HST spectroscopy of the double QSO HS 1216+5032 AB

Abstract

(Abridged) We report on HST FOS observations of the double QSO HS 1216+5032 AB (z(A)=1.455; z(B)=1.451, and angular separation theta=9 arcsec). An unusual broad-absorption-line (BAL) system is observed only in the B component: maximum outflow velocity of ~5000 km/s; probably a mixture of broad and narrow components. Observed ions are: HI, CII, CIII, NIII, NV, OVI, and possibly SIV and SVI. We also discuss two outstanding intervening systems: (1) a complex CIV system at z=0.72 of similar strength in A and B, with a velocity span of 1500 km/s; and (2) a possible strong MgII system at z=0.04 observed in B only. Assuming HS 1216+5032 is a binary QSO, the size of Ly-alpha forest clouds is constrained using S~ 80 /h_50 kpc at redshifts between z=1.15 and 1.45. Four Ly-alpha systems not associated with metal lines and producing lines with W_0>0.17 AA 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 non-evolving spheres, a maximum-likelihood analysis yields a most probable cloud diameter D=256 /h_50 kpc and 2 sigma bounds of 172<D<896 /h_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 maximum-likelihood approach, the equivalent width differences provide evidence for coherent structures. The suggestion that the size of Ly-alpha forest clouds increases with decreasing redshift is not confirmed.