One percent determination of the primordial deuterium abundance

TL;DR

A reanalysis of a near-pristine absorber at $z_{abs}=2.52564$ toward Q1243+307 combines archival and new HIRES data to measure the primordial deuterium-to-hydrogen ratio using eight D I lines. The study finds $log10(D/H) = -4.622 \,\pm\ 0.015$ for the system, which implies $log10(D/H)_P = -4.5974 \,\pm\ 0.0052$ or $(D/H)_P = (2.527 \,\pm\ 0.030) \times 10^{-5}$, i.e., a one-percent determination. Incorporating six prior high-precision and homogeneously analyzed D/H measurements and a BBN calculation with updated nuclear inputs, the inferred primordial abundance and the baryon density are consistent with the Planck CMB results within $2\sigma$. The work supports a negligible metallicity dependence of the primordial $D/H$ and demonstrates the power of high-resolution quasar absorption-line spectroscopy in constraining early-Universe physics.

Abstract

We report a reanalysis of a near-pristine absorption system, located at a redshift z_abs=2.52564 toward the quasar Q1243+307, based on the combination of archival and new data obtained with the HIRES echelle spectrograph on the Keck telescope. This absorption system, which has an oxygen abundance [O/H]=-2.769+/-0.028 (~1/600 of the Solar abundance), is among the lowest metallicity systems currently known where a precise measurement of the deuterium abundance is afforded. Our detailed analysis of this system concludes, on the basis of eight D I absorption lines, that the deuterium abundance of this gas cloud is log_10(D/H) = -4.622+/-0.015, which is in very good agreement with the results previously reported by Kirkman et al. (2003), but with an improvement on the precision of this single measurement by a factor of ~3.5. Combining this new estimate with our previous sample of six high precision and homogeneously analyzed D/H measurements, we deduce that the primordial deuterium abundance is log_10(D/H)_P = -4.5974+/-0.0052 or, expressed as a linear quantity, (D/H)_P = (2.527+/-0.030)x10^-5; this value corresponds to a one percent determination of the primordial deuterium abundance. Combining our result with a BBN calculation that uses the latest nuclear physics input, we find that the baryon density derived from BBN agrees to within 2 sigma of the latest results from the Planck CMB data.

Figures (3)

• Figure 1: The final combined and flux-calibrated spectrum of Q1243$+$307 is shown (black histogram) with the corresponding error spectrum (blue histogram) and zero level (green dashed line). The red tick marks above the spectrum indicate the locations of the Lyman series absorption lines of the sub-DLA at redshift $z_{\rm abs}=2.52564$. Note the exquisite signal-to-noise ratio of the combined spectrum, which varies from S/N $\simeq~80$ near the Ly$\alpha$ absorption line of the sub-DLA ($\sim4300$ Å) to S/N $\simeq~25$ at the Lyman limit of the sub-DLA, near 3215 Å in the observed frame.
• Figure 2: The Ly$\alpha$ profile of the absorption system at $z_{\rm abs}=2.52564$ towards the quasar Q1243$+$307 is shown (black histogram) overlaid with the best-fitting model profile (red line), continuum (long dashed blue line), and zero-level (short dashed green line). The top panels show the raw, extracted counts scaled to the maximum value of the best-fitting continuum model. The bottom panels show the continuum normalized flux spectrum. The label provided in the top left corner of every panel indicates the source of the data. The blue points show the normalized fit residuals, (data--model)/error, of all pixels used in the analysis, and the gray band represents a confidence interval of $\pm2\sigma$. The signal-to-noise ratio is comparable between the two datasets at this wavelength range, but it is markedly different near the high order Lyman series lines (see Figures \ref{['fig:lyseriesa']} and \ref{['fig:lyseriesb']}). The red tick marks above the spectra in the bottom panels show the absorption components associated with the main gas cloud (Components 2, 3, 4, 5, 6, 8, and 10 in Table \ref{['tab:compstruct']}), while the blue tick marks indicate the fitted blends. Note that some blends are also detected in Ly$\beta$--Ly$\epsilon$.
• Figure 3: The metal absorption lines that are used in our analysis are shown as a black histogram, overlaid with the best-fitting model (red line). The data and model are normalized to the best-fitting continuum model (long dashed blue line) and corrected for the fitted zero-level (short dashed green line). The red tick marks above each spectrum indicate the location of the absorption components seen in neutral gas (Components 3, 4, 5; see Table \ref{['tab:compstruct']}), while the blue tick marks indicate the absorption components that are only seen in ionized gas (remaining components; see Table \ref{['tab:compstruct']}). The number above each tick mark indicates the Component Number, which is listed in the first column of Table \ref{['tab:compstruct']}. The blue points below each spectrum are the normalized fit residuals, (data--model)/error, of all pixels used in the analysis, and the gray band represents a confidence interval of $\pm2\sigma$. The different profiles exhibited by the neutral (O i) and single ionized species (all remaining ions shown) are likely to be the result of ionized gas.