CDROM/APJ/V452/P238     HST Images of I Zw 18 Massive Stars (Hunter+ 1995)
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The Massive Stars of I Zw 18 as Seen in Hubble Space Telescope Images
      Diedre A. Hunter & Harley A. Thronson, Jr.
      <1995, ApJ, 452, 238>
      =1995ApJ...452..238H
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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.

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


File Summary:
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File Name    Lrecl    Records    Explanations
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table2.dat      98        398    The photometry
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Byte-by-byte Description of file: table2.dat
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   Bytes Format  Units  Label          Explanations
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   1- 6   I6     ---    Star           Star identification number
   7-13   F7.1   pix    X             *X pixel coordinate
  14-20   F7.1   pix    Y             *Y pixel coordinate
  21-27   F7.2   mag    F336W          []?=100.00 F336W magnitude
  28-34   F7.2   mag    e_F336W        []?=100.00 F336W magnitude uncertainty
  35-41   F7.2   mag    F555W          []?=100.00 F555W magnitude
  42-48   F7.2   mag    e_F555W        []?=100.00 F555W magnitude uncertainty
  49-55   F7.2   mag    F814W          []?=100.00 F814W magnitude
  56-62   F7.2   mag    e_F814W        []?=100.00 F814W magnitude uncertainty
  63-71   F9.2   mag    F336W-F555W    []?=100.00 F336W-F555W color
  72-80   F9.2   mag    e_F336W-F555W  []?=100.00 F336W-F555W color uncertainty
  81-89   F9.2   mag    F555W-F814W    []?=100.00 F555W-F814W color
  90-98   F9.2   mag    e_F555W-F814W  []?=100.00 F555W-F814W color uncertainty
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Notes for file: table2.dat
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X,Y:
  The X,Y coordinates are those in the F555W image, and the coordinate
  system is indicated in Figure 2 of the printed paper.
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(End)                                               Lee Brotzman [ADS] 11-Oct-95