  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.
