A new infrared view of evolved stars in IZw18

We report results from near- infrared imaging of IZw 1 8, the most metal- poor galaxy in the local universe, with NICMOS on board the Hubble Space Telescope ( HST). Observations were obtained in the broad F205W filter (similar to K) and in the medium- wide bands F171M and F180M, the latter which cover strong molecular absorption features in cool stars. The new data, together with previously obtained HST/ NICMOS images in the F110W (similar to J) and F160W (similar to H) filters, provide a census of the cool stellar population in this chemically unevolved galaxy. We find that stars as old as similar to 1 Gyr are required to explain the observed colour - magnitude diagrams. Combining broad and medium- wide band photometry, we have classified the observed stars. The observed stellar populations, to the depth of our data, are dominated by luminous red supergiant stars. However, an intermediate- age component is also present. We have identified carbon star candidates, and show that they dominate the stellar content for intermediate mass and age. We show that low metallicity intermediate- age stars have, for a given colour, a F110W (similar to J- band) excess compared to local carbon stars. The carbon stars identified in IZw 18 are the most distant ( more than 10 Mpc) resolved examples yet discovered.