The physics of broad absorption line quasars

Quasar feedback is considered a crucial process in galaxy formation and evolution; however, details of the physical process remain a mystery. Quasar outflow is one possible process. To assess the importance of quasar outflow we have performed a series of studies on broad absorption line (BAL) quasars, and have obtained the following results. （1）The UV absorption column density is an order of magnitude higher than previously believed, and low ionization BALs are formed in the high ionization zone. (2) In radio-quiet BAL quasars, the outflow is mainly confined to the equatorial directionwhile in radio-loud ones both equatorial and polar outflows are possible. (3) The polarization of resonantly scattered lines suggests that the outflow carries large angular momentum.