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Astron. Astrophys. 355, 891-899 (2000)

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A continuous low star formation rate in IZw 18? *

F. Legrand 1,5, D. Kunth 1, J.-R. Roy 2, J.M. Mas-Hesse  **  3 and J.R. Walsh  ***  4

1 Institut d'Astrophysique de Paris, CNRS, 98bis boulevard Arago, 75014 Paris, France
2 Département de physique and Observatoire du mont Mégantic, Université Laval, Québec Qc G1K 7P4, Canada
3 LAEFF-INTA, Apdo 50727, 28080 Madrid, Spain
4 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
5 Instituto Nacional de Astrofsica, Optica y Electrónica, Tonantzintla, Apartado Postal 51 y 216, 72000 Puebla, México

Received 6 September 1999 / Accepted 13 January 2000


Deep long-slit spectroscopic observations of the blue compact galaxy IZw 18 obtained with the CFH 3.6 m Telescope are presented. The very low value of oxygen abundance previously reported is confirmed and a very homogeneous abundance distribution is found (no variation larger than 0.05 dex) over the whole ionized region. We concur with Tenorio-Tagle (1996) and Devost et al. (1997) that the observed abundance level cannot result from the material ejected by the stars formed in the current burst, and propose that the observed metals were formed in a previous star formation episode. Metals ejected in the current burst of star formation remain most probably hidden in a hot phase and are undetectable using optical spectroscopy. We discuss different scenarios of star formation in IZw 18. Combining various observational facts, for instance the faint star formation rate observed in low surface brightness galaxies (Van Zee et al. 1997c), it is proposed that a low and continuous rate of star formation occurring during quiescent phases between bursts could be a significant source of metal enrichment of the interstellar medium.

Key words: galaxies: dwarf – galaxies: individual: IZw 18 – galaxies: ISM

* This research has made use of NASA's Astrophysics Data System Abstract Service.
** JMMH supported by Spanish CICYT under grant ESP95-0389-C02-02.
*** DK and JRR Visiting astronomers at Canada-France-Hawaii Telescope, which is operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France, and the University of Hawaii

Send offprint requests to: F. Legrand (legrand@iap.fr)

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000