Reexamining the Helium Abundance of I Zw 18

Abstract

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-deÐcient blue compact dwarf (BCD) galaxy known. In addition, the Ñux-calibrated spectrum of the faint C component, a blue irregular star-forming region D22A northwest of the NW component, is presented. The Hb 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 D2 ] 108 yr) to the youngest SE component (age D5 ] 106 yr). The spatial distribution of the nebular He II j4686 emission is investigated. It is conÐned to the older NW component, suggesting that hot postÈmain-sequence stars may be responsible for this emission. We derive from the He I j4471 and j6678 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 Skill-man & 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 j6678 emission line and to 0.199 ^ 0.008 from the He I j4471 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 pri-mordial helium mass fraction derived earlier by our group by extrapolation of the Y p \ 0.243 ^ 0.003 linear regressions of the Y-O/H and Y-N/H relations for a large sample of BCDs to O/H \ N/H \ 0.