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 H \alpha images detect little coincidence emission indicating an unusually high [ O iii ] /H \alpha emission ratio for such a large and apparently old remnant .
Low-dispersion optical spectra of several regions confirm large [ O iii ] /H \alpha 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 \simeq 50 kyr .