Molecular Hydrogen at z = 2.8108

Extract

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 Lyα forest region of 0528–250, which has a damped Lyα 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 of T_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.