Three supernova shells around a young star cluster in M33
  Using a specialized technique sensitive to the presence of expanding ionized
gas we have detected a set of three concentric expanding shells in an HII
region in the nearby spiral galaxy M33. After mapping the kinematics in
H{\alpha} with Fabry-Perot spectroscopy we used slit spectra to measure the
intensities of the [SII] doublet at {\lambda}{\lambda} 671.9, 673.1 nm and the
[NII] doublet at {\lambda}{\lambda} 645.8, 658.3 nm to corroborate the
kinematics and apply diagnostic tests using line ratios. These showed that the
expanding shells are shock dominated as would be the case if they had
originated with supernova explosions. Estimating their kinetic energies we find
fairly low values, indicating a fairly advanced stage of evolution. We obtain
density, mass and parent star mass estimates, which, along with the kinetic
energies, are inconsistent with the simplest models of shock-interstellar
medium interaction. We propose that the presence and properties of an
inhomogeneous medium offer a scenario which can account for these observations,
and discuss the implications. Comparing our results with data from the
literature supports the combined presence of an HII region and supernova
remnant material at the observed position.
