The deuterium to hydrogen abundance ratio toward a fourth QSO: HS 0105+1619

We report the measurement of the primordial D/H abundance ratio toward QSO HS 0105+1619. The column density of the neutral hydrogen in the z similar or equal to 2.536 Lyman limit system is high, log N-HI = 19.422 +/- 0.009 cm(-2), allowing for the deuterium to be seen in five Lyman series transitions. The measured value of the D/H ratio toward QSO HS 0105 + 1619 is found to be D/H = 2.54 +/- 0.23 x 10(-5). The metallicity of the system showing D/H is found to be similar or equal to 0.01 solar, indicating that the measured D/H is the primordial D/H within the measurement errors. The gas that shows D/H is neutral, unlike previous D/H systems that were more highly ionized. Thus, the determination of the D/H ratio becomes more secure since we are measuring it in different astrophysical environments, but the error is larger because we now see more dispersion between measurements. Combined with prior measurements of D/H, the best D/H ratio is now D/H = 3.0 +/- 0.4 x 10(-5), which is 10% lower than the previous value. The new values for the baryon-to-photon ratio and baryonic matter density derived from D/H are eta = 5.6 +/- 0.5 x 10(-10) and Omega (b), h(2) = 0.0205 +/- 0.0018, respectively.