The Star Formation History of the very metal poor BCD IZw18 from HST/ACS data

TL;DR

This study reconstructs the star formation history of the metal-poor galaxy IZw18 using HST data, revealing it is not a young galaxy and has experienced active star formation over a significant period, with implications for understanding early galaxy evolution.

Contribution

First detailed star formation history of IZw18 derived from deep HST/ACS data, showing it is not a primordial galaxy and has complex star formation episodes.

Findings

IZw18 started star formation over 1 Gyr ago.

Main body has high recent star formation rate (~1 M_sun/yr).

Secondary body was less active, with implications for galaxy evolution.

Abstract

We have derived the star formation history (SFH) of the blue compact dwarf galaxy IZw18 through comparison of deep HST/ACS data with synthetic color magnitude diagrams. A statistical analysis was implemented for the identification of the best-fit SFH and relative uncertainties. We confirm that IZw18 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 IZw18's main body we infer a lower limit of ~2 x 10^{6} M_sun for the mass locked-up in old stars. IZw18' 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_sun/yr (or ~2 x 10^{-5} M_sun 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 IZw18, 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.