Star formation in outer rings of S0 galaxies. III. UGC 5936 -- an S0
  with currently accreted satellite matter
  Though S0 galaxies are usually thought to be `red and dead', they demonstrate
often star formation organized in ring structures. We try to clarify the nature
of this phenomenon and its difference from star formation in spiral galaxies.
The luminous S0 galaxy with a large ring, UGC 5936, is studied here. By
applying long-slit spectroscopy along the major axis of UGC 5936, we have
measured gas and star kinematics, Lick indices for the main body of the galaxy,
and strong emission-line flux ratios in the ring. After inspecting the gas
excitation in the ring using line ratios diagnostic diagrams and having ensured
that it is ionized mostly by young stars, we have determined the gas oxygen
abundance by using popular strong-line methods. Also we have proved the spatial
proximity of the south-eastern dwarf satellite to UGC 5936 and have measured
its gas metallicity. The ionized gas of the ring is excited by young stars and
has solar metallicity. Star formation in the ring is rather prolonged, and its
intensity corresponds to the current HI content of UGC 5936 (to the
Kennicutt-Schmidt relation). The whole morphology of the HI distribution
implies current accretion of the cold gas from the satellite onto the outer
disc of UGC 5936; due to the satellite location and rotation in the plane of
the stellar disc of the host galaxy, the accretion is smooth and laminar
providing the favorable condition for star formation ignition.
