2017MNRAS.471.4527T

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 (SF) and/or active galactic nucleus (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 (10⁴² < L_X < 10⁴⁵ 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 VIMOS Ultra Deep Survey (VUDS). We measure mean velocity offsets of ∼- 150 km s^–1 in the SFGs, while in the AGN sample the velocity is much higher (∼- 950 km s^–1), suggesting that the AGN is boosting the outflow up to velocities that could not be reached only with the SF contribution. The sample of X-ray AGN has on average a lower SF rate 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.