Where do Wet, Dry, and Mixed Galaxy Mergers Occur? A Study of the Environments of Close Galaxy Pairs in the DEEP2 Galaxy Redshift Survey

TL;DR

This study investigates the environments of wet, dry, and mixed galaxy mergers at redshift 0.75-1.2, revealing that dry mergers predominantly occur in dense regions and significantly contribute to the mass growth of massive red galaxies.

Contribution

It provides new insights into the environmental dependence of different galaxy merger types and their role in galaxy evolution at z~1.

Findings

Dry mergers are more common in dense environments.

Wet merger rates show marginal dependence on environment.

Dry mergers contribute significantly to the mass assembly of red galaxies.

Abstract

We study the environment of wet, dry, and mixed galaxy mergers at 0.75 < z < 1.2 using close pairs in the DEEP2 Galaxy Redshift Survey. We find that the typical environment of mixed and dry merger candidates is denser than that of wet mergers, mostly due to the color-density relation. While the galaxy companion rate (Nc) is observed to increase with overdensity, using N-body simulations we find that the fraction of pairs that will eventually merge decreases with the local density, predominantly because interlopers are more common in dense environments. After taking into account the merger probability of pairs as a function of local density, we find only marginal environment dependence of the fractional merger rate for wet mergers over the redshift range we have probed. On the other hand, the fractional dry merger rate increases rapidly with local density due to the increased population of red galaxies in dense environments. We also find that the environment distribution of K+A galaxies is similar to that of wet mergers alone and of wet+mixed mergers, suggesting a possible connection between K+A galaxies and wet and/or wet+mixed mergers. We conclude that, as early as z ~ 1, high-density regions are the preferred environment in which dry mergers occur, and that present-day red-sequence galaxies in overdense environments have, on average, undergone 1.2+-0.3 dry mergers since this time, accounting for (38+-10)% of their mass accretion in the last 8 billion years. Our findings suggest that dry mergers are crucial in the mass-assembly of massive red galaxies in dense environments, such as Brightest Cluster Galaxies (BCGs) in galaxy groups and clusters.