A Newly Recognized Very Young Supernova Remnant in M83
  As part of a spectroscopic survey of supernova remnant candidates in M83
using the Gemini-South telescope and GMOS, we have discovered one object whose
spectrum shows very broad lines at H$\alpha$, [O~I] 6300,6363, and [O~III]
4959,5007, similar to those from other objects classified as `late time
supernovae.' Although six historical supernovae have been observed in M83 since
1923, none were seen at the location of this object. Hubble Space Telescope
Wide Field Camera 3 images show a nearly unresolved emission source, while
Chandra and ATCA data reveal a bright X-ray source and nonthermal radio source
at the position. Objects in other galaxies showing similar spectra are only
decades post-supernova, which raises the possibility that the supernova that
created this object occurred during the last century but was missed. Using
photometry of nearby stars from the HST data, we suggest the precursor was at
least 17 $\rm M_{sun}$, and the presence of broad H$\alpha$ in the spectrum
makes a type II supernova likely. The supernova must predate the 1983 VLA radio
detection of the object. We suggest examination of archival images of M83 to
search for evidence of the supernova event that gave rise to this object, and
thus provide a precise age.
