbiased CS in order to reveal the effect of galaxy interactions.