No evidence for [CII] halos or high-velocity outflows in z>6 quasar host
  galaxies
  We study the interstellar medium in a sample of 27 high-redshift quasar host
galaxies at z>6, using the [CII] 158um emission line and the underlying dust
continuum observed at ~1kpc resolution with ALMA. By performing uv-plane
spectral stacking of both the high and low spatial resolution data, we
investigate the spatial and velocity extent of gas, and the size of the
dust-emitting regions. We find that the average surface brightness profile of
both the [CII] and the dust continuum emission can be described by a steep
component within a radius of 2kpc, and a shallower component with a scale
length of 2kpc, detected up to ~10kpc. The surface brightness of the extended
emission drops below ~1% of the peak at radius of ~5kpc, beyond which it
constitutes 10-20% of the total measured flux density. Although the central
component of the dust continuum emission is more compact than that of the [CII]
emission, the extended components have equivalent profiles. The observed
extended components are consistent with those predicted by hydrodynamical
simulations of galaxies with similar infrared luminosities, where the dust
emission is powered by star formation. The [CII] spectrum measured in the mean
uv-plane stacked data can be described by a single Gaussian, with no observable
[CII] broad-line emission (velocities in excess of >500km/s), that would be
indicative of outflows. Our findings suggest that we are probing the
interstellar medium and associated star formation in the quasar host galaxies
up to radii of 10kpc, whereas we find no evidence for halos or outflows.
