High resolution radio recombination line observations

Abstract

Over the last decade very sensitive observations of radio recombination line emission using high angular resolution synthesis telescopes have become available. As a result it has now become possible to image the physical parameters deduced from radio recombination lines of individual sources. In the case of HII regions this work has resulted in detailed maps of radial velocities, the electron temperature and the abundance of singly ionized helium (Y⁺). Furthermore the influence on radio recombination line emission towards HII regions of variations in Y⁺, of pressure broadening and of the continuum optical depth has been demonstrated unequivocally. Other new results due to the high angular resolution are a better understanding of the properties of the emission from the partially ionized medium adjacent to HII regions (CII and H° regions). Finally the new observational possibilities show great promise for the study of the physics and kinematics of e.g. extragalactic objects.