A New Deep HST/ACS CMD of I Zw 18: Evidence for Red Giant Branch Stars
Abstract
We present results from new deep HST/ACS photometry of I Zw 18, the most metal-poor blue compact dwarf galaxy in the nearby universe. It has been previously argued that this is a very young system that started forming stars only ≲500 Gyr ago, but other work has hinted that older (≳1 Gyr) red giant branch (RGB) stars may exist in this galaxy. Our deeper data indeed reveal evidence for an RGB. Underlying old (≳1 Gyr) populations are therefore present in even the most metal-poor systems, implying that star formation started at z ≳ 0.1. The RGB tip (TRGB) magnitude and the properties of Cepheid variables identified from our program indicate that I Zw 18 is farther away (D = 19.0 ± 1.8 Mpc) than previously believed.
- Publication:
-
Stellar Populations as Building Blocks of Galaxies
- Pub Date:
- August 2007
- DOI:
- 10.1017/S1743921307008228
- arXiv:
- arXiv:astro-ph/0702216
- Bibcode:
- 2007IAUS..241..310A
- Keywords:
-
- galaxies: dwarf;
- irregular;
- starburst;
- stellar content;
- distances and redshifts;
- Cepheids;
- Astrophysics
- E-Print:
- 4 pages, 2 figures. Typos corrected. Proceedings of the IAU Symp. 241 "Stellar Populations as Building Blocks of Galaxies", held in La Palma in December 2006, eds. R. F. Peletier &