The search for faint radio supernova remnants in the outer Galaxy: five new discoveries

Abstract

Context. High resolution and sensitivity large-scale radio surveys of the Milky Way are critical in the discovery of very low surface brightness supernova remnants (SNRs), which may constitute a significant portion of the Galactic SNRs still unaccounted for (ostensibly the "missing SNR problem"). Aims. The overall purpose here is to present the results of a systematic, deep data-mining of the Canadian Galactic plane Survey (CGPS) for faint, extended non-thermal and polarized emission structures that are likely the shells of uncatalogued SNRs. Methods. We examine 5 × 5 degree mosaics from the entire 1420 MHz continuum and polarization dataset of the CGPS after removing unresolved "point" sources and subsequently smoothing them. Newly revealed extended emission objects are compared to similarly prepared CGPS 408 MHz continuum mosaics, as well as to source-removed mosaics from various existing radio surveys at 4.8 GHz, 2.7 GHz, and 327 MHz, to identify candidates with non-thermal emission characteristics. We integrate flux densities at each frequency to characterise the radio spectra behaviour of these candidates. We further look for mid-and high-frequency (1420 MHz, 4.8 GHz) ordered polarized emission from the limb brightened "shell"-like continuum features that the candidates sport. Finally, we use IR and optical maps to provide additional backing evidence. Results. Here we present evidence that five new objects, identified as filling all or some of the criteria above, are strong candidates for new SNRs. These five are designated by their Galactic coordinate names G108.5+11.0, G128.5+2.6, G149.5+3.2, G150.8+3.8, and G160.1−1.1. The radio spectrum of each is presented, highlighting their steepness, which is characteristic of synchrotron radiation. CGPS 1420 MHz polarization data and 4.8 GHz polarization data also provide evidence that these objects are newly discovered SNRs. These discoveries represent a significant increase in the number of SNRs known in the outer Galaxy second quadrant of longitude (90• < < 180 • ), and suggests that deep mining of other current and future Milky Way surveys will find even more objects and help to reconcile the difference between expected numbers of Galactic SNRs and the smaller number of currently known SNRs.

Discovery of a new supernova remnant G150.3+4.5

“…The two straight lines shown in the CGPS λ21 cm total intensity image indicate the emission, which does not seem to be connected to the eastern shell of G150.3+4.5. the λ6 cm and λ11 cm data, which are consistent with the result of Gerbrandt et al (2014). The western shell of G150.3+4.5 is not mentioned in Gerbrandt et al (2014).…”

“…Together with the flat spectral index they derived, the eastern shell was suggested to be an H ii region. We agree with Gerbrandt et al (2014) that the strong overall infrared emission does not appear to be correlated with the eastern shell of the new SNR G150.3+4.5.…”

“…the λ6 cm and λ11 cm data, which are consistent with the result of Gerbrandt et al (2014). The western shell of G150.3+4.5 is not mentioned in Gerbrandt et al (2014). It can be identified from all total intensity images at various wavelengths as we showed.…”

“…The new SNR G149.5+3.2 can also be identified in the lower right of G150.3+4.5. Gerbrandt et al (2014) demonstrate the non-thermal synchrotron emission nature of the eastern shell of G150.3+4.5 and propose that it is a new SNR. We tested it with the TT-plot method (Turtle et al 1962), using the Urumqi λ6 cm, the Effeslberg λ11 cm, and the Effelsberg λ21 cm data.…”

“…The eastern shell of G150.3+4.5 can be traced clearly. The overlap of the lower part of the western shell with the new SNR G149.5+3.2 (Gerbrandt et al 2014) is clearly seen. The upper part of the western shell, free of the contamination of G149.5+3.2, can be identified above b = 4 • .…”