EMERGE: Constraining merging probabilities and timescales of close galaxy pairs

TL;DR

This paper develops empirical models to accurately predict galaxy merger rates from close pair observations up to redshift 4, highlighting the importance of selection criteria and dynamical factors.

Contribution

It introduces new fitting formulae for merging probabilities and timescales based on observable pair properties, improving merger rate estimates.

Findings

Merging probabilities follow a logistic function.

Merging timescales are linearly related to projected separation and velocity difference.

Selection criteria significantly affect the representativeness of merger rate estimates.

Abstract

Theoretical models are vital for exploring the galaxy merger process, which plays a crucial role in the evolution of galaxies. Recent advances in modelling have placed tight constraints on the buildup of stellar material in galaxies across cosmic time. Despite these successes, extracting the merger rates from observable data remains a challenge. Differences in modelling techniques, combined with limited observational data, drive conflicting conclusions on the merging timescales of close pairs. We employ an empirical model for galaxy formation that links galaxy properties to the growth of simulated dark matter halos, along with mock lightcone galaxy catalogues, to probe the dependencies of pair merging probabilities and merging timescales. In this work, we demonstrate that the pair merging probabilities are best described by a logistic function and that mean merging timescales can be well approximated by a linear relation in the projected separation and line of sight velocity difference in observed pairs. Together, our fitting formulae can accurately predict merger rates from galaxy pairs to at least $z\sim4$ under a wide variety of pair selection criteria. Additionally, we show that some commonly used pair selection criteria may not represent a suitable sample of galaxies to reproduce underlying merger rates. Finally, we conclude from our analysis that observation timescales are primarily driven by dynamics and are not strongly impacted by the star formation properties of the component galaxies.