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

Abstract

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$\hat{I}\pm$ images reveal no strong coincident emission suggesting a high [O III]/H$\hat{I}\pm$ ratio. We also find no other [O III] emission nebulosity as bright as SDSO within several degrees of M31 and no filamentary H$\hat{I}\pm$ emission connected to SDSO. Optical spectra taken along the arc's northern limb reveal [O III] $\hat{I}»\hat{I}»$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 $(4\pm 2)\times 10^{-18}$ 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.

20 pages, 13 figures, and 1 table; Revised and accepted for publication in ApJ