The Pair Fraction of Massive Galaxies at 0 < z < 3

TL;DR

This study analyzes the evolution of the pair fraction of massive galaxies from redshift 0 to 3, finding a mild increase with redshift and estimating the number of major mergers, but suggesting other mechanisms influence galaxy size evolution.

Contribution

It provides new measurements of the pair fraction of massive galaxies at high redshift and models its evolution, contributing to understanding galaxy merger history.

Findings

Pair fraction at 1.7 < z < 3.0 is 0.15 ± 0.08.

Estimated average of 1.1 ± 0.5 major mergers per galaxy from z=3 to 0.

Merger rate shows no strong redshift evolution within uncertainties.

Abstract

Using a mass-selected ($M_{\star} \ge 10^{11} M_{\odot}$) sample of 198 galaxies at 0 < z < 3.0 with HST/NICMOS $H_{160}$-band images from the COSMOS survey, we find evidence for the evolution of the pair fraction above z ~ 2, an epoch in which massive galaxies are believed to undergo significant structural and mass evolution. We observe that the pair fraction of massive galaxies is 0.15 \pm 0.08 at 1.7 < z < 3.0, where galaxy pairs are defined as massive galaxies having a companion of flux ratio from 1:1 to 1:4 within a projected separation of 30 kpc. This is slightly lower, but still consistent with the pair fraction measured previously in other studies, and the merger fraction predicted in halo-occupation modelling. The redshift evolution of the pair fraction is described by a power law F(z) = (0.07 \pm 0.04) * (1+z) ^ (0.6 \pm 0.5). The merger rate is consistent with no redshift evolution, however it is difficult to constrain due to the limited sample size and the high uncertainties in the merging timescale. Based on the merger rate calculation, we estimate that a massive galaxy undergoes on average 1.1 \pm 0.5 major merger from z = 3 to 0. The observed merger fraction is sufficient to explain the number density evolution of massive galaxies, but insufficient to explain the size evolution. This is a hint that mechanism(s) other than major merging may be required to increase the sizes of the massive, compact quiescent galaxies from z ~ 2 to 0.