A consistent measure of the merger histories of massive galaxies using close-pair statistics I: Major mergers at $z<3.5$

Abstract

We use a large sample of $\sim 350,000$ galaxies constructed by combining the UKIDSS UDS, VIDEO/CFHT-LS, UltraVISTA/COSMOS and GAMA survey regions to probe the major merging histories of massive galaxies ($>10^{10}{\mathrm{M}}_{\odot }$) at $0.005<z<3.5$. We use a method adapted from that presented in Lopez-Sanjuan et al. (2014) using the full photometric redshift probability distributions, to measure pair $\text{fractions}$ of flux-limited, stellar mass selected galaxy samples using close-pair statistics. The pair fraction is found to weakly evolve as $\propto (1+z{)}^{0.8}$ with no dependence on stellar mass. We subsequently derive major merger $\text{rates}$ for galaxies at $>10^{10}{\mathrm{M}}_{\odot }$ and at a constant number density of $n>10^{-4}$ Mpc${}^{-3}$, and find rates a factor of 2-3 smaller than previous works, although this depends strongly on the assumed merger timescale and likelihood of a close-pair merging. Galaxies undergo approximately 0.5 major mergers at $z<3.5$, accruing an additional 1-4 $\times 10^{10}{\mathrm{M}}_{\odot }$ in the process. Major merger accretion rate densities of $\sim 2\times 10^{-4}$ ${\mathrm{M}}_{\odot }$ yr${}^{-1}$ Mpc${}^{-3}$ are found for number density selected samples, indicating that direct progenitors of local massive ($>10^{11}{\mathrm{M}}_{\odot }$) galaxies have experienced a steady supply of stellar mass via major mergers throughout their evolution. While pair fractions are found to agree with those predicted by the Henriques et al. (2014) semi-analytic model, the Illustris hydrodynamical simulation fails to quantitatively reproduce derived merger rates. Furthermore, we find major mergers become a comparable source of stellar mass growth compared to star-formation at $z<1$, but is 10-100 times smaller than the SFR density at higher redshifts.

26 pages, 18 figures, accepted to MNRAS

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