Heavy elements unveil the non primordial origin of the giant HI ring in
  Leo
  The origin and fate of the most extended extragalactic neutral cloud known in
the local Universe, the Leo ring, is still debated 38 years after its
discovery. Its existence is alternatively attributed to leftover primordial gas
with some low level of metal pollution versus enriched gas stripped during a
galaxy-galaxy encounter. Taking advantage of MUSE (Multi Unit Spectroscopic
Explorer) operating at the VLT, we performed optical integral field
spectroscopy of 3 HI clumps in the Leo ring where ultraviolet continuum
emission has been found. We detected, for the first time, ionized hydrogen in
the ring and identify 4 nebular regions powered by massive stars. These nebulae
show several metal lines ([OIII],[NII],[SII]) which allowed reliable measures
of metallicities, found to be close to or above the solar value. Given the
faintness of the diffuse stellar counterparts, less than 3 percent of the
observed heavy elements could have been produced locally in the main body of
the ring and not much more than 15 percent in the HI clump towards M96. This
inference, and the chemical homogeneity among the regions, convincingly
demonstrates that the gas in the ring is not primordial, but has been
pre-enriched in a galaxy disk, then later removed and shaped by tidal forces
and it is forming a sparse population of stars.
