The distribution of HII regions in the outer Galaxy

The positions of a large sample of HII regions in the outer Galaxy have been determined. Assuming R(0) = 10 kpc and rotation curve = 250 km/s, it is found that essentially all HII regions are within a radius of less than about 20 kpc. The dispersion in their distances from the galactic plane increases from 90 pc near the sun to greater than 300 pc at large R. There is also a strong suggestion that the HII regions follow the warp in the plane that is seen in HI. There are some indications that the mean diameter of HII regions descreases with increasing R.