The blue compact galaxy IZw 18 is a fiducial object - it has a very low metallicity and is currently experiencing an intense star formation episode. Its metallicity (Searle & Sargent 1972, Skillman & Kennicutt 1993) is the lowest (1/50 ) observed in this kind of objects and more metal-deficient blue compact dwarf galaxies have not been found despite extensive searches (Terlevich 1982, Terlevich et al. 1991, Masegosa et al. 1994, Izotov et al. 1994, Terlevich et al. 1996), with the possible exception of SBSG 0335-052W (Lipovetsky et al. 1999). This led Kunth & Sargent (1986) to suggest that during a single starburst event, the metals ejected by the massive ionizing stars are mixed within a short time scale in the HII region and lead, in few Myr, to a metallicity level comparable to that of IZw 18. Other studies have also shown that only one burst is sufficient to account for the oxygen abundance in IZw 18 (Alloin et al. 1978, Lequeux et al. 1981, Kunth et al. 1995). Thus if the metallicity measured in IZw 18 is solely the result of the metals produced in the current burst, a discontinuity in the spatial abundance distribution would be expected corresponding to the edge of the recently enriched region, typically a few hundred parsecs away from the young stellar core (Roy & Kunth 1995).
Early measurements of the abundance in the HI halo of IZw 18 by Kunth et al. (1994) indicated that the metallicity of the very massive neutral cloud in front of the ionizing cluster might be a factor of about 20 lower than in the HII region, strengthening the possibility of a sharp abundance drop. However the UV absorption lines they used were saturated, and this result remains very uncertain (Pettini & Lipman 1995). Moreover, recent HI observations by Van Zee et al. (1998) displayed lower velocity dispersion in the HI halo than those assumed by Kunth et al. (1994), leading to a metallicity comparable to the abundance in the central ionized region.
On the other hand, several observations of starburst galaxies (Kobulnicky & Skillman 1997, and references therein) have shown no significant gradient or discontinuity in the oxygen abundance distributions within the HII regions, except for the well established local overabundance of nitrogen in NGC5253 (Welch 1970, Walsh & Roy 1987, 1989, Kobulnicky et al. 1996, 1997). This corroborates models which predict that during a starburst, the heavy elements produced by the massive stars are ejected with high velocities into a hot phase, leaving the starburst region without immediate contribution to the enrichment of the insterstellar medium (Pantelaki & Clayton 1987, Tenorio-Tagle 1996, Devost et al. 1997, Kobulnicky & Skillman 1997, Pilyugin 1999). In this scenario, the metals observed now would have their origin in a previous star formation event, and an underlying old stellar population would be expected. Early observations of IZw 18 did not reveal clearly such an old population (Thuan 1983, Hunter & Thronson 1995), but recent reanalysis of HST archive data (Aloisi et al. 1999) has shown that stars older than 1 Gyr must be present. Moreover, Ostlin (1999) studied the resolved stellar population in the near infrared with NICMOS onboard HTS and found also that while the NIR colour magnitude diagram was dominated by stars 10-20 Myr old, numerous red AGB stars require a much higher age, in agreement with Aloisi et al. (1999). NICMOS data require stars older than 1 Gyr to be present and an age as high as 5 Gyr is favoured. This holds even if a distance slightly higher than the conventional 10 Mpc is adopted. This suggests that the present star formation episode in IZw 18 is not the first one. The rather high C/O ratio observed in IZw 18 (Garnett et al. 1997) could also suggest a carbon enrichment by an evolved population of intermediate mass stars. However, other starburst galaxies show quite lower C/O ratio (Garnett et al. 1995, Kobulnicky & Skillman 1998, Izotov & Thuan 1999), so this fact remains puzzling and controversial (Izotov & Thuan 1999). Considering the large uncertainties on the determinations of the stellar yields (Prantzos 1998, 1999) and on the determination of the C/O ratio (Izotov et al. 1997, Izotov & Thuan 1999), this may not be used as a strong evidence for an enrichment by an old stellar population.
Thus the mechanism responsible for the dispersal and mixing of newly synthesized elements in a starburst galaxy remains unclear, as well as the chain of star formation events responsible for the observed abundances. IZw 18, as the lowest abundance galaxy among starbursts, is an ideal laboratory to study these processes; its low metallicity is indicative of a rather "simple" star formation history, and one would expect the material ejected by the present massive stars to give high contrast in the abundances between the enriched and the non enriched zones. However, if the small companion galaxy northwest of I Zw18 has had an influence, as suggested by Dufour et al. (1996) through tidal effects or streaming gas resulting from a collision with the main body of I Zw18, the recent history of dispersal and mixing of elements may not be that easy to disentangle.
We conducted deep long slit spectroscopy of IZw 18 in order to measure the O/H abundances as far as possible from the central HII region of the NW knot, and to detect a discontinuity or systematic gradient in the metallicity distribution. Observations and reduction are described in Sect. 2; results are presented in Sect. 3 and 4 and the star formation history of this galaxy is discussed in the last sections.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000