The UKST H Survey - Scientific Background

Introduction

Why is H so important and useful to astrophysics? It provides a sensitive tracer of ionised hydrogen and thus of star formation, especially that of massive stars. To understand why this is so, it is useful to outline the recombination processes which follow the ionisation of hydrogen by ultra-violet photons from such stars. This is shown in the figure.

Since all the Lyman photons are re-absorbed and so are re-processed, every ionisation eventually gives a Balmer photon. Nearly half of these photons are in the H line, which is thus an excellent tracer of ionisation and thus of star formation.

Why is star formation so important? It is the dominant process in the present day evolution of spiral galaxies such as our own and presumably of the past evolution of old ellipticals. It is thus of great concern to astrophysicists who study :
a) galaxies
b) the interstellar medium, whose properties are largely determined by the effects of massive O and B stars
c) stellar evolution.

  
Figure 1: Balmer photons, in particular H, are the natural end-products of the recombination processes which occur within HII regions. In the diagram, dotted lines represent the Lyman photons which are reabsorbed by neutral hydrogen in the ground state, either within the HII region or in the surrounding ISM.

© Copyright Astronomical Society of Australia 1997