New Light on the Stellar Populations in I Zw 18: Deep Near‐Infrared Imaging

We present deep JHK images of IZw18, the most metal-deficient Blue CompactDwarf galaxy known, and analyze them in conjunction with archival HST/WFPC2optical images. To investigate the stellar populations, we have divided themain body of IZw18 into eight regions, and fit the optical, near-infrared(NIR), and hybrid optical-NIR colors of these and the C component withevolutionary synthesis models. The composite best fit is obtained for an agefor evolved stellar populations of <= 200Myr; fits with an older age of <=500Myr are less likely but possible. Our data show no evidence for stellarpopulations in IZw18 older than this, although as much as 22% of the stellarmass in older stars (4% in J light) could remain undetected. The colors of theyoung and intermediate-age stellar populations are significantly affected bywidespread and inhomogeneously distributed ionized gas and dust. Ionized gasemission is important in every region examined except the NW star cluster.Extinction is significant in both the NW and SE clusters. Red H-K, B-H, and V-Kcolors are not reliable indicators of old stellar populations because ionizedgas emission is also red in these colors. V-I, on the other hand, reliablyseparates stars from gas because the former are red (V-I >= 0.4) while thelatter is blue (V-I ~ -0.4).Comment: 29 pages, 9 figures, accepted for publication in ApJ (May, 2003