A collisional origin for the Leo ring
  Extended HI structures around galaxies are of prime importance to probe
galaxy formation scenarios. The giant HI ring in the Leo group is one of the
largest and most intriguing HI structures in the nearby Universe. Whether it
consists of primordial gas, as suggested by the apparent absence of any optical
counterpart and the absence of an obvious physical connection to nearby
galaxies, or of gas expelled from a galaxy in a collision is actively debated.
We present deep wide field-of-view optical images of the ring region obtained
with MegaCam on the CFHT. They reveal optical counterparts to several HI and UV
condensations along the ring, in the g', r', and i' bands, which likely
correspond to stellar associations formed within the gaseous ring. Analyzing
the spectral energy distribution of one of these star-forming regions, we found
it to be typical for a star-forming region in pre-enriched tidal debris. We
then use simulations to test the hypothesis that the Leo ring results from a
head-on collision between Leo group members NGC 3384 and M96. According to our
model which is able to explain, at least qualitatively, the main observational
properties of the system, the Leo ring is consistent with being a collisional
ring. It is thus likely another example of extended intergalactic gas made-up
of pre-enriched collisional debris.
