Far-UV and Optical Emissions from Three Very Large Supernova Remnants Located at Unusually High Galactic Latitudes

Galactic supernova remnants (SNRs) with angular dimensions greater than a few degrees are relatively rare, as are remnants located more than ten degrees off the Galactic plane. Here we report a UV and optical investigation of two previously suspected SNRs more than ten degrees in both angular diameter and Galactic latitude. One is a proposed remnant discovered in 2008 through 1420 MHz polarization maps near Galactic coordinates $l$ = 353, $b$ = $-$34. GALEX far UV (FUV) and H$\alpha$ emission mosaics show the object's radio emission coincident with a 11 x 14 degree shell of UV filaments surrounding a diffuse H$\alpha$ emission ring. Another proposed high latitude SNR is the 20 x 26 degree Antlia nebula (G275.5+18.4) discovered in 2002 through low-resolution all-sky H$\alpha$ and ROSAT soft X-ray emissions. GALEX UV and H$\alpha$ mosaics along with optical spectra indicate the presence of shocks throughout the Antlia nebula with estimated shock velocities of 70 to over 100 km s$^{-1}$. We also present evidence that it has collided with the northeast rim of the Gum Nebula. We find both of these nebulae are bona fide SNRs with ages less than 10$^{5}$ yr despite their unusually large angular dimensions. Using FUV and optical spectra and images, we also report finding an apparent new, high latitude SNR (G249.7+24.7) 4.5 degrees in diameter based on its UV and optical emission properties. We find this remnant's distance to be $\leq$400 pc based on the detection of red and blue shifted Na I absorption features in the spectra of two background stars.