Relationships between Galactic Radio Continuum and H Emission

Introduction

The proposed UKST H Survey will detect Galactic optical emission in the H line, which is produced by electronic transitions from the n=3 to the n=2 bound level in atomic Hydrogen. It will be of interest to compare this emission with galactic thermal continuum radio emission, which is formed by thermal electrons moving in the presence of protons (Hydrogen nuclei). In (low-density) interstellar thermal plasmas, the excitation of the n=3 level in Hydrogen and the ionization of atomic Hydrogen are intimately connected, and so at first sight we would expect that radiation in H and in the thermal radio continuum would be quite similar.

However, there are many physical mechanisms and instrumental characteristics which conspire to produce marked differences in the appearance of cosmic sources in optical H radiation and in the radio continuum. This paper outlines the basic theory of the relevant emission processes and points out some of the effects that complicate the interpretation of observed similarities and differences in the intensity distribution of optical and radio emission from the Galactic inter-stellar medium.

Primary references to the topics covered here include the papers in the volume edited by Menzel (1962), and the books by Osterbrock (1974), Aller (1987), Longair (1994) and Kirk et al.  (1994).

© Copyright Astronomical Society of Australia 1997