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(circle dot)/50), using Hubble Space Telescope/ACS images, the deepest ones ever obtained for this galaxy. The resulting I versus 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 less than or similar to 30 Myr) and blue and red supergiants (10 Myr less than or similar to age less than or similar to 100 Myr), but also an older evolved population of asymptotic giant branch (AGB) stars (100 Myr less than or similar to age less than or similar to 500 Myr). We derive a distance to I Zw 18 in the range 12.6-15 Mpc from the brightness of its AGB stars, with preferred values in the higher range. Red giant branch (RGB) stars are conspicuous by their absence, although, for a distance of I Zw 18 less than or equal to 15 Mpc, our imaging data go similar to1-2 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 similar to100-200 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 buildup of a young galaxy.