Physical Structure and Nature of Supernova Remnants in M101
  Supernova remnant (SNR) candidates in the giant spiral galaxy M101 have been
previously identified from ground-based H-alpha and [SII] images. We have used
archival Hubble Space Telescope (HST) H-alpha and broad-band images as well as
stellar photometry of 55 SNR candidates to examine their physical structure,
interstellar environment, and underlying stellar population. We have also
obtained high-dispersion echelle spectra to search for shocked high-velocity
gas in 18 SNR candidates, and identified X-ray counterparts to SNR candidates
using data from archival observations made by the Chandra X-ray Observatory.
Twenty-one of these 55 SNR candidates studied have X-ray counterparts, although
one of them is a known ultra-luminous X-ray source. The multi-wavelength
information has been used to assess the nature of each SNR candidate. We find
that within this limited sample, ~16% are likely remnants of Type Ia SNe and
~45% are remnants of core-collapse SNe. In addition, about ~36% are large
candidates which we suggest are either superbubbles or OB/HII complexes.
Existing radio observations are not sensitive enough to detect the non-thermal
emission from these SNR candidates. Several radio sources are coincident with
X-ray sources, but they are associated with either giant HII regions in M101 or
background galaxies. The archival HST H-alpha images do not cover the entire
galaxy and thus prevents a complete study of M101. Furthermore, the lack of HST
[SII] images precludes searches for small SNR candidates which could not be
identified by ground-based observations. Such high-resolution images are needed
in order to obtain a complete census of SNRs in M101 for a comprehensive
investigation of the distribution, population, and rates of SNe in this galaxy.
