Astron. Astrophys. 325, 1259-1263 (1997)

1. Introduction

It is thought that massive star formation in the universe may provide a significant fraction of the background radiation that maintains the diffuse intergalactic medium and the Lyman forest clouds highly ionized (e.g. Bechtold et al. 1987, Songaila et al. 1990, Miralda-Escudé & Ostriker 1990). This contribution would augment that of quasars, especially at high redshift where their number density is observed to decline. The contribution of quasars itself depends on how our picture of their evolution is distorted by dust obscuration (Fall & Pei 1993).

Direct observations of the Lyman continuum () radiation escaping from galaxies is however extremely difficult and, so far, only upper limits have been obtained with the Hopkins Ultraviolet Telescope (HUT) in four nearby star-forming galaxies (Leitherer et al. 1995). Attempts to understand how radiation leaks out from sites of star formation, ionizes the diffuse interstellar medium around and eventually escapes from a galaxy (e.g. Dove & Shull 1994, Patel & Wilson 1995a, b, Ferguson et al. 1996) have shown that the phenomenon is dominated by patchiness in the distribution of the neutral gas and should be highly random. Any quantitative assessment of the contribution of galaxies to the ionizing background would therefore require a large number of observations before a luminosity function is established. This uncertainty on the escape fraction is also a severe limitation for model predictions even though some of them are reasonably successful in linking the radiation produced by star formation to the rate of chemical enrichment in the universe (Cowie 1988, Songaila et al. 1990, Madau & Shull 1996).

In this paper we use the recent determination of the H luminosity density of nearby galaxies by Gallego et al. (1995) to estimate the contribution of galaxies to the diffuse radiation background at the Lyman limit and at . As a support to our approach, the H luminosity density will be compared with other tracers of the local star formation activity such as the luminosity density and the diffuse background in the far non-ionizing ultraviolet. From the comparison between the diffuse radiation at the Lyman limit predicted from galaxies and that measured from all sources of ionization, we will derive an upper limit to the effective escape fraction in the local universe.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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