A spectral stacking analysis to search for faint outflow signatures in
  z~6 quasars
  Outflows in quasars during the early epochs of galaxy evolution are an
important part of the feedback mechanisms potentially affecting the evolution
of the host galaxy. However, systematic observations of outflows are only now
becoming possible with the advent of sensitive mm telescopes. In this study we
use spectral stacking methods to search for faint high velocity outflow signal
in a sample of [C II] detected, z ~ 6 quasars. We search for broad emission
line signatures from high-velocity outflows for a sample of 26 z ~ 6 quasars
observed with ALMA, with a detection of the [C II] line. The observed emission
lines of the sources are dominated by the host galaxy, and outflow emission is
not detected for the individual sources. We use a spectral line stacking
analysis developed for interferometric data to search for outflow emission. We
stack both extracted spectra and the full spectral cubes. We also investigate
the possibility that only a sub-set of our sample contributes to the stacked
outflow emission. We find only a tentative detection of a broad emission line
component in the stacked spectra. When taking a region of about 2 arcsec around
the source central position of the stacked cubes, the stacked line shows an
excess emission due to a broad component of 1.1-1.5 sigma, but the significance
drops to 0.4-0.7 sigma when stacking the extracted spectra from a smaller
region. The broad component can be characterised by a line width of full width
half max FWHM > 700 km/s. Furthermore, we find a sub-sample of 12 sources the
stack of which maximises the broad component emission. The stack of this
sub-sample shows an excess emission due to a broad component of 1.2-2.5 sigma.
The stacked line of these sources has a broad component of FWHM > 775 km/s.
Deeper ALMA observations are necessary to confirm the presence of a broad
component in the individual spectra.
