The primordial deuterium abundance at z = 2.504 from a high
  signal-to-noise spectrum of Q1009+2956
  The spectrum of the $z_{\rm em} = 2.63$ quasar Q1009+2956 has been observed
extensively on the Keck telescope. The Lyman limit absorption system $z_{\rm
abs} = 2.504$ was previously used to measure D/H by Burles & Tytler using a
spectrum with signal to noise approximately 60 per pixel in the continuum near
Ly {\alpha} at $z_{\rm abs} = 2.504$. The larger dataset now available combines
to form an exceptionally high signal to noise spectrum, around 147 per pixel.
Several heavy element absorption lines are detected in this LLS, 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 {\alpha} absorption from a low column density H I cloud,
reducing the expected D/H precision. We find D/H =
$2.48^{+0.41}_{-0.35}\times10^{-5}$ 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)$_{\rm p} = (2.545 \pm 0.025)\times10^{-5}$. The
corresponding mean baryonic density of the Universe is $\Omega_{\rm b}h^2 =
0.02174\pm0.00025$. The quasar absorption data is of the same precision as, and
marginally inconsistent with, the 2015 CMB Planck (TT+lowP+lensing)
measurement, $\Omega_{\rm b}h^2 = 0.02226\pm0.00023$. Further quasar and more
precise nuclear data are required to establish whether this is a random
fluctuation.
