We have acquired deep long slit spectroscopy of the metal poor dwarf star forming galaxy IZw 18. We confirm the very low metal content of the galaxy, and show that no significant abundance gradient nor inhomogeneities larger than dex are present in IZw 18 on scales of 50 pc to 600pc. This is in apparent contradiction with the hypothesis of instantaneous local pollution proposed by Kunth & Sargent (1986). Instead, this supports a picture where metals ejected in the current burst of star formation escape into a hot halo hidden phase in the halo, follow a long excursion while cooling and come back much later into the central galactic region (or escape into the intergalactic medium). This also implies that star formation has been occurring previous to the current burst. Based on different observational facts, we propose that the metals in IZw 18 are the result of a mild continuous star formation rate. The generalization of this model to all gas clouds can account for the scarcity of local galaxies with a metallicity lower than IZw 18, for the increase with time of the metallicity of the most underabundant DLA systems, and for the apparent absence of HI clouds without optical counterparts. If starbursts appear as important episodes in the history of galaxies, the low continuous star formation regime, dominant during the quiescent inter-burst periods, cannot be neglected.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000