First Comparison of Ionization and Metallicity in Two Lines of Sight toward HE 1104−1805 AB atz=1.66

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Abstract
Using new Hubble Space Telescope Faint Object Spectrograph, the New Technology Telescope ESO Multi Mode Instrument, and Keck HIRES spectra of the gravitationally lensed double QSO HE 1104-1805 AB (zem=2.31) and assuming UV photoionization by a metagalactic radiation field, we derive physical conditions (ionization levels, metal abundances, and cloud sizes along the lines of sight) in five C IV+Mg II absorption systems clustered around z=1.66 along the two lines of sight. Three of these systems are associated with a damped Lyα (DLA) system with log N(H I)=20.85, which is observed in the UV spectra of the bright QSO image, A. The other two systems are associated with a Lyman-limit system with log N(H I)=17.57 seen in the fainter image, B. The C IV and Mg II line profiles in A resemble those in the B spectra and span Δv≈360 km s-1. The angular separation, θ=3.195'', between A and B corresponds to a transverse proper separation of S⊥=8.3 h−150 kpc for q0=0.5 and a lens at z=1. Assuming that the relative metal abundances in these absorption systems are the same as those observed in the DLA system, we find that the observed N(C IV)/N(Mg II) ratios imply ionization parameters of logΓ=-2.95 to -2.35. Consequently, these clouds should be small (0.5-1.6 kpc with a hydrogen density of nH0.01 cm-3) and relatively highly ionized. The absorption systems to B are found to have a metallicity 0.63 times lower than the metallicity of the gas giving rise to the DLA system, ZDLA1/10 Z☉. We detect O VI at z=1.66253 in both QSO spectra but no associated N V. Our model calculations lead us to conclude that the C IV clouds should be surrounded by large (~100 kpc), highly ionized low-density clouds (nH~10−4 cm-3) in which O VI, but only weak C IV, absorption occurs. In this state, logΓ≥-1.2 reproduces the observed ratio of N(O VI)/N(N V)>60. These results are discussed in view of the disk/halo and hierarchical structure formation models.