Spectroscopy of PKS 0528−250: New Limits on CO Absorption and Emission

We have obtained a moderate resolution spectrum of the quasar PKS 0528-250with the Red Channel Spectrograph on the Multiple Mirror Telescope (MMT) inorder to study a damped Lyman alpha absorption line system at z = 2.8115. We obtain a new upper limit for the CO column density for the z = 2.8108velocity component in the z = 2.8115 damped Lyman alpha system. The ionizationof different species in this component rules out a quasar spectral energydistribution (SED) as the ionization field,and implies an ultraviolet radiationfield intensity a few times that of the Milky Way value. The estimated totalnumber density is n(H) about 20 cm^{-3}. The physical size for the z = 2.8108component implied by these models is about 40 parsecs. The ionization ofdifferent species also suggests a structure with a hot intercloud mediumassociated with a H I cloud in this component, that is, most low ionized ionsare from the cold medium where photoionization and photodissociation dominates.The highly ionized species may be from the intercloud medium where collisionalionization dominates. We also present newly identified Ni II absorption linesin the z = 2.1408 and z = 2.8115 damped Ly$\alpha$ systems. The deriveddepletion of nickel by dust confirms previous results that the dust-to-gasratio in these two damped Lyman alpha systems is about 10% of the Milky Wayratio. Millimeter wavelength observations obtained at the NRAO 12 metertelescope provide new upper limits on CO (3-2) emission in the z = 2.8115damped Lyman alpha system.