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

Published online by Cambridge University Press:  01 December 2006

A. Aloisi ,
F. Annibali ,
J. Mack ,
M. Tosi ,
R. van der Marel ,
G. Clementini ,
R. A. Contreras ,
G. Fiorentino ,
M. Marconi  and
I. Musella
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A. Aloisi
Affiliation:
Space Telescope Science Institute, Baltimore, USA email: aloisi@stsci.edu On Assignment from the Space Telescope Division of the European Space Agency
F. Annibali
Affiliation:
Space Telescope Science Institute, Baltimore, USA email: aloisi@stsci.edu
J. Mack
Affiliation:
Space Telescope Science Institute, Baltimore, USA email: aloisi@stsci.edu
M. Tosi
Affiliation:
INAF – Osservatorio Astronomico di Bologna, Bologna, Italy
R. van der Marel
Affiliation:
Space Telescope Science Institute, Baltimore, USA email: aloisi@stsci.edu
G. Clementini
Affiliation:
INAF – Osservatorio Astronomico di Bologna, Bologna, Italy
R. A. Contreras
Affiliation:
INAF – Osservatorio Astronomico di Bologna, Bologna, Italy
G. Fiorentino
Affiliation:
INAF – Osservatorio Astronomico di Bologna, Bologna, Italy
M. Marconi
Affiliation:
INAF – Osservatorio Astronomico di Capodimonte, Napoli, Italy
I. Musella
Affiliation:
INAF – Osservatorio Astronomico di Capodimonte, Napoli, Italy
A. Saha
Affiliation:
National Optical Astronomy Observatories, Tucson, USA
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Abstract

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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.

Keywords

galaxies: dwarfirregularstarburststellar contentdistances and redshiftsCepheids
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 2 , Symposium S241: Stellar Populations as Building Blocks of Galaxies , December 2006 , pp. 310 - 314
Copyright
Copyright © International Astronomical Union 2007

References

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