An uncatalogued optical HII region at the outskirts of the Galaxy

Details An uncatalogued optical HII region at the outskirts of the Galaxy An uncatalogued optical HII region at the outskirts of the Galaxy

2004-02-20

arxiv.org/abs/astro-ph/0402483v1

Astron.Astrophys. 420 (2004) 225-232

Astronomy and Astrophysics

Astrophysics

8 pages, 9 figures (some of which degraded due to size constraints), accepted on 2004/02/12 for publication in Astronomy and A

Scientific paper

10.1051/0004-6361:20041013

We present NOT optical observations of a clump (l = 127.9435 deg, b = +1.8298 deg), embedded in an extended, irregularly shaped, diffuse optical nebula. This condensation shows an emission-line spectrum typical of classic HII 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 arcmin-sized HII region, V_LSR = -71$\pm$12 km/s, poses it far out in the Galaxy, beyond the Perseus arm. The location of this region in the Galaxy is supported by HI structures visible at comparable radial velocity on CGPS data. We argue that this HII 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 HII 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 HII region, ~ 70 pc in size, probably photoionized by an association of OB stars and surrounded by a ring of neutral hydrogen.

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