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 similar to 22 " northwest of the NW component, is presented. The H beta 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 similar to 2 x 10(8) yr) to the youngest SE component (age similar to 5 x 10(6) yr). The spatial distribution of the nebular He II lambda 4686 emission is investigated. It is confined to the older NW component, suggesting that hot post-main-sequence stars may be responsible for this emission. We derive from the He I lambda 4471 and lambda 6678 emission lines a helium mass fraction Y in the SE component equal to 0.242 +/- 0.009. This value is higher than the value Y = 0.230 +/- 0.009 derived by Skillman & Kennicutt and Y = 0.231 +/- 0.010 by Pagel and coworkers for the SE component, although all values are consistent within the errors. By contrast, the helium mass fraction in the NW component, which has been used until now in the primordial helium abundance determination, is lower, equal only to 0.233 +/- 0.008 from the He I lambda 6678 emission line and to 0.199 +/- 0.008 from the He I lambda 4471 emission line. We argue that the lower Y-values and the large discrepancy between the values derived from the two lines in the NW component are caused by strong underlying stellar absorption in the He I lines. This absorption is much weaker in the SE component, and its Y-value should be the one to use for primordial helium abundance determination. The new Y-value in I Zw 18 is in agreement with the primordial helium mass fraction Y-p = 0.243 +/- 0.003 derived earlier by our group by extrapolation of the linear regressions of the Y-O/H and Y-N/H relations for a large sample of BCDs to O/H = N/H = 0.