First Comparison of Ionization and Metallicity in Two Lines of Sight toward HE 1104-1805 AB at z=1.66

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 (z_em=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 n_H<~0.01 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, Z_DLA~=1/10 Z_solar. 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 (n_H~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.

Based on observations made at ESO, La Silla, Chile, the Anglo Australian Telescope, and the NASA/ESA HST, obtained at the STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555. Also based on observations made at the W. M. Keck Observatory, which is operated as a scientific partnership between the California Institute of Technology and the University of California; it is made possible by the generous support of the W. M. Keck Foundation.