HD molecules at high redshift

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 SDSS J143912.04+111740.5. This is a unique system in which H 2 and CO molecules are also detected. The chemical enrichment of this gas derived from Zn ii and S ii is as high as in the Sun. We measure N(HD)/2N(H 2) = 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 to obtain an independent constraint on the cosmological-time variability of µ, the proton-to-electron mass ratio.