3-kpc arm
Editor: J. C. Cersosimo,
Ionized Gas from the
3-kpc arm
Detections of H166a radio recombination line probably associated with the 3 kpc arm has been reported on 1990. Using the data, along with those published by Lockman at 1.4 GHz and by Caswell and Hynes at 5 GHz, we analyze the distribution of the ionized gas in the inner galaxy. s a result we find that low-density ionized gas is located along the 3-kpc arm. The presence of this gas argues that the HII region are in a latest stage of development. Almost always, the dense HII regions observed at 5 GHz by Caswell at al do not show emission of the H166a line indicating that these HII regions are in an earliest phase of development. This work inferred the presence of diffuse HII regions.
H166a line emitting regions (filled circles) and dense HII regions (triangles) in the inner region of the Galaxy between 338 and 360 longitude. The locus of the 3 kpc arm and of the tilted arc are shown. Filled triangles define the locus of the tilted arc.