H[TINF]2[/TINF], C [CSC]i[/CSC], Metallicity, and Dust Depletion in the [CLC][ITAL]z[/ITAL][/CLC] = 2.34 Damped L[CLC]y[/CLC]α Absorption System toward QSO 1232+0815

Abstract

We report the detection of strong molecular hydrogen (H2) absorption lines in the z = 2.34 damped Lyα absorber (DLA) toward QSO 1232+0815. This is the second detection in our survey for high-redshift molecules. The new ultraviolet spectrum of the QSO 1232+0815 at 0.9 Å resolution, taken with the Multiple Mirror Telescope Blue Channel Spectrograph, shows the υ = 0-0 up to 10-0 Lyman bands and also the Werner υ = 0-0 band of H2 associated with the z = 2.34 DLA. We have estimated the total H2 column density in this system. It ranges from ≈3 × 1019 cm-2 to ≈3 × 1017 cm-2 depending on the Doppler parameters, b = 6 or 10 km s-1. Based on the best fit with b = 6 km s-1, the estimated kinetic temperature is TK ≈ 80 K. The measurements of the abundance of zinc and iron in the same DLA show that the metallicity measured by the relatively undepleted element zinc is [Zn/H] = -0.86, while the relative abundance ratio [Fe/Zn] is -1.04, indicating dust depletion. Combining our work with previous results on H2 and relative heavy-element depletion, we find that there is a correlation between them, which suggests that the formation of H2 on dust grains is perhaps the dominant formation process in high-redshift DLAs. Detections of strong absorption from the ground and excited states of neutral carbon (C I) in the z = 2.34 DLA are also presented. The excitation temperature between the two fine-structure levels of C I is 15.7 ± 3.5 K, an upper limit for the cosmic microwave background radiation temperature at z = 2.34. This value is consistent with the prediction by the standard big bang cosmology.