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Description
Blue compact dwarf galaxies are tiny galaxies that are dominated by intense star-forming regions. Thus, they have been thought to represent a different and extreme environment for star formation compared to the Milky Way and many other nearby galaxies. In this paper we use images obtained with the Hubble Space Telescope to resolve the main body of one of these galaxies, I Zw 18, into stars for the first time. This galaxy is also one of the most metal-poor galaxies known and is sometimes argued to be evolutionarily young. Broadband colors are used to determine the bulk characteristics of the resolved stellar population to an F555W magnitude of about 26, or M_(V,0)~-4 (O9.5 V star). Narrow-band images are used to look for emission characteristic of Wolf-Rayet stars. Color-magnitude diagrams reveal a broad main sequence of massive stars as well as blue and red supergiants, although there is a surprising lack of Wolf-Rayet stars. About half the massive stars are located in two groups, corresponding to the two knots of emission identified in ground-based images, and the rest are distributed between these regions and in the outer parts of the galaxy. We find that the northwestern of these two regions resolves in Halpha images into a small H II region plus a complicated shell structure that encircles the northern stellar association. There are additional, larger loops and filaments of ionized gas up to 450pc from the center of the galaxy. While the shell could be as young as the stars it encircles, the larger ionized gas structures must be older for reasonable models, and their presence implies that there has been a previous generation of massive stars at least several tens of millions of years ago. However, we find no evidence in these data for stars that must be older than this. The larger sites of most recent star formation are located northwest and south of the central part of the galaxy. Thus, we find that I Zw 18 is somewhat more complicated than had previously been thought, both in terms of its star formation history and the state of its interstellar medium. At the same time, however, the stellar populations look relatively normal, and the spatial concentration of massive stars is closer to that of large OB associations in nearby galaxies rather than to that exemplified by the compact cluster R136 in the Large Magellanic Cloud.
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