6. HELIUM

The high abundance of ⁴He allows accurate measurements in many locations. However, ⁴He is also produced by stars, and since such high accuracy is required, the primordial abundance is best measured in locations with the least amounts of stellar production. High accuracy is desired, since D/H predicts Y_p to within 0.0014 (Y_p / Y_p = 0.006, 95% confidence), which is well beyond the typical accuracy of astronomical abundance determinations. In the local ISM, the amount of ⁴He from stars is about Y = 0.01 - 0.04; much less than Y_p , but ten times the desired accuracy for Y_p .

Helium has been seen in the intergalactic medium, where Carbon abundances are < 0.01 solar, and possibly zero in much of the volume. Strong absorption is seen from the He II Ly line at 304 Å in the redshifted spectra of quasars [114], however it is difficult to obtain an abundance from these measurements, because nearly all He is He III which is unobservable, and we do not know the ratio He II/He to within an order of magnitude. However, the strength of the He II absorption does mean that there is abundant He in the intergalactic gas [115], which has very low metal abundances, which is consistent with BBN, and probably not with a stellar origin for the ⁴He..

The best estimates of the primordial abundance of He are from ionized gas surrounding hot young stars (H II regions) in small galaxies. The two galaxies with the lowest abundances have 1/55 and 1/43 of the solar abundance. The ⁴He and H abundances come from the strengths of the emission lines which are excited by photons from near by hot stars.

Values for Y_p from these extragalactic H II regions have been reported with small errors for more than 25 years, e.g.:

• Y_p = 0.216 ± 0.02 [44]

• Y_p = 0.230 ± 0.004 [116]

• Y_p = 0.234 ± 0.008 [117]

• Y_p = 0.236 ± 0.005 [118]

• Y_p = 0.228 ± 0.005 [119]

• Y_p = 0.234 ± 0.002 ± 0.005 [46]. (random, and systematic errors)

• Y_p = 0.246 ± 0.0014 (95% prediction from low D/H and SBBN).

These values are lower than the value now predicted by low quasar D/H and they appear incompatible, because of the small errors. However Skillman et al. [120] argued that errors could be much larger than quoted, allowing Y_p < 0.252, and Pagel [121] (and private communication 1994) agreed this was possible.

The measurement of Y_p involves three steps. Emission line flux ratios must be measured to high accuracy, which requires good detector linearity and flux calibration, and corrections for reddening and stellar He I absorption. These fluxes must be converted to an abundance, which requires correction for collisional ionization and neutral He. Correction for unseen neutral He depends on the spectral senegy distribution adopted for the ionizing radiation and might change Y_p by 1 - 2 percent. Then the primordial abundance must be deduced from the Y values in different galaxies.

Izotov, Thuan & Lipovetsky [122], [123] have been pursuing a major observational program to improve the determination of Y_p . They have found many more low metallicity galaxies and have been reporting consistently higher Y_p values, most recently in their clear and persuasive paper [1]:

• Y_p = 0.244 ± 0.002 from regression with O/H and

• Y_p = 0.245 ± 0.001 from regression with N/H.

The four main reasons why these values are higher are as follows, in order of importance [1], [124], [125], (Skillman, and Thuan personal communication 1998).

1. When stellar He I absorption lines underlying He emission lines are not recognized, the derived Y_p is too low. This is a important for IZw18 [124] which has the lowest metallicity and hence great weight in the derivation of Y_p , and perhaps for many other galaxies.

2. The emission line fluxes must be corrected for collisional excitation from the metastable level. At low abundances, which correlate with high temperatures, these corrections can be several percent. The amount of correction depends on the density. There are no robust ways to measure these densities, and differing methods, used by different groups, give systematically different results. Izotov and Thuan [124] solve for the He II density, while Olive, Skillman and Steigman [46] use an electron density from the S II lines.

3. Izotov & Thuan [124] have spectra which show weaker lines, and they use the five brightest He lines, while Olive et al. [46] usually use only HeI 6678.

4. Izotov & Thuan [1], [124] correct for fluorescent enhancement, which increases the Y values from for a few galaxies.

For these reasons Izotov & Thuan [124] obtain higher Y values for individual galaxies which have also been observed by Olive, Skillman & Steigman [46], and Izotov & Thuan [124] find a shallower slope for the regression to zero metal abundance (see [11] Fig 6). And most importantly, using higher quality Keck telescope spectra, they obtain high Y_p = 0.2452 ± 0.0015 (random errors), from the two galaxies with the lowest metal abundances [126].

These measurement difficulties, combined with the recent improvements, lead most to conclude that the Y_p is in accord with the SBBN. The Izotov & Thuan [1] values are very close to the low D/H predictions, while the lower Y_p quoted by Olive [7], 0.238 ± 0.002 ± 0.005, is also consistent when the systematic error is used.

It is clear that the systematic errors associated with the Y_p estimates have often been underestimated in the past, and we propose that this is still the case, since two methods of analyzing the same Helium line fluxes give results which differ by more than the quoted systematic errors. While the Izotov & Thuan [124] method has advantages, we do not know why the method used by Olive, Skillman & Steigman [46] should give incorrect answers. Hence the systematic error should be larger than the differences in the results: 0.007 using the most recent values, or 0.011 using earlier results.