Molecular hydrogen in a damped lyman-alpha system at Zabs = 4.224

Abstract

We present the direct detection of molecular hydrogen at the highest redshift known today (zabs = 4.224) in a Damped Lyman-α (DLA) system toward the quasar PSS J 1443+2724. This absorber is remarkable for having one of the highest metallicities amongst DLA systems at zabs > 3, with a measured iron abundance relative to Solar of −1.12 ± 0.10. We provide for the first time in this system accurate measurements of Ni, Mg ii, S ii and Ar i column densities. The sulfur and nitrogen abundances relative to Solar, −0.63 ± 0.10 and −1.38 ± 0.10 respectively, correspond exactly to the primary nitrogen production plateau. H2 absorption lines are detected in four different rotational levels (J = 0, 1, 2 and 3) of the vibrational ground-state in three velocity components with total column densities of logN(H2) = 17.67, 17.97, 17.48 and 17.26 respectively. The J = 4 level is tentatively detected in the strongest component with logN(H2) ∼ 14. The mean molecular fraction is log f = −2.38 ± 0.13, with f = 2N(H2)/(2N(H2) + N(Hi)). We also measure logN(HD)/N(H2) < −4.2. The excitation temperatures T01 for the two main components of the system are 96 and 136 K respectively. We argue that the absorbing galaxy, whose star formation activity must have started at least 2− 5× 108 yrs before z = 4.224, is in a quiescent state at the time of observation. The density of the gas is small, nH ≤ 50 cm−3 , and the temperature is of the order of T ∼ 90 − 180 K. The high excitation of neutral carbon in one of the components can be explained if the temperature of the Cosmic Microwave Background Radiation has the valueexpected at the absorber redshift, T = 14.2 K. These observations demonstrate the feasibility to study H2 at the highest redshifts provided high enough spectral resolution and good S/N ratio are achieved.