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J.M. Cannon, E.D. Skillman (University of Minnesota), D.R. Garnett (University of Arizona), R.J. Dufour (Rice University)
We present new WFPC2 narrow-band imaging of the blue compact dwarf galaxy I Zw 18, which is host to the lowest-metallicity HII regions known. Images at H\alpha, H\beta, [S~II], [O~II] and [O~III] are combined with archival broad-band images to create a comprehensive dataset covering a broad wavelength range. The large HII regions in the galaxy are cataloged and converted into ionizing source equivalents. We find significant dust concentrations in both the ``Northwest'' and ``Southeast'' regions of the galaxy, with H\alpha/H\beta flux ratios elevated to levels as high as 3.4 in some areas. The dust distribution is inhomogeneous and varies considerably with position throughout the galaxy, similar to results found for the dwarf galaxy NGC 5253 (Calzetti et al., 1997).
To date, direct evidence of an ancient stellar population (i.e., ~10 Gyr) in I Zw 18 has remained elusive. However, a recent NICMOS study by Östlin (2000) reveals the presence of AGB stars of age greater than 1 Gyr and best-fit by a population older than 5 Gyr. The presence of dust within I Zw 18 could be taken as further evidence of an older stellar population, in agreement with arguments given by Garnett et al. (1997). Together, these clues suggest that I Zw 18 may not be a galaxy undergoing its first burst of star formation. Finally, this detection of differential reddening in I Zw 18 complements the reported detection of large amounts of dust in SBS 0335-052 (Thuan, Sauvage and Madden, 1999). It is interesting that the two lowest-metallicity galaxies known both contain appreciable amounts of dust.
\vspace{1 cm} This research was supported, in part, by NASA through grant GO-6536 from STScI and LTSARP grant NAG5-9221.