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 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 (×3.5) and the fraction of starbursts (20 per cent) peak in the postmerger 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.

SPECTROSCOPIC STUDY OF STAR-FORMING GALAXIES IN FILAMENTS AND THE FIELD ATz∼ 0.5: EVIDENCE FOR ENVIRONMENTAL DEPENDENCE OF ELECTRON DENSITY

“…At small scales (a few tens of kpc), there is substantial evidence from observations and simulations for a decrement of metallicity and an enhancement of star-formation activity in galaxy close pairs, merging, and interacting systems compared to isolated, field galaxies, mostly attributed to the interaction-induced inflow of metal-poor gas from the periphery of interacting galaxies to the center, diluting their metal content, and increasing their gas fuel for star-formation (Mihos & Hernquist 1996;Kewley et al 2006;Ellison et al 2008Ellison et al , 2013Michel-Dansac et al 2008;Rupke et al 2010;Sol Alonso et al 2010;Perez et al 2011;Scudder et al 2012b;Ly et al 2014). At intermediate group scales where galaxy interactions are more common compared to cluster and field environments (Perez et al 2009;Tonnesen & Cen 2012)-owing to a combination of (1) a lower velocity dispersion of group galaxies relative to their cluster counterparts and (2) a higher number density of group galaxies compared to the field systems, which provide an ideal condition for interactions)-there is also evidence for a deficit of metals in group galaxies compared to control samples in the field (Lara-López et al 2013b), possibly owing to a higher fraction of interacting galaxies.…”

Supernovae and their host galaxies – II. The relative frequencies of supernovae types in spirals

“…Lambas et al 2012), and postmerging/remnant (single galaxies that exhibit signs of a past interaction, with a strong or already relaxed disturbance, see e.g. Ellison et al 2013 andLotz et al 2008, respectively). Fig.…”

Galaxy pairs in the SDSS – XIII. The connection between enhanced star formation and molecular gas properties in galaxy mergers

“…The most evident effect driven by galaxy encounters is probably the triggering of new episodes of star formation, which can occur both in the pre-merger regime between first pericentre and coalescence (e.g. Nikolic et al 2004;Patton et al 2011;Scudder et al 2012;Ellison et al, 2008aEllison et al, , 2013b, and in the post-merger phase, where the two nuclei of the interacting galaxies have merged together (e.g. Ellison et al 2013a;Kaviraj et al 2012Kaviraj et al , 2014.…”