The high star formation rate (SFR hereafter) and low neutral gas content deduced for dwarf star-forming galaxies imply consumption time-scales of about 10⁹yr (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 ( $M_B\ge-18^{\mathrm{m}}$ ) galaxies with compact sizes whose spectra are similar to those of low metallicity H II 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).

Optical imaging (both broad- and narrow-band) and near infrared images have been obtained to map the different regions and get insight into the star formation history of every region. In addition, we have obtained very high quality spectra with spatial resolution. The spectra of 22 star-forming knots and of free interim were taken with 10 distinct slit positions and orientations and moderate and high spectral resolution.

After briefly introduce the previous results on Mrk 86 in Sect. 2, we will describe the observations and data reduction in Sect. 3. The data analysis is presented in Sect. 4. We describe the evolutionary synthesis models in Sect. 5. In Sect. 6 we give some results. Finally, our conclusions are presented in Sect. 7.

In Gil de Paz et al. (2000a; Paper II hereafter) we derive the physical properties of the star-forming regions and stellar populations of Mrk 86 comparing our photometric data with the evolutionary synthesis models presented in this paper. Electron densities and temperatures and chemical abundances of the ionized gas in the galaxy star-forming regions will be also obtained. Finally, a global interpretation of the galaxy will be given.

**Table 1:** Collection of the published parameters of Mrk 86
Parameter	Data	Reference
$v_{\hbox{$\odot$ }}$	447 kms^-1	(1)
v_LG	522 kms^-1	(1)
v_LG	460 kms^-1	(2)
Distance	6.9Mpc	(3)
B	12.8	(4)
f(1510 Å)	$0.87\, 10^{-14}\,{\rm erg\,m}^{-2}\,{\rm s}^{-1}~{\mbox \AA}^{-1}$	(5)
S(2.8 cm)	$7\pm2$ mJy	(6)
S(6.3 cm)	$18\pm4$ mJy	(7)
S(1.2 cm)	$11\pm4$ mJy	(7)
f(12 $\mu$ m)	0.25Jy	(8)
f(25 $\mu$ m)	0.42Jy	(8)
f(60 $\mu$ m)	3.15Jy	(8)
f(100 $\mu$ m)	6.26Jy	(8)
L_IR	0.3510⁹ $L_{\hbox{$\odot$ }}$	(8)
J	13.25	(9)
H	12.57	(9)
K	12.37	(9)
L	12.63	(9)
M(H I)	2.410⁸ $M_{\hbox{$\odot$ }}$	(10)
$M_{\rm T}$	5.810⁸ $M_{\hbox{$\odot$ }}$	(10)
f(H I 21 cm)	2.1Jykms^-1	(11)
$FWHM_{\mathrm{H{\sc i}}}$	93kms^-1	(11)
I_CO	$0.83\pm0.12$ Kkms^-1	(12)

(1) Bottinelli et al. (1990); (2) Rood & Dickel (1976); (3) Sharina et al. (1999); (4) Dultzin-Hacyan et al. (1990); (5) Fanelli et al. (1988); (6) Klein et al. (1991); (7) Klein et al. (1984); (8) Thronson & Telesco (1986); (9) Thuan (1983, measured with 7 $.\!\!^{\prime\prime}$ 8 apertures); (10) Thuan & Martin (1981); (11) Verter (1985); (12) Sage et al. (1992).

1 Introduction