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5.1.3 The Companion of IZw18

Zwicky noted what he called a ``flare'' at the north west of IZw18, which has turned out to be a separate galaxy. Halpha images showed nebulosity close to the centre of this putative companion galaxy, suggesting that it had the same redshift as IZw18 (Dufour and Hester 1990, Östlin et al. 1995). This was later confirmed by spectroscopy (Petrosian et al. 1997, Dufour et al. 1996). The HST imaging study by Dufour et al. (1996) was able to resolve several young (~ 80 Myr) luminous stars, see also Aloisi et al. (1999). It is located in the same H I cloud as IZw18 and its location coincides with a peak in the H I column density (van Zee et al. 1998a).

In view of its position, just a kpc away from IZw18, its chemical composition would be of considerable interest. This galaxy is fainter than IZw18 but still very blue. However spectra have failed to unambiguously detect any oxygen lines, and consequently its metallicity is not known. The problem is that the galaxy has very low central surface brightness (µB = 22.7 mag/arcsec2, Östlin 1999c) making spectroscopy difficult.