2020A&A...640A..38R

Context. Isolated early-type galaxies 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.
Aims. 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 isolated early-type galaxies, and we quantitatively revise their galaxy classification.
Methods. We observed 20 (out of 104) isolated early-type galaxies, selected from the AMIGA catalog, with the 4KCCD camera at the Vatican Advanced Technology Telescope in the Sloan Digital Sky Survey g and r bands. These are the deepest observations of a sample of isolated early-type galaxies so far: on average, the light profiles reach µ_g≃28.11±0.70mag/arcsec² and µ_r≃27.36±0.68mag/arcsec². The analysis was performed using the AIDA package, providing point spread function-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.
Results. Our re-classification suggests that the sample is composed of bona fide early-type galaxies 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.
Conclusions. 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 isolated early-type galaxies have experienced such events. Because they are isolated (after 2-3Gyr), these galaxies are the cleanest environment in which to study phenomena connected with events like these.