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

Abstract

We present deep JHK images of I Zw 18, the most metal deficient blue compact dwarf galaxy known, and analyze them in conjunction with archival Hubble Space Telescope/Wide Field Planetary Camera 2 optical images. To investigate the stellar populations, we have divided the main body of I Zw 18 into eight regions and fitted the optical, near-infrared (NIR), and hybrid optical-NIR colors of these and the C component with evolutionary synthesis models. The composite best fit is obtained for an age for evolved stellar populations of >~200 Myr; fits with an older age of >~500 Myr are less likely but possible. Our data show no evidence for stellar populations in I Zw 18 older than this, although as much as 22% of the stellar mass in older stars (4% in J light) could remain undetected. The colors of the young and intermediate-age stellar populations are significantly affected by widespread and inhomogeneously distributed ionized gas and dust. Ionized gas emission is important in every region examined except the northwest star cluster. Extinction is significant in both the northwest and southeast clusters. Red H-K, B-H, and V-K colors are not reliable indicators of old stellar populations because ionized gas emission is also red in these colors. V-I, on the other hand, reliably separates stars from gas because the former are red (V-I>=0.4) while the latter is blue (V-I~-0.4).