GALEX UV properties of the polar ring galaxy MCG-05-07-001 and the shell
  galaxies NGC 1210 and NGC 5329
  Systems of shells and polar rings in early-type galaxies are considered "bona
fide" tracers of mass accretion and/or mergers. Their high frequency in low
density environments suggests that such episodes could drive the evolution of
at least a fraction of the early-type galaxy population. Their UV emission is
crucial to test whether these galaxies host ongoing/recent star formation. We
used far and near ultraviolet, optical, near infrared images, HI maps, and line
strength indices to investigate the nuclear and outer regions of the galaxies
as well as the regions where fine structures are present. The GALEX Near (NUV)
and Far UV (FUV) images of MCG-05-07-001 and NGC 1210 show complex tidal tails
and debris structures. The UV morphology of both galaxies appears so different
from the optical one that the early-type classification may not apply. In both
GALEX bands the polar ring of MCG-05-07-001 is the dominant feature, whereas an
extended tidal tail dominates the FUV bands of NGC 1210. In MCG-05-07-001 and
NGC 1210 there is a strong correlation between structures detected in the FUV
and NUV bands and in HI. NGC 5329 does not show evidence of shells in the UV.
We try to constrain the age of the accretion episode or merger which gave rise
to the shells and polar rings with the aid of composite stellar populations
that take the presence of dust into account. The presence of HI in both
MCG-05-07-001 and NGC 1210 argues in favour of wet mergers. Models suggest the
presence of very young stellar populations in MCG-05-07-001: the observations
could be explained in the framework of a conspicuous burst of star formation
that occurred <=1 Gyr ago and involved a large fraction of the galaxy mass. Our
models suggest that also the nuclei of NGC 1210 and NGC 5329 could have been
rejuvenated by an accretion episode about 2-4 Gyr ago. (abridged)
