Where Do Wet, Dry, and Mixed Galaxy Mergers Occur? A Study of the Environments of Close Galaxy Pairs in the Deep2 Galaxy Redshift Survey

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, aiming to establish a clear picture of how the cosmic evolution of various merger types relate to the observed large-scale extra-galactic environment and its role in the growth of redsequence galaxies. We find that the typical environment of mixed and dry mergers is denser than that of wet mergers, mostly due to the color-density relation. While the galaxy companion rate (N c ) 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. In other words, while wet mergers transform galaxies from the blue cloud into the red sequence at a similar fractional rate across different environments (assuming that the success rate of wet mergers to yield red galaxies does not depend on environment), the dry and mixed mergers are most effective in overdense regions. 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. Based on our results, we therefore expect that the properties, including structures and masses, of red-sequence galaxies should be different between those in underdense regions and in overdense regions since the dry mergers are significantly more important in dense environments. 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.

MASSIV: Mass Assembly Survey with SINFONI in VVDS

“…The T MM from Kitzbichler & White (2008) are typically longer than others in the literature (e.g., Patton & Atfield 2008;Lin et al 2010) or similar to those from N-body/hydrodynamical simulations (Lotz et al 2010b,a). Thus, we expect, if anything, a shorter T MM , which implies a larger number density of ETGs due to mergers.…”

“…We measured the median-weighted stellar mass from MASSIV sources in each of the three redshift bins under study, and estimated the merger time scale for these stellar masses. These time scales already include the factor C m (see Patton & Atfield 2008;Bundy et al 2009;Lin et al 2010;López-Sanjuan et al 2011), so we take C m = 1 in the following. In addition, López-Sanjuan et al (2011) show that the time scales Table 2.…”

Galaxy interactions

“…On the other hand, different authors (e.g. Kauffmann et al 2004;Baldry et al 2006;Cooper et al 2006;Skibba et al 2009;Lin et al 2010;Patton et al 2011) show that paired galaxies are found in higher density environments than their control sample and that interacting galaxies are redder before the interaction starts, owing to the older stellar population present in higher density environments. However, pairs and isolated galaxies in our samples are selected from equal density environments i.e.…”

“…Similar results were found by Ellison et al (2010), who analyzed the dependence of interactions on cluster-centric distance, finding that for close pairs the fraction of asymmetric galaxies (associated with tidal disruptions) is highest in the cluster centers. Moreover, Lin et al (2010) showed that the galaxy pair fraction increases with density. We also study the luminosity galaxy ranking in pairs within groups (Fig.…”

Investigating merging galaxies by using Pan-STARRS images

“…Mergers with composite colors in the elliptical region are classified as dry mergers, while mergers with combined colors in the starburst region are considered as wet mergers, and those objects with merged colors in the Sbc/Scd regions are regarded as mix mergers. It should be noted that our classifications are on the basis of blurred-together colors of both components of a merging system (e.g., luminosity-weighted combined colors), which is different from the original definition of "gas-rich wet mergers" or "gas-poor dry mergers" (e.g., Lin et al 2010). For comparison, we show an example of mergers of PS1 r -and y -band, CFHT r -band, and SDSS r -band in Fig.…”