Physical Structure and Nature of Supernova Remnants in M101
Supernova remnant (SNR) candidates in the giant spiral galaxy M101 have been previously identified from groundbased Hα and [S ii] images. We have used archival Hubble Space Telescope (HST) Hα and broadband 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 ultraluminous 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/H ii 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 H ii regions in M101 or background galaxies. The archival HST Hα images do not cover the entire galaxy and thus prevents a complete study of M101. Furthermore, the lack of HST [S ii] 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.
The Astronomical Journal, 143:85 (14pp), 2012 April.