Remnants of recent mergers in nearby early-type galaxies and their classification

Abstract

We search for signatures of recent galaxy close interactions and mergers in a sample of 202 early-type galaxies in the local universe from the public SDSS Stripe82 deep images (μr ∼ 28.5 mag arcsec−2). Using two different methods to remove galaxies’ smooth and symmetric light distribution, we identify and characterize 11 distinct types of merger remnants embedded in the diffuse light of these early-type galaxies. We discuss how the morphology of merger remnants can result from different kinds of minor and major mergers, and estimate the fraction of early-type galaxies in the local universe with evidence of recent major (27 per cent) and minor (57 per cent) mergers. The merger fractions deduced are higher than in several earlier surveys. Among remnants, we find that shells are the dominant merger debris (54 per cent) associated with early-type galaxies, resulting from both major and minor mergers, with those characteristics of major mergers being significant (24 per cent of shell host galaxies). The most uncommon merger-related structures are boxy isophotes of the stellar distribution and the presence of disc fragments near the cores of galaxies. We develop a classification scheme for these fine structures that may be used to infer their likely genesis histories. The classification is primarily based on the mass ratios of the merged galaxies. This work, when combined with predictions from numerical simulations, indicates that most (if not all) early-type galaxies in the local Universe are continually evolving as a result of (minor) merger activities.