Expanding shells of shocked neutral hydrogen around compact HII regions

¹ National Research Council of Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, Penticton, British Columbia, V2A 6K3, Canada
² Department of Physics and Astronomy, The University of Calgary, 2500 University Dr. NW, Calgary, AB, T2N 1N4 Canada e-mail: charles.kerton@nrc.ca

Corresponding author: R. Kothes, roland.kothes@nrc.ca

Received: 20 February 2002
Accepted: 30 April 2002

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. – ISM: kinematics and dynamics