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

  • A. Aloisi (a1) (a2), F. Annibali (a1), J. Mack (a1), M. Tosi (a3), R. van der Marel (a1), G. Clementini (a3), R. A. Contreras (a3), G. Fiorentino (a3), M. Marconi (a4), I. Musella (a4) and A. Saha (a5)...

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.

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References

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Keywords

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

  • A. Aloisi (a1) (a2), F. Annibali (a1), J. Mack (a1), M. Tosi (a3), R. van der Marel (a1), G. Clementini (a3), R. A. Contreras (a3), G. Fiorentino (a3), M. Marconi (a4), I. Musella (a4) and A. Saha (a5)...

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