High-Resolution CCD Photometry of HII Galaxies

Summary

Introduction

HII galaxies are dwarf emission-line galaxies undergoing violent star formation. They are characterized by having giant HII regions which dominate their observable properties at optical wavelengths. Most HII galaxies are contained as a subsample of Blue Compact Galaxies (BCGs), but due to the different selection criteria only a small percentage of BCGs are HII galaxies. We will stick to the name “HII galaxies” to refer to the systems selected by objective prism surveys and having strong emission lines.

Various studies of their spectroscopic properties in optical wavelengths have revealed systems of very low heavy-element abundance and high rates of star formation. Earlier morphological studies have suggested that a large proportion of the sample of HII galaxies observed are starlike and isolated (Melnick 1987). For these, no clear indication of the mechanisms which may have triggered the burst is apparent. This together with the spectroscopic properties have made workers in the field since their discovery pose the question of whether these systems may be truly young galaxies or periodic bursts followed by long quiescent periods in the lifetime of the galaxy. Reviews of the general statistical properties of HII galaxies can be found in the Spectrophotometric Catalogue of HII Galaxies (hereafter SCHG, Terlevich et al. 1991; Melnick 1992).

We have pursued the line of work of surface photometry to address the questions of morphology and dynamics (Telles & Terlevich 1993; Telles, Melnick & Terlevich 1994) of HII galaxies as well as the questions of the age of the underlying systems and possible importance of the immediate environment in the onset of the present burst of star formation in HII galaxies.