The cosmological baryon density from the deuterium to hydrogen ratio towards QSO absorption systems: D/H towards Q1243+3047

TL;DR

<p>We measure the deuterium-to-hydrogen ratio in a $z=2.526$ LLS toward Q1243+3047 using Keck/HIRES data, implementing a comprehensive, end-to-end modeling framework that treats the continuum, Ly$\alpha$ forest, and absorber velocity structure simultaneously. The analysis yields $\log (D/H) = -4.617^{+0.058}_{-0.048}$ (i.e., $D/H = 2.42^{+0.35}_{-0.25} \times 10^{-5}$) with $\log N_{\rm HI} = 19.73 \pm 0.04$, and $\log N_{\rm DI,total} = 15.113^{+0.042}_{-0.026}$, implying a primordial baryon density $\Omega_b h^2 = 0.0214 \pm 0.0020$ and $\eta = 5.9 \pm 0.5\times 10^{-10}$ in good agreement with pre-MAP CMB results. By combining this measurement with five QSOs, the inferred primordial D/H is $\log (D/H) = -4.556 \pm 0.064$ (D/H $=2.78^{+0.44}_{-0.38} \times 10^{-5}$), though the data show a dispersion larger than individual errors, suggesting some underestimated uncertainties in several measurements. The work reinforces D/H as a robust baryometer and highlights methodological advances (B-spline continuum modeling, grid-based D/H exploration, and joint continuum/Ly$\alpha$ forest/DLA fitting) that improve error estimation and reliability of primordial abundance inferences.</p>

Abstract

We report the detection of Deuterium absorption at redshift 2.525659 towards Q1243+3047. We describe improved methods to estimate the Deuterium to Hydrogen abundance ratio (D/H) in absorption systems, including improved modeling of the continuum level, the Ly-alpha forest and the velocity structure of the absorption. Together with improved relative flux calibration, these methods give D/H = 2.42^+0.35_-0.25 x 10^-5 cm^-2 from our Keck-I HIRES spectra of Q1243+3047, where the error is from the uncertainty in the shape of the continuum level and the amount of D absorption in a minor second component. The measured D/H is likely the primordial value because the [O/H] = -2.79 +/- 0.05. This absorption system has a neutral Hydrogen column density of 19.73 +/- 0.04 cm^-2, it shows five D lines and is mostly ionized. The best estimate of the primordial D/H is 2.78^+0.44_-0.38 x 10^-5, from the log D/H values towards five QSOs. The dispersion in the five values is larger than we expect from their individual measurement errors and we suspect this is because some of these errors were underestimated. We observe a trend in D/H with neutral H column density that we also suspect is spurious. The D/H corresponds to a baryon-to-photon ratio ETA = 5.9 +/- 0.5 x 10^-10 and a cosmological baryon density Omega_b h^2 = 0.0214 +/- 0.0020 (9.3%) that agrees with values from the pre-MAP measurements of the anisotropy of the Cosmic Microwave Background.

Figures (32)

• Figure 1: The spectra of Q1243+3047 from the KAST spectrograph (top), HIRES (middle) and ESI (bottom). We show the complete wavelength coverage for the Kast and HIRES spectra, but not for the ESI, which extends to 10,000 Å . We have applied relative flux calibration to all three spectra. The emission lines blend to give a continuously undulating continuum level from 4400 -- 5000 Å . The vertical marks above the Kast and HIRES spectra show the positions of the Lyman series lines in the absorption system at $z = 2.526$ that gives the D/H value. The Ly$\alpha$ absorption line of this system, from which we get the H I column density, is near 4285 Å , just to the left of the peak of the Ly$\alpha$ emission line. We do not plot most pixels, to reduce the file size.
• Figure 2: Expansion of the Kast, ESI and HIRES spectra from Figure 1. The Ly$\alpha$ absorption near 4285 Å is from the system in which we measure D/H.
• Figure 3: Most of the metal absorption lines near $z = 2.526$. We shifted the Al II 1670 spectrum, which is the only one from ESI, by $-16.5$ km s$^{-1}$ to correct a likely error. We see three types of components, grouped by ionization; the low ionization lines represented by O I alone, intermediate ions C II, Si II and high ionization C IV and Si IV.
• Figure 4: Voigt profile fits to the lines of the low ionization ions. The line parameters used to generate these profiles are given in Table \ref{['dhlinetab']}. The data is our combined HIRES spectrum, with $1\sigma$ errors shown just above the zero flux level. Flux is in units of $10^{-16}$ erg s$^{-1}$ cm$^{-2}$ Å $^{-1}$. The O I absorption shows a well defined component 1 at $v =0.0$ km s$^{-1}$, and additional absorption in component 2 near $+13$ km s$^{-1}$. C II and Si II show several components, including a main component near $v = 0$ km s$^{-1}$ for C II (1335) or 3 km s$^{-1}$ for C II (1036) and Si II, and a second component near 16 km s$^{-1}$ . We see no absorption near --82 km s$^{-1}$.
• Figure 5: As Figure \ref{['lowions']}, but here we show the lines of the high ionization ions. Again, there is no evidence of metal line absorption near --82 km s$^{-1}$.