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T.G. Pannuti (MIT), N. Duric (UNM)
We present a multi-wavelength (X-ray, optical and radio) study of the resident supernova remnant (SNR) populations of the Sculptor Group galaxies NGC 300 and NGC 7793 and the northern spiral NGC 6946. We have observed these galaxies at the wavelengths of 6 and 20 cm with the Very Large Array (VLA), and complemented this data with our own optical images and archived X-ray observations made with the Positional Sensitive Proportional Counter (PSPC) aboard the ROSAT satellite. We have searched for X-ray and radio emission from previously-known optically-identified SNRs, and searched for new candidate X-ray and radio SNRs. We find that remarkably few of the optically-identified SNRs possess counterparts at either of the other two wavelengths. Our search for new candiate X-ray and radio SNRs has produced fourteen new candidate radio SNRs and two candidate X-ray SNRs in NGC 300, and five candidate radio SNRs in NGC 7793. Very limited intersection is seen between the sets of X-ray, optical and radio-selected SNRs in these galaxies, and such a result may reflect possible selection effects inherent in surveys at each wavelength. Optical surveys favor the detection of SNRs located in low density regions, and are more likely to detect SNRs produced by the explosion of low mass stars in Type Ia supernovae. In contrast, X-ray and radio surveys favor the detection of SNRs situated in high density regions and therefore are more likely to detect SNRs produced by the explosion of high mass stars in Type II supernovae. These results indicate the need of a multi-wavelength campaign to detect a maximum number of SNRs in a galaxy of interest.
We have also examined the relationship between SNRs and diffuse radio emission in NGC 6946 using VLA and Effelsberg observations of that galaxy. The diffuse emission is far better correlated with the positions of the radio-selected SNRs than the positions of the optically-identified SNRs. If we interpret the separate sets of SNRs identified through optical and radio observations to correspond to SNRs parented by Type Ia and Type II supernovae, respectively, and if the diffuse emission in that galaxy corresponds to relativistic cosmic rays accelerated by SNRs, our results therefore indicate that the SNRs associated with Type II supernovae are the agents responsible for cosmic ray acceleration within this galaxy, and by extension other galaxies as well.
The author(s) of this abstract have provided an email address for comments about the abstract: tpannuti@space.mit.edu