Molecular Hydrogen in a Damped Lyalpha System at zabs=4.224

International audienceWe present a direct detection of molecular hydrogen at the highest redshift known today (zabs=4.224) in a damped Lyalpha (DLA) system toward the quasar PSS J1443+2724. This absorber is remarkable for having one of the highest metallicities among 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 N I, 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 logN(H2) of 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 logf=-2.38+/-0.13, with f=2N(H2)/[2N(H2)+N(H I)]. We also measure log[N(HD)/N(H2)