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

Abstract

Using new HST/FOS, NTT/EMMI, and Keck/HIRES spectra of the gravitationally-lensed double QSO HE 1104-1805 AB (z=2.31, θ=3.2), 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 CIV+MgII absorption systems clustered around z=1.66 along the two lines of sight. Three of these systems are associated with a damped Ly-alpha (DLA) system with logN(HI)=20.85, which is observed in A. The other two systems are associated with a Lyman-limit system with logN(HI)=17.57 (B). The CIV and MgII line profiles in A resemble those in the B spectra, and span Δv=360 km/s. Assuming that the relative metal abundances in these absorption systems are the same as observed in the DLA system, we find that the observed N(CIV)/N(MgII) 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 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)= 0.1*Z(solar). We detect OVI at z=1.66253 in both QSO spectra, but no associated NV. Our model calculations lead us to conclude that the CIV clouds should be surrounded by large (>= 100 kpc) highly ionized low-density clouds (n_H\sim e-4 cm-3), in which OVI, but only weak CIV absorption occurs. In this state, logΓ>=-1.2 reproduces the observed ratio of N(OVI)/N(NV)>60. These results are discussed in view of the disk/halo and hierarchical structure formation models.

LaTeX, 49 pages including 10 figures, minor changes introduced. Now in press at ApJ (1999 March 10 issue)

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