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Wide-field Halpha images of the radio faint Galactic supernova remnantG182.4+4.2 reveal a surprisingly extensive and complex emission structure, withan unusual series of broad and diffuse filaments along the remnant'ssouthwestern limb. Deep [O III] 5007 images reveal no appreciable remnantemission with the exception of a single filament coincident with thewesternmost of the broad southwest filaments. The near total absence of [O III]emission suggests the majority of the remnant's optical emission arises fromrelatively slow shocks (<70 km/s), consistent with little or no associatedX-ray emission. Low-dispersion optical spectra of several regions in theremnant'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 witha shock-heated origin. We find G182.4+4.2 to be a relatively large (d~50 pc at4 kpc) and much older (age ~40 kyr) supernova remnant than previouslyestimated, whose weak radio and X-ray emissions are related to its age, lowshock velocity, and location in a low density region some 12 kpc out from theGalactic centre.

Detection of Extensive Optical Emission from the Extremely Radio Faint Galactic Supernova Remnant G182.4+4.3