  A few percent of quasars show strong associated MgII absorption, with
velocities (v_off) lying within a few thousand km/s from the quasar systemic
redshift. These associated absorption line systems (AALs) are usually
interpreted as absorbers that are either intrinsic to the quasar and its host,
or arising from external galaxies clustering around the quasar. Using composite
spectra of ~1,800 MgII AAL quasars selected from SDSS DR7 at 0.4<~ z<~2, we
show that quasars with AALs with v_off<1500 km/s have a prominent excess in
[OII]3727 emission (detected at >7sigma) at rest to the quasar host, compared
to unabsorbed quasars. We interpret this [OII] excess as due to enhanced star
formation in the quasar host. Our results suggest that a significant fraction
of AALs with v_off<1500 km/s are physically associated with the quasar and its
host, and are most likely large-scale flows with typical velocities of a few
hundred km/s. AAL quasars also have dust reddening lying between normal quasars
and the so-called dust-reddened quasars. We suggest that the unique properties
of AAL quasars can be explained if they are the transitional population from
heavily dust-reddened quasar to normal quasars in the formation process of
quasars and their hosts. This scenario predicts a larger fraction of young
bulges, disturbed morphologies and interactions of AAL quasar hosts compared to
normal quasars. The intrinsic link between associated absorbers and quasar
hosts opens a new window to probe massive galaxy formation and galactic-scale
feedback processes, and provides a crucial test of the evolutionary picture of
quasars.
