Deuterium Abundance in the Most Metal-Poor Damped Lyman alpha System: Converging on Omega_baryons

TL;DR

This study leverages a very metal-poor damped Lyα system toward Q0913+072 to measure the primordial deuterium abundance from resolved DI Lyman-series lines, obtaining log(D/H) = -4.56 ± 0.04. By aggregating seven high-redshift D/H measurements and applying bootstrap/Bayesian analyses, the authors determine ⟨log(D/H)⟩_p ≈ -4.55 ± 0.03 and derive Ω_b,0 h^2(BBN) ≈ 0.0213 ± 0.0010, in good agreement with CMB constraints. They demonstrate how the D/H_p prior tightens ΛCDM parameter estimates from WMAP5, notably lowering n_s slightly (to ≈0.956–0.959) and constraining tensor modes (r < ~0.16–0.26, 95% c.l.), while highlighting sensitivity to the error model of D/H measurements. The results indicate a converged, robust baryon density and primordial deuterium abundance, with Planck anticipated to further sharpen these cosmological inferences.

Abstract

The most metal-poor DLA known to date, at z = 2.61843 in the spectrum of the QSO Q0913+072, with an oxygen abundance only about 1/250 of the solar value, shows six well resolved D I Lyman series transitions in high quality echelle spectra recently obtained with the ESO VLT. We deduce a value of the deuterium abundance log (D/H) = -4.56+/-0.04 which is in good agreement with four out of the six most reliable previous determinations of this ratio in QSO absorbers. We find plausible reasons why in the other two cases the 1 sigma errors may have been underestimated by about a factor of two. The addition of this latest data point does not change significantly the mean value of the primordial abundance of deuterium, suggesting that we are now converging to a reliable measure of this quantity. We conclude that <log (D/H)_p> = -4.55+/-0.03 and Omega_b h^2 (BBN) = 0.0213+/-0.0010 (68% confidence limits). Including the latter as a prior in the analysis of the five year data of WMAP leads to a revised best-fitting value of the power-law index of primordial fluctuations n_s = 0.956+/-0.013 (1 sigma) and n_s < 0.990 with 99% confidence. Considering together the constraints provided by WMAP 5, (D/H)_p, baryon oscillations in the galaxy distribution, and distances to Type Ia supernovae, we arrive at the current best estimates Omega_b h^2 = 0.0224+/-0.0005 and n_s = 0.959+/-0.013.

Figures (8)

• Figure 1: Observed profiles (black histograms) and fitted Voigt profiles (continuous red lines) of absorption lines in the Lyman series of the $z_{\rm abs} = 2.61843$ DLA in Q0913+072. The $y$-axes of the plots show relative intensity. The two vertical tick marks in each panel indicate the expected locations of the main absorption component of the DLA in, respectively, H i (at $v = 0$ km s$^{-1}$ in the plots) and D i (at $v = -81.6$ km s$^{-1}$). A second absorption component, centred at $v = -12.4$ km s$^{-1}$, is resolved in the metal lines associated with this DLA, but the two components are blended in the intrinsically broader H i and D i absorption. Additional H i absorption is seen at positive velocities relative to the DLA, but its column density is $\sim 100$ times lower than that of the DLA, and therefore does not contribute to the observed D i absorption lines (see text).
• Figure 2: Apparent optical depth analysis: column density as a function of velocity for the four unsaturated and unblended D i lines in the $z_{\rm abs} = 2.61843$ DLA in Q0913+072. The transitions shown in the legend are in increasing order of $f \lambda$ from the top. Values of $N_{\rm TOT}$(D i) are collected in Table \ref{['tab:N_aod']}.
• Figure 3: Top: Portion of the normalised spectrum of Q0913+072 encompassing the damped Ly$\alpha$ line at $z_{\rm abs} = 2.61843$ (black histogram). The continuous red line shows the theoretical profile for a neutral hydrogen column density $\log [N$(H i)/cm$^{-2}] = 20.34$. Bottom: Expanded portion of the upper plot on a scale chosen to illustrate the sensitivity of the damping wings to the column density of neutral hydrogen. The short-dash green lines correspond to the $\pm 0.04$ limits to the best fitting value $\log [N$(H i)/cm$^{-2}] = 20.34$. In both panels the $y$-axis is residual intensity.
• Figure 4: Measures of deuterium abundance in high redshift QSO absorbers. Only cases were the deuterium absorption is clearly resolved from nearby spectral features are shown here (see text). Blue circles denote systems observed from the ground with 8-10 m telescopes and echelle spectrographs, while the red triangle refers to lower resolution observations made with the Hubble Space Telescope. Absorption systems discussed in the text are labelled with the name of the background QSO. The horizontal lines are drawn at the weighted mean value of $\log {\rm (D/H)}$ and its error, as determined with the bootstrap method.
• Figure 5: Probability distributions of cosmological parameters deduced from the analysis of WMAP 5 data alone (red dashed line) and including $\Omega_{\rm b,0}h^2{\rm (BBN) }$ as a prior (black continuous line).