An uncatalogued optical H II region in the outskirts of the Galaxy

Abstract

We present NOT optical observations of a clump (l = 127. ◦ 9435 , b =+ 1. ◦ 8298) embedded in an extended, irregularly shaped, diffuse optical nebula. This condensation shows an emission-line spectrum typical of classic H  regions. Although its location on the sky coincides with a nearby extended photoionized region recently identified by Cichowolski et al. (2003) in radio data from the Canadian Galactic Plane Survey (CGPS), the systemic velocity of this ≈1 � -sized H  region, VLSR = −71 ± 12 km s −1 , poses it far out in the Galaxy, beyond the Perseus arm. The location of this region in the Galaxy is supported by H  structures visible at comparable radial velocity on CGPS data. We argue that this H  region might belong to an outer Galactic arm. The emission line ratios of the surrounding extended nebula, whose radial velocity is consistent with that of the small H  region, are typical of photoionized gas in the low density limit. Smaller clumps of comparable surface brightness are visible within the optical boundaries of the extended, faint nebula. After comparison of the optical data with far infrared and radio observations, we conclude that this nebula is an H  region, ∼70 pc in size, probably photoionized by an association of OB stars and surrounded by a ring of neutral hydrogen.