An ATCA radio-continuum study of the Small Magellanic Cloud - IV. A multifrequency analysis of the N 66 region

Traditional identification of supernova remnants (SNRs) include the use of radio spectral index, optical spectral studies (including strong [SII], [NII], [OI], [OII] and [OIII] lines) and X-ray co-identifications. Each of these can have significant limitations within the context of a particular SNR candidate and new identification methods are continually sought. In this paper, we explore subtraction techniques by Ye, Turtle and Kennicutt to remove thermal emission estimated from Ha flux from radio-continuum images. The remaining non-thermal emission allows the identification of SNRs embedded within these HII regions. Subtraction images of the N66 region in the Small Magellanic Cloud (SMC) using H alpha wide-field optical CCD images from the Curtis Schmidt Telescope and the recent Australia Telescope Compact Array (ATCA)/Parkes radio-continuum (1420, 2370, 4800 and 8640 MHz) data are presented as an example. These show three SNRs (B0057-724, B0056-724 and B0056-725) separated from their surrounding HII radio emission. 2.3-m dual-beam spectrograph long-slit spectra from selected regions within N66 suggest the presence of an additional SNR with no radio or X-ray emission. Radio spectral index, [SII]/H alpha ratio and archived Chandra images of N66 combine to give a more coherent picture of this region, confirming B0057-724 as an SNR. The N66 nebula complex is divided into 10 components, composed separately of these SNRs and HII regions.