The distance to the C component of I Zw 18 and its star formation history

Abstract

Aims. We analyzed the resolved stellar population of the C component of the extremely metal-poor dwarf galaxy I Zw18 in order to evaluate its distance and star formation history as accurately as possible. In particular, we aimed at answering the question of whether this stellar population is young. Methods. We developed a probabilistic approach to analyzing high-quality photometric data obtained with the Advanced Camera for Surveys of the Hubble Space Telescope. This approach gives a detailed account of the various stochastic aspects of star formation. We carried out two successive models of the stellar population of interest, paying attention to how our assumptions could affect the results. Results. We found a distance to the C component of I Zw 18 as high as 27 Mpc, a significantly higher value than those cited in previous works. The star formation history we inferred from the observational data shows various interesting features: a strong starburst that lasted for about 15 Myr, a more moderate one that occurred ≈100 Myr ago, a continuous process of star formation between both starbursts, and a possible episode of low level star formation at ages over 100 Myr. The stellar population studied is likely ≈125 Myr old, although ages of a few Gyr cannot be ruled out. Furthermore, nearly all the stars were formed in the last few hundreds of Myr.

I Zw 18 as morphological paradigm for rapidly assembling high-zgalaxies

“…This conclusion concurs with the one of Izotov et al (2001a) who infer an upper age of ∼100 Myr for I Zw 18 C from spectral synthesis models. Jamet et al (2010) reach a similar conclusion, that I Zw 18 C is younger than ≈125 Myr, from a probabilistic analysis of HST ACS CMDs.…”

THE STAR FORMATION HISTORY OF THE VERY METAL-POOR BLUE COMPACT DWARF I Zw 18 FROMHST/ACS DATA

“…We confirm that I Zw 18 has started forming stars earlier than ∼1 Gyr ago, and possibly at epochs as old as a Hubble time. Thus it is not a truly young galaxy at its first bursts of SF, as argued in previous studies (e.g., Izotov & Thuan 2004;Jamet et al 2010;Papaderos & Ostlin 2012). 2.…”

“…In this paper we use HST/ACS data in combination with the method of synthetic CMDs to quantitatively derive the SFH in I Zw 18. Previous similar studies have been presented by Aloisi et al (1999) and by Jamet et al (2010). However, the SFH derived by Aloisi et al (1999) used ACS data (only the Thuan data set) for the derivation of the SFH in the secondary body of I Zw 18, but inferred a distance of ∼27 Mpc from the CMD analysis, incompatible with the distance of ∼18 Mpc obtained from the Cepheid light curves.…”

“…The high current SFR in I Zw 18 explains why this galaxy is so blue and has a high ionized gas content, resembling primeval galaxies in the early universe. Detailed chemical evolution models are required to quantitatively check whether the SFH from the synthetic CMDs can explain the low measured element abundances, both in the ionized (e.g., Garnett et al 1997;Izotov & Thuan 1998) and in the neutral interstellar medium (Aloisi et al 2003;Lecavelier des Etangs et al 2004;James et al 2013), or if galactic winds with loss of metals are needed, as originally suggested by Matteucci & Tosi (1985), Pilyugin (1993), and Marconi et al (1994).…”

Concluding Remarks

“…This is a great step toward the very low metallicities of the distant Universe. Photometric data have recently been obtained from the HST for a part of this galaxy (Jamet et al 2010).…”