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

TL;DR

This study identifies and classifies various merger remnants in nearby early-type galaxies using deep SDSS images, revealing high recent merger activity and diverse morphological features indicative of ongoing galaxy evolution.

Contribution

It introduces a new classification scheme for merger remnants based on their morphology and merger history, supported by analysis of a large galaxy sample.

Findings

27% of galaxies show evidence of recent major mergers

57% of galaxies show evidence of recent minor mergers

Shell structures are the most common merger remnants, present in 54% of cases

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 ($\mu_r \sim 28.5$ mag arcsec$^{-2}$). Using two different methods to remove galaxies' smooth and symmetric light distribution, we identify and characterize eleven 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%) and minor (57%) mergers. The merger fractions deduced are higher than in several earlier surveys. Among remnants, we find that shells are the dominant merger debris (54%) associated with early-type galaxies, resulting from both major and minor mergers, with those characteristics of major mergers being significant (24% of shell host galaxies). The most uncommon merger-related structures are boxy isophotes of the stellar distribution and the presence of disk 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.