G315.1+2.7: a new Galactic SNR from the AAO/UKST HAlpha survey

Abstract

New narrow-band HAlpha imaging and subsequent optical spectra confirm G315.1+2.7, a previously identified candidate supernova remnant (SNR), as a bona-fide Galactic SNR. Present observations are based on independent discovery of filamentary optical emission nebulosity on images of the AAO/UKST HAlpha survey of the southern Galactic plane which were found to co-incide with existing multi-frequency radio detections. Separate medium and high dispersion spectra were taken across two locations of this 11 arcmin N-S aligned optical filament. The resulting spectral signatures were found to strongly confirm the SNR identification based on standard emission line ratio discriminators which characterise emission from shock heated gas. The average observed ratios of [SII]/HAlpha=1.13, [NII]/HAlpha=1.43 and [SII] 6717/6731=1.46, together with the simultaneous detection of [OII] at 3727A, [OIII] 5007A and [OI] 6300A, all point to an SNR origin of the observed optical emission. Careful scrutiny of the low-resolution but high sensitivity SHASSA HAlpha survey also revealed a low-level but distinct optical emission arc. This arc precisely correlates with the large, 2.5 degree, north-south angular extent of the proposed new SNR also seen as a fractured structure in the extant radio data. G315.1+2.7 was detected previously at 2400 and 4800 MHz and at 408 and 1420 MHz. We also identified associated radio emission at 843 MHz from the now publicly available SUMSS survey. On the basis of optical imaging and spectra and radio observations at five frequencies, we identify G315.1+2.7 as a new Galactic SNR. The large projected angular extent of the new remnant, together with the distance estimate of ~1.7 kpc and diameter of ~80pc, make G315.1+2.7 one of the largest remnants known.