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

Abstract

We have derived the star formation history (SFH) of the blue compact dwarf galaxy I Zw 18 through comparison of deep HST/ACS data with synthetic color-magnitude diagrams (CMDs). A statistical analysis was implemented for the identification of the best-fit SFH and relative uncertainties. We confirm that I Zw 18 is not a truly young galaxy, having started forming stars earlier than ∼1 Gyr ago, and possibly at epochs as old as a Hubble time. In I Zw 18's main body we infer a lower limit of ≈2 × 10 6 M for the mass locked up in old stars. I Zw 18's main body has been forming stars very actively during the last ∼10 Myr, with an average star formation rate (SFR) as high as ≈1 M yr −1 (or ≈2 × 10 −5 M yr −1 pc −2 ). On the other hand, the secondary body was much less active at these epochs, in agreement with the absence of significant nebular emission. The high current SFR can explain the very blue colors and the high ionized gas content in I Zw 18, 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, or if galactic winds with loss of metals are needed.

J0811+4730: the most metal-poor star-forming dwarf galaxy known

“…We obtain aperturecorrected stellar masses 10 7.34 M⊙ and 10 6.30 M⊙ for the NW and SE components, respectively. The total stellar mass of the I Zw 18 main body derived by SED fitting is therefore 10 7.38 M⊙, consistent with the lower limit derived by Annibali et al (2013) from the CMD.…”

“…This SFG has been selected because its stellar mass can be determined by at least another indepen-dent method. Annibali et al (2013) have used the CMD of its resolved stellar populations to derive a stellar mass of >10 7.24 M⊙ of its main body, adopting a distance of 18.2 Mpc (Aloisi et al 2007). This is to be compared with the stellar mass obtained from the SED fitting described above.…”

Hubble Space Telescope Observations of Accretion-Induced Star Formation in the Tadpole Galaxy Kiso 5639

“…I Zw 18 is one of the lowest metallicity local galaxies (Searle & Sargent 1972;Izotov et al 2009). Our estimates for supernovae clearing in Kiso 5639 are consistent with similar conclusions about star formation in I Zw 18 based on HST WFPC2 images (Hunter & Thronson 1995;Annibali et al 2013;Lebouteiller et al 2013).…”

STELLAR POPULATIONS AND STAR FORMATION HISTORY OF THE METAL-POOR DWARF GALAXY DDO 68∗

“…the blue supergiants. A similar problem was found for another very low-metallicity dwarf galaxy, I Zw 18, for which we derived the SFH (Annibali et al 2013). As already discussed in that study, possible explanations for the observed discrepancy are: i) the completeness behavior with magnitude is not properly taken into account, because of the severe crowding of this central region; ii) some of the most luminous blue stars in the CMD are not actually single objects but blends of two or more blue stars; iii) the timescales for the evolution of massive stars in the brightest post-MS phase are not properly modeled.…”

“…It is important to notice that at distances of few Mpc the SFH older than 1 Gyr necessarily relies on RGB stars, therefore the corresponding uncertainties are huge (see Appendix B of Weisz et al 2011). Together with the mentioned NGC 1569 (Greggio et al 1998;Angeretti et al 2005) and NGC 1705 (Annibali et al 2003(Annibali et al , 2009), we include in this comparison I Zw 18 (Annibali et al 2013), I Zw 36 (Schulte-Ladbeck et al 2001 and MrK 187 (Schulte-Ladbeck et al 2000). The shown galaxies are too far away even to detect the He-burning phase of stars with ages between 0.5 and 1 Gyr, hence the resulting SFHs are rather uncertain also in that age range.…”