Galaxy Zoo: The properties of merging galaxies in the nearby Universe - local environments, colours, masses, star-formation rates and AGN activity

TL;DR

This study analyzes the properties of merging galaxies in the nearby universe, revealing that internal galaxy features influence merger detectability and star formation activity, with implications for galaxy evolution models.

Contribution

It provides a detailed analysis of merging galaxy properties, highlighting the role of internal characteristics in merger detection and star formation, and links transition mass to disc survival.

Findings

Merging galaxies show a wider colour spread with a blue tail from star formation.

Star formation rates double in spiral mergers but are unchanged in ellipticals.

Environmental distributions of mergers are similar to control samples, suggesting internal factors drive detectability.

Abstract

Following the study of Darg et al. (2009; hereafter D09a) we explore the environments, optical colours, stellar masses, star formation and AGN activity in a sample of 3003 pairs of merging galaxies drawn from the SDSS using visual classifications from the Galaxy Zoo project. While D09a found that the spiral-to-elliptical ratio in (major) mergers appeared higher than that of the global galaxy population, no significant differences are found between the environmental distributions of mergers and a randomly selected control sample. This makes the high occurrence of spirals in mergers unlikely to be an environmental effect and must, therefore, arise from differing time-scales of detectability for spirals and ellipticals. We find that merging galaxies have a wider spread in colour than the global galaxy population, with a significant blue tail resulting from intense star formation in spiral mergers. Galaxies classed as star-forming using their emission-line properties have average star-formation rates approximately doubled by the merger process though star formation is negligibly enhanced in merging elliptical galaxies. We conclude that the internal properties of galaxies significantly affect the time-scales over which merging systems can be detected (as suggested by recent theoretical studies) which leads to spirals being `over-observed' in mergers. We also suggest that the transition mass $3\times10^{10}{M}_{\astrosun}$, noted by \citet{kauffmann1}, below which ellipticals are rare could be linked to disc survival/destruction in mergers.