Detection of extensive optical emission from the extremely radio faint Galactic supernova remnant G182.4+4.3

Wide-field H α 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 south-western 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 south-west 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^-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]/Hα 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.