QSO Emission Line Redshift Differences

Redshift differences between QSO emission lines reflect the kinematical differences between the regions where the lines are produced. Gaskell (1982) discovered a systematic redshift difference between the broad lines of high ionization and the narrow forbidden lines that amounted to a blueshifting of C IV λ1549 by 600 km s −1 . Gaskell obtained this result basically by comparing C IV emission and Mg II λ2798 emission in one sample of QSOs and by comparing Mg II emission and the narrow forbidden lines in a different sample. So the distribution of C IV emission redshifts relative to the narrow forbidden lines was not sampled directly by Gaskell. By observing QSOs with redshifts near 1.3, it is possible to directly sample the redshift difference between C IV and the narrow forbidden lines. This redshift difference and its distribution should provide kinematical information on the clouds of gas that give rise to the broad lines of high ionization. The narrow forbidden lines are the preferred velocity reference because they may reflect the systemic velocity of the QSO.