2023RNAAS...7..265R

We report the discovery of a previously uncatalogued arch-shaped filamentary nebula at the outer part of the Triangulum galaxy (M33) centered at α = 1^h34^m25^s, $\delta =+30^\circ 20^{\prime} 17^{\prime\prime} $ (ICRS). This discovery stems from meticulous observations employing deep exposures of M33, using both Hα and [O III] narrow-band filters. The nebula, designated as Roig1 Prades Sky, exhibits an Hα surface brightness of $23.9\,\mathrm{mag}\,{\mathrm{arcsec}}^{-2}$. Its sky projected location is 21' away from the M33 galactic center toward the southeast direction with an extent of (120 × 440) ± 30 pc. Deep spectroscopic observations are required to unveil its real nature.