Detection of Extensive Optical Emission from the Extremely Radio Faint
  Galactic Supernova Remnant G182.4+4.3
  Wide-field Halpha images of the radio faint Galactic supernova remnant
G182.4+4.2 reveal a surprisingly extensive and complex emission structure, with
an unusual series of broad and diffuse filaments along the remnant's
southwestern limb. Deep [O III] 5007 images reveal no appreciable remnant
emission with the exception of a single filament coincident with the
westernmost of the broad southwest filaments. The near total absence of [O III]
emission suggests the majority of the remnant's optical emission arises from
relatively slow shocks (<70 km/s), consistent with little or no associated
X-ray emission. Low-dispersion optical spectra of several regions in the
remnant's main emission structure confirm a lack of appreciable [O III]
emission and indicate [S II]/Halpha line ratios of 0.73 - 1.03, consistent with
a shock-heated origin. We find G182.4+4.2 to be a relatively large (d~50 pc at
4 kpc) and much older (age ~40 kyr) supernova remnant than previously
estimated, whose weak radio and X-ray emissions are related to its age, low
shock velocity, and location in a low density region some 12 kpc out from the
Galactic centre.
