Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation

We present V and I photometry of the resolved stars in the most metal-deficient blue compact dwarf galaxy known, I Zw 18 (Z⊙/50), using Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest ones ever obtained for this galaxy. The resulting I vs. V − I color-magnitude diagram (CMD) reaches limiting magnitudes V = I = 29 mag. It reveals a young stellar population of blue main-sequence (MS) stars (age . 30 Myr) and blue and red supergiants (10 Myr . age . 100 Myr), but also an older evolved population of asymptotic giant branch (AGB) stars (100 Myr . age . 500 Myr). We derive a distance to I Zw 18 in the range 12.6 Mpc – 15 Mpc from the brightness of its AGB stars, with preferred values in the higher range. The red giant branch (RGB) stars are conspicuous by their absence, although, for a distance of I Zw 18 ≤ 15 Mpc, our imaging data go ∼ 1 – 2 mag below the tip of the RGB. Thus, the most evolved stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young galaxy. Several star formation episodes can be inferred from the CMDs of the main body and the C component. There have been respectively three and two episodes in these two parts, separated by periods of ∼ 100 – 200 Myr. In the main body, the younger MS and massive post-MS stars are distributed over a larger area than the older AGB stars, suggesting that I Zw 18 is still forming from the inside out. In the C component, different star formation episodes are spatially distinct, with stellar population ages decreasing from the northwest to the southeast, also suggesting the ongoing build-up of a young galaxy. Subject headings: galaxies: stellar content – galaxies: ISM – galaxies: starburst – galaxies: dwarf – galaxies: individual (I Zw 18) – galaxies: photometry