Published online by Cambridge University Press: 25 May 2016
Although a connection between certain classes of QSO absorption line systems and gas associated with galaxies was hypothesized long ago, the first systematic evidence supporting this was provided by Bergeron & Boissé (1991). Observing QSOs with known Mgll absorption lines at zabs < zQSO, they identified galaxies near the QSO sightline and spectroscopically confirmed that their redshifts matched those of the MgII absorption doublet.