Tentative Discovery of a New Supernova Remnant in Cepheus: Unveiling an Elusive Shell in the Spitzer Galactic First Look Survey

We have discovered an axially symmetric, well-defined shell of material in the constellation of Cepheus, based on imaging acquired as part of the Galactic First Look Survey with the Spitzer Space Telescope. The 86'' x 75'' object exhibits brightened limbs on the minor axis, and is clearly visible at 24 microns, but is not detected in the 3.6, 4.5, 5.8, 8.0, 70, or 160 micron images. Followup with 7.5 - 40 micron spectroscopy reveals the shell to be composed entirely of ionized gas, and that the 24 micron imaging traces solely [O IV] 25.89 micron emission. The spectrum also exhibits weaker [Ne III], [S III], and very weak [Ne V] emission. No emission from warm dust is detected. Spectral cuts through the center of the shell and at the northern limb are highly consistent with each other. The progenitor is not readily identified, but with scaling arguments and comparison to well-known examples of evolved stellar objects, we find the observations to be most straightforward to interpret in terms of a young supernova remnant located at a distance of at least 10 kpc, some 400 pc above the Galactic disk. If confirmed, this would be the first SNR discovered initially at infrared wavelengths.