Search

Deep optical emission-line images are presented for nine known plus three new Galactic supernova remnants (SNRs), all but one having at least one angular dimension greater than one degree. Wide-field images taken in H$\alpha$ and [O III] $\lambda$5007 reveal many new and surprising remnant structures including large remnant shock extensions and `breakout' features not seen in published optical or radio data. These images represent over 12,000 individual images totaling more than 1000 hours of exposure time taken over the last two years mainly using small aperture telescopes which detected fainter nebular line emissions than published emission-line images. During the course of this imaging program, we discovered three new SNRs, namely G107.5-5.1 (the Nereides Nebula), G209.9-8.2, and G210.5+1.3, two of which have diameters >1.5 degrees. Besides offering greater structural detail on the nine already known SNRs, a key finding of this study is the importance of [O III] emission-line imaging for mapping the complete shock emissions of Galactic SNRs., Comment: 53 pages, 40 figures, 3 tables. Accepted for publication in the Astrophysical Journal Supplement Series

Drechsler et al. (2023) reported the unexpected discovery of a 1.5 degree long [O III] emission nebula 1.2 degrees southeast of the M31 nucleus. Here we present additional images of this large emission structure, called SDSO, along with radial velocity and flux measurements from low-dispersion spectra. Independent sets of [O III] images show SDSO to be composed of broad streaks of diffuse emission aligned NE-SW. Deep H$\alpha$ images reveal no strong coincident emission suggesting a high [O III]/H$\alpha$ ratio. We also find no other [O III] emission nebulosity as bright as SDSO within several degrees of M31 and no filamentary H$\alpha$ emission connected to SDSO. Optical spectra taken along the arc's northern limb reveal [O III] $\lambda\lambda$4959,5007 emissions matching the location and extent seen in our [O III] images. The heliocentric velocity of this [O III] nebulosity is $-9.8 \pm 6.8$ km s$^{-1}$ with a peak surface brightness of $(4\pm2) \times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ ($\sim$0.55 Rayleigh). We discuss SDSO as a possible unrecognized supernova remnant, a large and unusually nearby planetary nebula, a stellar bow shock nebula, or an interaction of M31's outer halo gas with high-velocity circumgalactic gas. We conclude that galactic origins for SDSO are unlikely and favor instead an extragalactic M31 halo--circumgalactic cloud interaction scenario, despite the nebula's low radial velocity. We then describe new observations that may help resolve the nature and origin of this large nebulosity so close to M31 in the sky., Comment: 20 pages, 13 figures, and 1 table; Revised and accepted for publication in ApJ