Expanding shells of shocked neutral hydrogen around compact HII regions

Abstract

By comparing radial velocities of radio bright compact Hii regions with their Hi absorption profiles, we discovered expanding shells of neutral hydrogen around them. These shells are revealed by absorption of the radio continuum emission from the Hii regions at velocities indicating greater distances than the observed radial velocity. We believe that these shells are shock zones at the outer edge of the expanding ionized region. Additionally we found evidence for a velocity inversion inside the Perseus arm caused by a spiral shock, which results in a deep absorption line in the spectra of compact Hii regions behind it. In this paper we present a novel technique for the detec- tion and study of expanding Hi structures around compact Hii regions that uses 21-cm Hi absorption spectra. The technique allows Hi surrounding Hii regions to be studied even if the Hii region is not resolved. In a number of cases we detect absorp- tion features that are associated with shocked expanding lay- ers of photodissociated gas. We focus on compact Hii regions within the observed area of the Canadian Galactic Plane Survey