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Astron. Astrophys. 361, 465-479 (2000)

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1. Introduction

The high star formation rate (SFR hereafter) and low neutral gas content deduced for dwarf star-forming galaxies imply consumption time-scales of about 109 yr (Fanelli et al. 1988; Thuan & Martin 1981), much shorter than the age of the Universe. Searle et al. (1973) suggested that either these objects are truly young systems or they have an intermittent star formation history with short intense star-forming episodes followed by long quiescent phases. Although this question remained unanswered during decades (Thuan 1983; Campbell & Terlevich 1984; Loose & Thuan 1985), nowadays, most of the studies on dwarf star-forming systems, including Blue Compact Dwarf galaxies (BCD hereafter), have revealed the existence of an evolved underlying population (Kunth et al. 1988; Hoffman et al. 1990; Papaderos et al. 1996b; Doublier 1998; Norton & Salzer 1997).

Therefore, the understanding of the mechanism, or mechanisms, governing this star formation regulation is the stepping stone of the dwarf star-forming galaxies evolution. Moreover, this mechanism constitutes the missing link between the different dwarf star-forming galaxies (Silk et al. 1987; Burkert 1989; Drinkwater & Hardy 1991; Papaderos et al. 1996b).

There are two different approaches to address these questions. On the one hand, statistical analysis of a sample of these objects will allow the study of the relationships between fundamental parameters and properties of the star-forming dwarf galaxies (Marlowe et al. 1995; Papaderos et al. 1996a, 1996b). On the other hand, the detailed analysis of individual low-redshift objects with multiple regions of star formation is fundamental to reconstruct their star formation histories. In particular, we can obtain a better understanding of possible effects of merging or some internal process as self-propagation that could originate the spreading of star formation, and the effects of these star-forming events on the future star formation.

Among dwarf galaxies, Blue Compact Dwarf galaxies conform the subpopulation where the star-forming events are most violent. Blue Compact Dwarfs are low-luminosity ([FORMULA]1 galaxies with compact sizes whose spectra are similar to those of low metallicity HII regions (Searle & Sargent 1972; Kunth & Sargent 1986; Thuan & Martin 1981). Their spectra are characterized by emission lines over a blue continuum which implies the existence of a large fraction of OB stars and an intense star-forming activity.

The Blue Compact Galaxy Mrk 86=NGC 2537 (Shapley & Ames 1932; Markarian 1969), also known as Arp 6 (Arp 1966), constitutes an excellent laboratory to test the BCD star formation history since its star-forming regions populate all the galaxy. This object is the propotype of the iE galaxies, the most important BCD galaxies subclass, that conform 70 per cent of the BCD galaxies (Thuan & Martin 1981; Thuan 1991).

In Gil de Paz et al. (2000a; Paper I hereafter) we presented observational data for the underlying population and star-forming regions of Mrk 86. We provided sizes, BVRJHK magnitudes, colors and emission-line fluxes for most of these regions. The evolutionary synthesis models used in this paper were also extensively described.

Now, we describe the procedure employed to compare the colors measured and those predicted by the evolutionary synthesis models in Sect. 2. The properties of the underlying stellar population, central starburst and recent star-forming regions are studied in Sects. 3.1, 3.2 and 3.3, respectively. Electron densities, temperatures and metal abundances for the brightest star-forming regions in the galaxy are given in Sect. 3.4. Finally, in Sect. 4, we provide a global interpretation of the short- and mid-term star formation history of the galaxy. The main conclusions from this study are summarized in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000