Galaxy pairs in the Sloan Digital Sky Survey - VIII: The observational properties of post-merger galaxies

TL;DR

This study analyzes 10,800 galaxy pairs and 97 post-mergers from SDSS, revealing that post-mergers exhibit peak star formation, lower metallicity, and increased AGN activity, indicating recent coalescence effects.

Contribution

It provides observational evidence of enhanced star formation and AGN activity in post-merger galaxies, with detailed analysis of their properties compared to control samples.

Findings

Post-mergers show 3.5 times higher central SFR.

20% of post-mergers are starbursts.

AGN activity is 3.75 times more frequent in post-mergers.

Abstract

In order to investigate the effects of galaxy mergers throughout the interaction sequence, we present a study of 10,800 galaxies in close pairs and a smaller sample of 97 post-mergers identified in the Sloan Digital Sky Survey. We find that the average central star formation rate (SFR) enhancement (x 3.5) and the fraction of starbursts (20 per cent) peak in the post-merger sample. The post-mergers also show a stronger deficit in gas phase metallicity than the closest pairs, being more metal-poor than their control by -0.09 dex. Combined with the observed trends in SFR and the timescales predicted in merger simulations, we estimate that the post-mergers in our sample have undergone coalescence within the last few hundred Myr. In contrast with the incidence of star-forming galaxies, the frequency of active galactic nuclei (AGN) peaks in the post-mergers, outnumbering AGN in the control sample by a factor of 3.75. Moreover, amongst the galaxies that host an AGN, the black hole accretion rates in the closest pairs and post-mergers are higher by a factor of ~3 than AGN in the control sample. These results are consistent with a picture in which star formation is initiated early on in the encounter, with AGN activity peaking post-coalescence.