Molecular Hydrogen at Z = 2.8108
The Lyman and Werner bands of H 2 in interstellar gas provide information about gas temperature, density and the ultraviolet radiation field, and possibly about dust content. This is especially useful for high redshift QSO absorption systems, where usually the only data available arise from absorption lines. We present 25 – 50 km s-1 resolution data taken with the CTIO 4m telescope plus echelle spectrograph of the Lvα forest region of 05208–250, which has a damped Lvα absorption system at z = 2.81. Using a χ2 profile fitting routine (Lanzetta & Bowen 1992, ApJ, 391, 48), we find an H 2 fraction of ~ 10-2, an order of magnitude below that of Galactic diffuse interstellar clouds. This may be caused by some combination of a less efficient H 2 formation rate or an increased H 2 dissociation rate. Using the relative populations of the J″ = 0,1 rotational levels, we derive a kinetic temperature ofT K = 136 ± 16 K. The total velocity spread as traced by sensitive metal transitions is 250 km s-1, consistent with a highly inclined, rotating ensemble of clouds associated with a luminous spiral galaxy. A representative section of the spectrum is shown below, binned at roughly the Nyquist rate with the χ2 fit to H 2 of the z = 2.8108 absorption system.