DeepHubble Space TelescopeACS Observations of I Zw 18: a Young Galaxy in Formation

Abstract

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/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 ≲ 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 12.6-15 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 ≤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 buildup of a young galaxy.