The blue compact galaxy IZw18 (also known as Markarian 116) was first described by Zwicky in 1966. IZw18 is the galaxy with the lowest known metallicity as derived from the ionised gaseous component. Its oxygen abundance is only ~ 1/50 of that of the Sun. Thus, often being referred to as ``the most metal-poor galaxy known'', it is still two orders of magnitude more metal rich than the most metal-poor stars in the Milky way. It is intriguing that while IZw18 was the first BCG (together with II Zw40) in which ionised gas abundances were investigated (Searle & Sargent 1972), it remains still the most metal-poor BCG known, despite large efforts in searching for more metal-poor ones.
Neutral hydrogen in IZw18 was detected by Chamaraux (1977), and further investigated using aperture synthesis by Lequeux and Viallefond (1980) who derived MH II = 7 . 107 M. Viallefond et al. (1987) mapped the galaxy in H I with VLA and from the velocity field inferred a mass Mdyn 9 . 108 M. Van Zee et al. (1998a) made a high resolution VLA study of IZw18 which revealed a complex HI morphology and velocity field. A complex velocity field was also found in the ionised component (Martin 1996, Petrosian et al. 1997). Molecular gas has not been detected, not surprising given the low metallicity. Moreover, low extinction is reported in most studies and the galaxy is not detected by the InfraRed Astronomical Satellite IRAS (IZw18 has not been observed with the Infrared Satellite Observatory, ISO), indicating a low dust content. Deep spectra revealed that IZw18 contains Wolf-Rayet stars (Fig. 7; Legrand et al. 1997b; Izotov et al. 1997a) showing that such can exist even at very low metallicities. HST imaging in the optical (Hunter and Thronson 1995, Dufour et al. 1996) and near infrared (Östlin 1999a) resolves the galaxy into individual luminous stars, and shows that the star formation has been active for at least 30 Myrs, with indications of an even older stellar population (see below). In Fig. 5 we show an H and a V-band image, and in Fig. 6 an optical spectrum of IZw18.
Figure 5. Left: Continuum subtracted H image of IZw18. Note the complex structure and very extended filaments. The dimension is 45 x 45 arcseconds, north is up, east is left. In the upper right corner, the faint H nebula in the companion galaxy is visible. The image was obtained at the Nordic Optical Telescope with 0.65'' seeing (Östlin). Right: A V-band (F555W) image of IZw18 obtained with the Planetary Camera of WFPC2 onboard HST (cf. Hunter and Thronson 1995). The dimension is 17 x 17 arcseconds, North is up left, east is down left. (Obtained from the HST archive.) At a distance of 10 Mpc, 1 arcsecond corresponds to 48 pc.
Figure 6. An spectrum of IZw18, with the most important emission lines labelled. Note the very blue continuum. That IZw18 is a very low abundance object can be seen by noting the following: [O III] 4959, 5007 and [O II] 3727 are rather weak with respect to H, while [O III] 4363 is relatively strong, and moreover the nitrogen and sulphur lines are very weak. (Plotted from a spectrum provided by F. Legrand).