The new infra-red view of evolved stars in IZw18

Abstract

We report results from near-infrared imaging of IZw18, 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 ($\sim$ 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 ($\sim$ J) and F160W ($\sim$ H) filters, provide a census of the cool stellar population in this chemically unevolved galaxy. We find that stars as old as $\sim 1$ 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 ($\sim$ J-band) excess compared to local carbon stars. The carbon stars identified in IZw18 are the most distant (more than 10 Mpc) resolved examples yet discovered.