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

Abstract

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