HD molecules at high redshift - A low astration factor of deuterium in a solar-metallicity DLA system at z = 2.418

Abstract

We present the detection of deuterated molecular hydrogen (HD) in the remote Universe in a damped Lyman-α cloud at Z_abs = 2.418 toward the quasar SDSSJ143912.04+111740.5. This is a unique system in which H₂ and CO molecules are also detected. The chemical enrichment of this gas derived from ZnII and SII is as high as in the Sun. We measure N(HD)/2N(H₂) = 1.5^+0.6_-0.4 × 10^-5, which is significantly higher than the same ratio measured in the Galaxy and close to the primordial D/H ratio estimated from the WMAP constraint on the baryonic matter density (Ω_b). This indicates a low astration factor of deuterium that contrasts with the unusually high chemical enrichment of the gas. This can be interpreted as the consequence of an intense infall of primordial gas onto the associated galaxy. Detection of HD molecules at high-z also opens the possibility of obtaining an independent constraint on the cosmological-time variability of μ, the proton-to-electron mass ratio.