Molecular Hydrogen Abundance in the Dust-Free Damped Ly-$\alpha$ Galaxy at z = 3.4

Abstract.

New results from the search for H₂ absorption in the damped Ly$\alpha$ galaxy at redshift z = 3.4 toward QSO 0000-2620 (z _em = 4.1) are reported. The high-resolution ( $\lambda /\Delta\lambda = 48,000$) spectra of Q0000-2620 were obtained using the Ultraviolet - Visual Echelle Spectrograph (UVES) on the 8.2m ESO Kueyen telescope. The ortho-H₂ column density is found to be $N(J = 1) = (5.55\pm 1.35) \times 10^{13}$ cm^-2 ($2\sigma$ C.L.). The combination of N(J = 1) with the limits available for other low rotational levels restricts the excitation temperature T _ex in the range (290 - 540) K. This gives the total H₂ column density of N(H $_2) = (8.75\pm 1.25) \times 10^{13}$ cm^-2 and the corresponding fraction of hydrogen atoms bound in molecules of f(H $_2) = (6.8\pm 2.0) \times 10^{-8}$.