Primordial helium, spectrophotometric technique, and I Zwicky 18

This paper is largely a warning to theorists and an appeal to observers. The most often quoted estimates of the primordial helium abundance are optimistic in the sense that quoted uncertainties usually do not include some potentially serious systematic errors. As a very nonroutine topic, primordial helium demands unusual care in the acquisition, reduction, analysis, and exposition of data. Here we illustrate the situation by describing emission-line observations of the highly relevant dwarf galaxy I Zw 18, which is interesting in its own right as a nearly primordial object. The helium mass fraction that we find for I Zw 18 is (0.212 +- 0.015) multiplied by a correction factor (perhaps about 1.1) which must be determined by specific observations and calculations beyond the scope of this paper. The standard error given here is more pessimistic, and we believe more realistic, than those quoted in most previous papers on primordial helium. Additional observational work should be done on I Zw 18 and on several other faint emission-line galaxies that have the most nearly primordial compositions in their class. We attempt to make the measurement process clear for those readers who are interested in primordial helium but who are unfamiliar with techniquesmore » of nebular spectroscopy and analysis.« less