HST/NICMOS Observations of IZw18 - revealing a population of old AGB stars

Abstract

I present the first results from a HST/NICMOS imaging study of the most metal-poor blue compact dwarf galaxy IZw18. The near infrared color-magnitude diagram (CMD) is dominated by two populations, one 10-20 Myr population of red super giants and one 0.5-5 Gyr population of asymptotic giant branch stars. Stars older than 1 Gyr are required to explain the observed CMD at the adopted distance of 12.6 Mpc, showing that IZw18 is not a young galaxy. The results hold also if the distance to IZw18 i s significantly larger. This rules out the possibility that IZw18 is a truly young galaxy formed recently in the local universe.