Spectroscopy of PKS 0528-250: New limits on CO absorption and emission

We have obtained a moderate-resolution spectrum of the quasar PKS 0528-250 with the Red Channel Spectrograph on the Multiple Mirror Telescope in order to study a damped Ly alpha absorption-line system at z = 2.8115. We obtain a new upper limit for the CO column density for the z = 2.8108 velocity component in the z = 2.8115 damped Ly alpha system. The ionization of different species in this component rules out a quasar spectral energy distribution as the ionization field and implies an ultraviolet radiation field intensity a few times that of the Milky Way value. The estimated total number density is n(H) similar to 20 cm(-3). The physical size for the z = 2.8108 component implied by these models is about 40 pc. The ionization of different species also suggests a structure with a hot intercloud medium associated with a H I cloud in this component, that is, most low ionized ions are from the cold medium where photoionization and photodissociation dominates. The highly ionized species may be from the intercloud medium where collisional ionization dominates. We also present newly identified Ni II absorption lines in the z = 2.1408 and z = 2.8115 damped Ly alpha systems. The derived depletion of nickel by dust confirms previous results that the dust-to-gas ratio in these two damped Ly alpha systems is about 10% of the Milky Way ratio. Millimeter-wavelength observations obtained at the NRAO 12 m telescope provide new upper limits on CO (3-2) emission in the z = 2.8115 damped Ly alpha system.