The Distribution of Hii Regions

Abstract

The distribution of optically observed Hii regions and OB stars with galactic longitude indicates that it is primarily determined by extinction by interstellar dust. Thus optical observations can, at the best, reveal the local structure in the vicinity of the sun. Radio observations, on the other hand, are not affected by dust. Thus the distribution of galactic radio sources, which peaks in the northern part at about l ^II = 17°.5, must be related to the large-scale structure of our Galaxy. Two radio recombination line surveys of the northern and southern sky yield kinematic distances. If only the ‘giant Hii regions’ are retained, the following distribution is obtained: (1) Only 5 giant Hii regions are found within the 4 kpc arm. (2) The bulk of the giant Hii regions is concentrated in a ring between 4 and 6 kpc from the galactic center. (3) There are other concentrations of giant Hii regions indicating the existence of the Sagittarius and Perseus arm. (4) The three features revealed by optical observations of Hii regions in the vicinity of the sun cannot be matched with the large-scale distribution outlined by giant Hii regions. This is particularly true for the so-called Orion arm. (5) At distances beyond 13 kpc from the galactic center virtually no giant Hii regions are found. (6) The surface density of giant Hii regions attains its maximum between 4 and 8 kpc; the surface density of neutral hydrogen (Hi) attains its maximum between 11 and 15 kpc, but the actual space density of Hi in the region 4 to 8 kpc may still be rather high.

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