Spheroidal post-mergers in the local Universe

TL;DR

This study investigates spheroidal post-merger galaxies in the local universe, revealing their bluer colors, increased AGN activity, and diverse progenitor types, providing insights into galaxy evolution after mergers.

Contribution

It identifies and characterizes spheroidal post-mergers, highlighting their properties and the evolution of activity phases post-merger, which was less understood before.

Findings

20 of 30 SPMs show LINER or Seyfert activity

AGNs increase in post-mergers compared to ongoing mergers

Most SPMs result from major mergers involving late-type galaxies

Abstract

Galaxy merging is a fundamental aspect of the standard hierarchical galaxy formation paradigm. Recently, the Galaxy Zoo project has compiled a large, homogeneous catalogue of 3373 mergers, through direct visual inspection of the entire SDSS spectro- scopic sample. We explore a subset of galaxies from this catalogue that are spheroidal 'post-mergers' (SPMs) - where a single remnant is in the final stages of relaxation after the merger and shows evidence for a dominant bulge, making them plausible progenitors of early-type galaxies. Our results indicate that the SPMs have bluer colours than the general early-type galaxy population possibly due to merger-induced star formation. An analysis using optical emission line ratios indicates that 20 of our SPMs exhibit LINER or Seyfert-like activity (68%), while the remaining 10 galaxies are classified as either star forming (16%) or quiescent (16%). A comparison to the emission line activity in the ongoing mergers from Darg et al. indicates that the AGN fraction rises in the post-mergers, suggesting that the AGN phase probably becomes dominant only in the very final stages of the merger process. The optical colours of the SPMs and the plausible mass ratios for their progenitors indicate that, while a minority are consistent with major mergers between two early-type galaxies, the vast majority are remnants of major mergers where at least one progenitor is a late-type galaxy.