AGN-enhanced outflows of low-ionization gas in star-forming galaxies at
  1.7<z<4.6
  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 (42<log(L_X)<45 erg/s)
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 -150 km/s in the SFGs while in the AGN sample the velocity is much
higher (-950 km/s), 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.
