Wide-field H \alpha 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 Å 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 ( \leq 70 km s ^ { -1 } ) , 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 ] /H \alpha 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 \sim 50 pc at 4 kpc ) and much older ( age \sim 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 .