Implications from HI Composition and Lyα Emission of HII Galaxies

Summary

From HST observations we have detected a damped Lyα absorption and metal lines that originate from HI gas in front of a star cluster in the HII galaxy I Zw 18. We have found that this neutral gas is 1000 times more metal-deficient than solar and 30 times less than the O/H abundance of the HII region itself. We discuss the implications of these results for the scenario of star formation, the mixing of synthetised elements and the Population III hypothesis. Moreover no Lyman emission has been detected from the ionized gas. Such a surprising result is discussed in the context of the search for young HII galaxies at high redshift.

Introduction

We know the importance of blue compact dwarf galaxies in the context of galaxy evolution. These objects were first identified as a class by Searle & Sargent (1972) who recognized their unusual spectroscopical nature making them indistinguishable from giant extragalactic HII regions. Also named HII galaxies they are the closest objects we can find to being pure starburst galaxies (Melnick 1987). Their optical and ultraviolet continuum is dominated by the O and B star population. In one HII galaxy a broad emission typical for WR stars has been first detected at about 4650 Å by Allen, Wright & Goss (1976). Kunth & Sargent (1981) later discovered such a feature in the dwarf galaxy NGC 3115, estimated the number of WR stars (found to be large) and concluded that their presence was further strong evidence that star formation in HII galaxies did occur in bursts rather than continuously.