The primordial helium abundance from a new sample of metal-deficient blue compact galaxies

Abstract

We use high-quality spectrophotometric observations of 10 low-metallicity blue compact galaxies (BCGs) with oxygen abundance ranging from 12 + log(O/H) = 7.37 to 8.04 to determine the primordial helium abundance. We take special care into investigating the physical effects which may effect such a determination. We find that underlying stellar absorption, enhancement of helium lines due to self-absorption, and corrections for neutral and doubly ionized helium to be negligible. The main physical mechanism changing the He I line intensities from their recombination values is collisional excitation. To correct for it, we calculate the electron number density in the He(+) zone by a self-consistent procedure which constrains the He I lambda 5876/lambda 4471, lambda 6678/lambda 4471 and lambda 7065/lambda 4471 line ratios to have their recombination values, after correction for collisional enhancement. The density sensitive He I lambda 7065 line plays an important role in this respect. The usual practice of using the electron number density derived from the (S II) lambda 6717/lambda 6731 ratio underestimates the collisional enhancement correction and hence overestimates the helium abundance by approximately 5%. We find that Brocklehurst's (1972) He I recombination emissivities give a smaller dispersion of the data points in the Y versus O/H and Y versus N/H planes and within the derived He abundances from different lines in a given galaxy than Smits's old (1991a, b) emissivities. The dispersion is comparable when Smits's new (1994) emissivities are used. By taking the mean of the intercepts of both Y versus O/H and Y versus N/H linear regressions at O/H = N/H = 0, we determine a primordial helium mass fraction Yp = 0.229 +/- 0.004, with Brocklehurst's emissivities, and Yp = 0.240 +/- 0.005 with Smits's new emissivities. Taking the mean Y of the four most metal-deficient BCGs (Z less than Z(solar)/15) in our sample gives Yp = 0.233 +/- 0.003 with Brocklehurst's emissivities. Adding the four known BCGs with Z less than Z(solar)/15 from the literature yields: Yp = 0.232 +/- 0.002. These determinations are consistent with the lower limit of Yp = 0.236 required by standard big bang nucleosynthesis theory. We obtain a slope dY/dZ = 5.8 +/- 1.7 with Brocklehurst's emissivities and dY/dZ = 5.8 +/- 4.4 with Smits' new emissivities, larger than the values predicted by closed-box models, but consistent with those given by chemical evolution models with differential galactic winds.