Astronomy and Astrophysics Is I Zw 18 a Young Galaxy?

Hubble Space Telescope (HST) colour magnitude diagrams in B, V and R along with long-slit Multiple Mirror Telescope (MMT) spectrophotometric data are used to investigate the evolutionary status of the nearby blue compact dwarf (BCD) galaxy I Zw 18. We find that the distance to I Zw 18 should be as high as 20 Mpc, twice the previously accepted distance, to be consistent with existing observational data on the galaxy: colour-magnitude diagrams, the high ionization state of the gas and presence of WR stars in the main body, and the ionization state of the C component. The spectral energy distribution (SED) of the main body of I Zw 18 is consistent with that of a stellar population with age <∼ 5 Myr. However, the presence of large-scale shells observed around the main body suggests that star formation began ∼ 20 Myr at the NW end and propagated in the SE direction. Our analysis of colour-magnitude diagrams and of the spectral energy distribution of the C component implies that star formation in this component started <∼ 100 Myr ago at the NW end, propagated to the SE and stopped ∼ 15 Myr ago. Thus, I Zw 18 is likely to be one of the youngest nearby extragalactic objects.