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Astron. Astrophys. 337, 338-348 (1998)

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1. Introduction

Dwarf irregulars galaxies (DIG) and blue compact galaxies (BCG) are very interesting objects to study galaxy evolution, for they are easier to model because of their generally small sizes and simple structures. Their stellar populations appear to be mostly young, their metallicity is low and their gas content is large. All these features indicate that these galaxies are poorly evolved objects, and may have undergone discontinuous or gasping star formation activity. In particular, BCG are the most luminous among dwarf irregular galaxies: their very low metal content, large gas fraction and very blue colours, suggest that star formation may have proceeded in short and intense bursts of activity (Searle et al. 1978). On the other hand, DIG seem to show a `gasping' activity of star formation (Tosi et al. 1992). In the last few years a great deal of theoretical models for DIG and BCG appeared in the literature. By means of analytical models, Matteucci and Chiosi (1983) first studied the effects of galactic winds on the chemical evolution of these galaxies, and suggested, among other hypotheses, that galactic winds of variable intensity could explain the observed spread in the physical properties of these galaxies. In the following years, by means of numerical models, Matteucci and Tosi (1985), Pilyugin (1992, 1993) and Marconi et al. (1994), studied the effects of galactic winds on the chemical evolution of such galaxies and confirmed the previous results. In these papers the possibility of metal enriched winds was also considered and favoured relative to the normal galactic winds. This metal enriched wind hypothesis, in fact, allowed them to explain the observed He abundances versus metal abundances. However, none of these studies had taken into account either the energy injected by supernovae and stellar winds into the ISM or the presence of dark matter halos which are of fundamental importance for the development of a galactic wind. The galactic winds were just assumed to occur every time the starburst was active.

Dark matter in dwarf irregular galaxies seems to be quite important as shown by recent data indicating an increasing amount of dark matter with decreasing luminosity (Skillman 1996), and therefore it cannot be ignored in chemical evolution models. Kumai and Tosa (1992) also explored the possibility of dark matter in these galaxies in a very simple way, and suggested that various amounts of dark matter could explain the observed spread in the [FORMULA] relation.

The energetics of supernovae and stellar winds and their interaction with the ISM, is a difficult problem, still not quite understood. The only existing models of chemical evolution taking into account the stellar energetics relative to the occurrence of galactic winds, are those developed to study the chemical and photometric evolution of elliptical galaxies (Arimoto and Yoshii 1987; Matteucci and Tornambé 1987; Angeletti and Giannone 1990; Ferrini and Poggianti 1994; Bressan et al. 1994; Gibson 1994, 1997). In these models, some specific assumptions relative to the amount of energy transferred into the ISM are made. Gibson (1994) has shown that the energetics from stellar winds is negligible in massive ellipticals whereas it could be important in smaller ones. For this reason it seems worthwhile to explore the effects of both supernovae and stellar winds on the evolution of dwarf irregulars and blue compact galaxies.

The aim of this paper is to take into account both the stellar energetics and the presence of dark matter in modelling the chemical evolution of dwarf irregular galaxies. The development of a galactic wind will therefore be calculated in detail and the existence of metal enriched or differential winds will be taken into account. By differential wind we intend a wind carrying out only some heavy elements, in particular those produced by type II supernovae which are the predominant supernovae during a starburst (Marconi et al. 1994) and eject material at much higher velocities than normal stellar winds. Therefore, under this assumption elements such as nitrogen and helium, which are restored by low and intermediate mass stars through stellar winds, will not leave the star forming region whereas oxygen and the other [FORMULA]-elements, ejected during type II supernova explosions are likely to be ejected outside the region and perhaps outside the galaxy. Iron will only in part leave the star forming region since only a fraction of this element is produced by SNe II, whereas the bulk of it comes from SNe Ia. It should be again said that the differential wind assumption has proven to be the most viable solution to the helium problem in blue compact galaxies (see Pilyugin 1993 and Marconi et al. 1994).

The paper is organized as follows: in Sect. 2. we will present the observational data concerning dwarf irregulars and blue compact galaxies, in Sect. 3. we will describe the chemical evolution model and the nucleosynthesis prescriptions, in Sect. 4. we will give our model results and finally in Sect. 5. we will draw some conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998