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A New Deep HST/ACS CMD of I Zw 18: Evidence for Red Giant Branch Stars

A. Aloisi·F. Annibali·J. Mack·M. Tosi·R. van der Marel·G. Clementini·R. Contreras·G. Fiorentino·M. Marconi·I. Musella·A. Saha
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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.

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A. Aloisi, F. Annibali, J. Mack, M. Tosi, R. van der Marel, G. Clementini, R. Contreras, G. Fiorentino, M. Marconi, I. Musella, A. Saha. 2007-02-09. A New Deep HST/ACS CMD of I Zw 18: Evidence for Red Giant Branch Stars. https://doi.org/10.1017/s1743921307008228

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