HST spectroscopy of the double QSO HS 1216 + 5032 AB ?

We report on Hubble Space Telescope Faint Object Spectrograph observations of the double QSO HS 1216+5032 AB (ze(A) = 1.455; ze(B) = 1.451, and angular separation θ = 9. ′′1). The spectral coverage is 910 Å to 1340 Å in the QSO rest-frame. An unusual broad-absorption-line (BAL) system is observed only in the B component: maximum outflow velocity of ∼ 5 000 km s−1; probably a mixture of broad and narrow components. Observed ions are: H i, C ii, C iii, N iii, N v, O vi, and possibly S iv and S vi. We also discuss two outstanding intervening systems: (1) a complex C iv system at z = 0.72 of similar strength in A and B, with a velocity span of 1500 km s−1 along the lines of sight (LOSs; LOS separation:S⊥[z = 0.72] ≈ 75 h−1 50 kpc); and (2) a possible strong Mg ii system at z = 0.04 observed in B only, presumably arising in a damped Lyα system. We assume HS 1216+5032 is a binary QSO but discuss the possibility of a gravitational lens system. The size of Lyα forest clouds is constrained using S⊥ ≈ 80 h−1 50 kpc at redshifts between z = 1.15 and 1.45. Four Lyα systems not associated with metal lines and producing lines with W0 > 0.17 Å are observed in both spectra, while five appear in only one spectrum. This sample, although scarce due to the redshift path blocked out by the BALs in B, allows us to place upper limits on the transverse cloud sizes. Modelling the absorbers as nonevolving spheres, a maximum-likelihood analysis yields a most probable cloud diameter D = 256 h−1 50 kpc and 2σ bounds of 172 < D < 896 h−1 50 kpc. If the clouds are modelled as filamentary structures, the same analysis yields lower transverse dimensions by a factor of two. Independently of the maximumlikelihood approach, the equivalent width differences provide evidence for coherent structures. The suggestion that the size of Lyα forest clouds increases with decreasing redshift is not confirmed. Finally, we discuss two za ≈ ze Lyα systems observed in both QSO spectra.