G315.1+2.7:: a new Galactic supernova remnant from the AAO/UKST Hα survey

New narrow-band H alpha 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 Anglo-Australian Observatory/United Kingdom Schmidt Telescope H alpha survey of the southern Galactic plane which were found to coincide with existing multifrequency radio detections. Separate medium- and high-dispersion spectra were taken across two locations of this 11-arcmin north-south (NS) aligned optical filament. The resulting spectral signatures were found to strongly confirm the SNR identification based on standard emission-line ratio discriminators which characterize emission from shock-heated gas. The average observed ratios of S II/H alpha = 1.13, N II/H alpha = 1.43 and SII 6717/6731 = 1.46, together with the simultaneous detection of [O II] 3727, [O III] 5007 and [O I] 6300 angstrom, all point to a SNR origin of the observed optical emission. There is also an excellent positional coincidence between the new H alpha filament and the north-east radio arc of G315.1+2.7 seen at several frequencies. Careful scrutiny of the low-resolution but high-sensitivity Southern H alpha Sky Survey Atlas also revealed a low-level but distinct optical emission arc. This arc precisely correlates with the large, 2.5 degrees, NS 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 Sydney University Molonglo Sky 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 similar to 1.7 kpc and diameter of similar to 80 pc, make G315.1+2.7 one of the largest remnants known.