A New Deep HST/ACS CMD of I Zw 18: Evidence for Red Giant Branch Stars
arXiv:astro-ph/0702216 · doi:10.1017/S1743921307008228
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 Myr 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.
arXiv comment4 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 & A. Vazdekis
References in corpus (5)
- Heavy element abundances in blue compact galaxies
- A step toward the calibration of the RGB Tip as a Standard Candle
- Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation
- The Star Formation History of IZw18
- Do young galaxies exist in the Local Universe? - Red Giant Branch detection in the metal-poor dwarf SBS 1415+437