The primordial deuterium abundance at zabs = 2.504 from a high signal-to-noise spectrum of Q1009+2956

Abstract

The spectrum of the zem = 2.63 quasar Q1009+2956 has been observed extensively on the Keck telescope. The Lyman limit absorption system zabs = 2.504 was previously used to measure D/H by Burles & Tytler using a spectrum with signal to noise (S/N) approximately 60 per pixel in the continuum near Ly α at zabs = 2.504. The larger data set now available combines to form an exceptionally high S/N spectrum, around 147 per pixel. Several heavy element absorption lines are detected in this Lyman limit system, providing strong constraints on the kinematic structure. We explore a suite of absorption system models and find that the deuterium feature is likely to be contaminated by weak interloping Ly α absorption from a low column density Hi cloud, reducing the expected D/H precision. We find D/H |$=2.48^{+0.41}_{-0.35} \times 10^{-5}$| for this system. Combining this new measurement with others from the literature and applying the method of Least Trimmed Squares to a statistical sample of 15 D/H measurements results in a ‘reliable’ sample of 13 values. This sample yields a primordial deuterium abundance of (D/H)p = (2.545 ± 0.025) × 10−5. The corresponding mean baryonic density of the Universe is Ωbh2 = 0.02174 ± 0.00025. The quasar absorption data are of the same precision as, and marginally inconsistent with, the 2015 CMB Planck (TT+lowP+lensing) measurement, Ωbh2 = 0.02226 ± 0.00023. Further quasar and more precise nuclear data are required to establish whether this is a random fluctuation.