Observational evidence revealing the main mechanisms that accelerate quasar outflows has proven difficult to obtain , due to the complexity of the absorption features that this gas produces in the spectra of the emission sources .
We build 36 composite outflow spectra , covering a large range of outflow and quasar parameters , by stacking broad ( > 450 { km s ^ { -1 } } ) absorption line systems in the spectra of SDSS-III/BOSS DR12 quasars .
The two lines of the atomic doublet of C iv , with a separation of \approx 497 { km s ^ { -1 } } , as well as those of other species appear well resolved in most of our composites .
This agrees with broad outflow troughs consisting in the superposition of narrow absorbers .
We also report on the ubiquitous detection of the radiative-acceleration signature known as line locking in all our composite outflow spectra , including one spectrum strictly built from broad absorption line ( BAL ) systems .
This is the first line-locking detection in BAL composite spectra .
Line locking is driven by the C iv atomic doublet , and is visible on the blue side of most strong absorption transitions .
Similar effects from the doublets of O vi , Si iv , or N v , however , seem to not be present .
Our results confirm that radiation pressure is a prevalent mechanism for accelerating outflows in quasars .