Spectroscopy of the QSO pair Q1228 + 076/Q1228 + 077

A further case is reported of a pair of quasars in which the higher redshift member of the pair shows absorption lines at a redshift very close to the emission redshift of the other QSO, which is situated nearby on the plane of the sky. In this case Q1228 + 077 (zsub(em) = 2.391) has a definite absorption system at zsub(a) = 1.8971, which differs by 2000 +- 100 km s-1 from the emission redshift of Q1228 + 076 (1.878) which lies 3.4 arcmin away on the plane of the sky. This is direct evidence that the absorption system in question is due to intervening matter unrelated to the background quasar. The spatial and velocity separations above are consistent with absorption in the halo of a galaxy in a cluster which also contains the foreground quasar. (author)