A new infrared view of evolved stars in IZw 18

¹ Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden e-mail: ostlin@astro.su.se
² School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK
³ Observatoire Astronomique de Strasbourg (UMR 7550), 11 rue de l'Université, 67000 Strasbourg, France

Received: 28 April 2004
Accepted: 31 October 2004

Abstract

We report results from near-infrared imaging of IZw 18, 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 (~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 (~J) and F160W (~H) filters, provide a census of the cool stellar population in this chemically unevolved galaxy. We find that stars as old as ~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 (~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.