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Outflows in quasars during the early epochs of galaxy evolution are animportant part of the feedback mechanisms potentially affecting the evolutionof the host galaxy. However, systematic observations of outflows are only nowbecoming possible with the advent of sensitive mm telescopes. In this study weuse spectral stacking methods to search for faint high velocity outflow signalin a sample of [C II] detected, z ~ 6 quasars. We search for broad emissionline signatures from high-velocity outflows for a sample of 26 z ~ 6 quasarsobserved with ALMA, with a detection of the [C II] line. The observed emissionlines of the sources are dominated by the host galaxy, and outflow emission isnot detected for the individual sources. We use a spectral line stackinganalysis developed for interferometric data to search for outflow emission. Westack both extracted spectra and the full spectral cubes. We also investigatethe possibility that only a sub-set of our sample contributes to the stackedoutflow emission. We find only a tentative detection of a broad emission linecomponent in the stacked spectra. When taking a region of about 2 arcsec aroundthe source central position of the stacked cubes, the stacked line shows anexcess emission due to a broad component of 1.1-1.5 sigma, but the significancedrops to 0.4-0.7 sigma when stacking the extracted spectra from a smallerregion. The broad component can be characterised by a line width of full widthhalf max FWHM > 700 km/s. Furthermore, we find a sub-sample of 12 sources thestack of which maximises the broad component emission. The stack of thissub-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 broadcomponent in the individual spectra.

A spectral stacking analysis to search for faint outflow signatures in z~6 quasars