Star-forming rings in lenticular galaxies: origin of the gas
  Rings in S0s are enigmatic features which can however betray the evolutionary
paths of particular galaxies. We have undertaken long-slit spectroscopy of five
lenticular galaxies with UV-bright outer rings. The observations have been made
with the Southern African Large Telescope (SALT) to reveal the kinematics,
chemistry, and the ages of the stellar populations and the gas characteristics
in the rings and surrounding disks. Four of the five rings are also bright in
the H-alpha emission line, and the spectra of the gaseous rings extracted
around the maxima of the H-alpha equivalent width reveal excitation by young
stars betraying current star formation in the rings. The integrated level of
this star formation is 0.1-0.2 solar mass per year, with the outstanding value
of 1 solar mass per year in NGC 7808. The difference of chemical composition
between the ionized gas of the rings which demonstrate nearly solar metallicity
and the underlying stellar disks which are metal-poor implies recent accretion
of the gas and star formation ignition; the star formation history estimated by
using different star formation indicators implies that the star formation rate
decreases with e-folding time of less than 1 Gyr. In NGC 809 where the UV-ring
is well visible but the H-alpha emission line excited by massive stars is
absent, the star formation has already ceased.
