Fast and energetic winds are invoked by galaxy formation models as essential processes in the evolution of galaxies .
These outflows can be powered either by star-formation and/or AGN activity , but the relative dominance of the two mechanisms is still under debate .
We use spectroscopic stacking analysis to study the properties of the low-ionization phase of the outflow in a sample of 1330 star-forming galaxies ( SFGs ) and 79 X-ray detected ( 10 ^ { 42 } { < } L _ { X } { < } 10 ^ { 45 } erg s ^ { -1 } ) Type 2 AGN at 1.7 { < } z { < } 4.6 selected from a compilation of deep optical spectroscopic surveys , mostly zCOSMOS-Deep and VUDS .

We measure mean velocity offsets of { \sim } -150 km s ^ { -1 } in the SFGs while in the AGN sample the velocity is much higher ( { \sim } -950 km s ^ { -1 } ) , suggesting that the AGN is boosting the outflow up to velocities that could not be reached only with the star-formation contribution .
The sample of X-ray AGN has on average a lower SFR than non-AGN SFGs of similar mass : this , combined with the enhanced outflow velocity in AGN hosts , is consistent with AGN feedback in action .

We further divide our sample of AGN into two X-ray luminosity bins : we measure the same velocity offsets in both stacked spectra , at odds with results reported for the highly ionized phase in local AGN , suggesting that the two phases of the outflow may be mixed only up to relatively low velocities , while the highest velocities can be reached only by the highly ionized phase .