Primordial deuterium abundance at z_abs = 2:504 towards Q1009+2956

The z abs = 2.504 Lyman Limit absorption system (LLS) towards Q1009+2956 has previously been used to estimate the primordial deuterium abundance. Since the initial measurement by Burles & Tytler [1], this quasar has been observed extensively with the Keck telescope, providing a substantial increase in signal-to-noise (from 60 to 147 at continuum level of Ly-α at z abs = 2.504). We explore a set of different models for the absorption system and find that the deuterium feature is contaminated by Ly-α absorption from a low column density HI cloud. This significantly limits precision to which one can estimate the D/H ratio in this LLS. Our final result for this system $\text{D/H}=2.48_{-0.35}^{+0.41}\times {{10}^{-5}}$ has the same relative uncertainty of 17% as the previous estimate by [1] despite the far higher signal-to-noise of our dataset. A weighted mean of 13 D/H measurements available in the literature (including our result) gives the primordial deuterium abundance of (D/H)p = (2.545 ± 0.025) × 10−5 and the corresponding baryon density of the Universe of Ωb h 2 = 0.02174 ± 0.00025 marginally inconsistent with the 2015 Planck CMB data. More measurements of the deuterium abundance in quasar spectra are needed to check this tension