THE ASTROPHYSICAL JOURNAL, 452:238-252, 1995 October 10

     THE MASSIVE STARS OF I Zw 18 AS SEEN IN HUBBLE SPACE TELESCOPE IMAGES

                               DEIDRE A. HUNTER
       Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001

                                      AND

                            HARLEY A. THRONSON, JR.
    Wyoming Infrared Observatory, University of Wyoming, Laramie, WY 82071


                                   ABSTRACT

        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 450 pc 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.

     Subject headings: galaxies: compact -- galaxies: individual (I Zw 18)
                       -- galaxies: stellar content -- stars: early-type