Reexamining the Helium Abundance of I Zw 18

We present high signal-to-noise 4 m KPNO Mayall telescope optical spectrophotometry of the two brightest northwest (NW) and southeast (SE) star-forming components in I Zw 18, the most metal-deficient blue compact dwarf (BCD) galaxy known. In addition, the flux-calibrated spectrum of the faint C component, a blue irregular star-forming region ~22'' northwest of the NW component, is presented. The Hβ equivalent width increases from the C component, through the NW component, to the SE component, indicating an age sequence and propagating star formation from the oldest C component (age ~2 × 108 yr) to the youngest SE component (age ~5 × 106 yr). The spatial distribution of the nebular He II λ4686 emission is investigated. It is confined to the older NW component, suggesting that hot post-main-sequence stars may be responsible for this emission.