2018MNRAS.478.1987H

Narrow passband optical images of the large Galactic supernova remnant G179.0+2.6 reveal a faint but nearly complete emission shell dominated by strong [O III] 4959, 5007 Å line emission. The remnant's optical emission, which consists of both diffuse and filamentary features, is brightest along its southern and northeastern limbs. Deep Hα images detect little coincidence emission indicating an unusually high [O III]/Hα emission ratio for such a large and apparently old remnant. Low-dispersion optical spectra of several regions confirm large [O III]/Hα line ratios with typical values around 10. The dominance of [O III] emission for the majority of the remnant's optical filaments suggests shock velocities above 100 km s^–1 are present throughout most of the remnant, likely reflecting a relatively low-density ambient ISM. The remnant's unusually strong [O III] emission adds to the remnant's interesting set of properties which include a thick radio emission shell, radial polarization of its radio emission like that typically seen in young supernova remnants, and an unusually slow-rotating gamma-ray pulsar with a characteristic spin-down age ≃50 kyr.