Evolution of the Major Merger Galaxy Pair Fraction at z < 1

TL;DR

This study analyzes the evolution of the major merger galaxy pair fraction at redshifts below 1, revealing a flatter evolution and a lower pair fraction at low redshift than previously thought, based on a large, near-infrared selected galaxy sample.

Contribution

It provides the largest near-infrared selected galaxy pair sample at low redshift and refines the evolution of the major merger pair fraction with a flatter trend than earlier estimates.

Findings

Major-merger pair fraction is approximately 2% across a wide luminosity range.

The evolution of the pair fraction with redshift is flatter (m=0.7±0.1) than previously reported.

A typical L* galaxy has experienced about 0.2-0.8 major mergers since z=1.

Abstract

We present a study of the largest available sample of near-infrared selected (i.e., stellar mass selected) dynamically close pairs of galaxies at low redshifts ($z<0.3$). We combine this sample with new estimates of the major-merger pair fraction for stellar mass selected galaxies at $z<0.8$, from the Red Sequence Cluster Survey (RCS1). We construct our low-redshift $K-$band selected sample using photometry from the UKIRT Infrared Deep Sky Survey (UKIDSS) and the Two Micron All Sky Survey (2MASS) in the $K-$band ($\sim 2.2~\mu$m). Combined with all available spectroscopy, our $K-$band selected sample contains $\sim 250,000$ galaxies and is $> 90\%$ spectroscopically complete. The depth and large volume of this sample allow us to investigate the low-redshift pair fraction and merger rate of galaxies over a wide range in $K-$band luminosity. We find the major-merger pair fraction to be flat at $\sim 2\%$ as a function of $K-$band luminosity for galaxies in the range $10^8 - 10^{12} L_{\odot}$, in contrast to recent results from studies in the local group that find a substantially higher low-mass pair fraction. This low-redshift major-merger pair fraction is $\sim 40-50\%$ higher than previous estimates drawn from $K-$band samples, which were based on 2MASS photometry alone. Combining with the RCS1 sample we find a much flatter evolution ($m = 0.7 \pm 0.1$), in the relation $f_{\rm{pair}} \propto (1+z)^m$, than indicated in many previous studies. These results indicate that a typical $L\sim L^*$ galaxy has undergone $\sim 0.2-0.8$ major mergers since $z=1$ (depending on the assumptions of merger timescale and percentage of pairs that actually merge).