The Spatial and Emission Properties of the Large [O iii] Emission Nebula Near M31

Abstract

Drechsler et al. reported the unexpected discovery of a 1.°5 long [O iii] emission nebula 1.°2 southeast of the M31 nucleus. Here we present additional images of this large emission arc, called the Strottner–Drechsler–Sainty Object (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 northeast–southwest. Deep Hα images reveal no strong coincident emission suggesting a high [O iii]/Hα ratio. We also find no other [O iii] emission nebulosity as bright as SDSO within several degrees of M31 and no filamentary Hα emission connected to SDSO. Optical spectra taken along the nebula’s northern limb reveal [O iii] λ λ4959, 5007 emissions matching the location and extent seen in our [O iii] images. The heliocentric velocity of this [O iii] nebulosity is −9.8 ± 6.8 km s−1 with a peak surface brightness of (4 ± 2) × 10−18 erg s−1 cm−2 arcsec−2 (∼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 with Local Group 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.

Deep Optical Emission-line Images of Nine Known and Three New Galactic Supernova Remnants

“…(4) Lastly, we note that the recent revolution in emission-line imaging by both professionals and amateurs brought on by new sensitive detectors and high-throughput narrow bandpass filter technology will likely significantly increase the depth of future optical emission-line surveys, both along and far away from the Galactic plane. The recent discovery of a very large and faint [O III] emission nebula near M31 (Drechsler et al 2023;Fesen et al 2023) is just one sign of the discovery power of deep imaging of an otherwise very well-studied object through the combination of hundreds of exposures taken using small and modest aperture telescopes and processed by powerful software.…”

“…Consequently, it has been said that images taken by amateurs can "far out-distance the images produced by most professional astronomers" (Abraham et al 2017). Moreover, small aperture [O III] imaging with total exposure times of ∼50-100 hr, have shown that brightness levels as low as ;0.2 Rayleigh are possible (Fesen et al 2023) thereby approaching the depth of WHAM in Hα but with a resolution of a few arcseconds instead of WHAM's ∼1°r esolution.…”