We present V and I photometry of the resolved stars in the most metal-deficient blue compact dwarf galaxy known , I Zw 18 ( Z _ { \odot } /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 \lesssim 30 Myr ) and blue and red supergiants ( 10 Myr \lesssim age \lesssim 100 Myr ) , but also an older evolved population of asymptotic giant branch ( AGB ) stars ( 100 Myr \lesssim age \lesssim 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 \leq 15 Mpc , our imaging data go \sim 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 \sim 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 .