Morphology and surface photometry of a sample of isolated early-type galaxies from deep imaging

TL;DR

This study uses deep imaging to analyze the structure of isolated early-type galaxies, revealing signs of past interactions like shells and tails, and revising their classification.

Contribution

It provides the deepest surface photometry of isolated early-type galaxies and identifies prevalent fine structures indicating past interactions.

Findings

Shells are present in about 60% of the sample.

Most galaxies are best fitted with a bulge plus disc model.

Fine structures suggest past mergers or accretion events.

Abstract

Isolated early-type galaxies (iETGs) are evolving in unusually poor environments for this morphological family, which is typical of cluster inhabitants. We investigate the mechanisms driving the evolution of these galaxies. Several studies indicate that interactions, accretions, and merging episodes leave their signature on the galaxy structure, from the nucleus down to the faint outskirts. We focus on revealing such signatures, if any, in a sample of iETGs, and we quantitatively revise their galaxy classification. We observed 20 (out of 104) iETGs, selected from the AMIGA catalog, with the 4KCCD camera at the VATT in the SDSS g and r bands. These are the deepest observations of a sample of iETGs so far. The analysis was performed using the AIDA package, providing PSF-corrected 2D surface photometry up to the galaxy outskirts. The package provides a model of the 2D galaxy light distribution, which after model subtraction enhances the fine and peculiar structures in the residual image of the galaxies. Our re-classification suggests that the sample is composed of bona fide ETGs spanning from ellipticals to late-S0s galaxies. Most of the surface brightness profiles are best fitted with a bulge plus disc model, suggesting the presence of an underlying disc structure. The residuals obtained after the model subtraction show the nearly ubiquitous presence of fine structures, such as shells, stellar fans, rings, and tails. Shell systems are revealed in about 60% of these galaxies. Because interaction, accretion, and merging events are widely interpreted as the origin of the fans, ripples, shells and tails in galaxies, we suggest that most of these iETGs have experienced such events. Because they are isolated (after 2-3 Gyr), these galaxies are the cleanest environment in which to study phenomena connected with events like these.