Far beyond stacking: Fully bayesian constraints on sub-microJy radio
  source populations over the XMM-LSS-VIDEO field
  Measuring radio source counts is critical for characterizing new
extragalactic populations, brings a wealth of science within reach and will
inform forecasts for SKA and its pathfinders. Yet there is currently great
debate (and few measurements) about the behaviour of the 1.4-GHz counts in the
microJy regime. One way to push the counts to these levels is via 'stacking',
the covariance of a map with a catalogue at higher resolution and (often) a
different wavelength. For the first time, we cast stacking in a fully bayesian
framework, applying it to (i) the SKADS simulation and (ii) VLA data stacked at
the positions of sources from the VIDEO survey. In the former case, the
algorithm recovers the counts correctly when applied to the catalogue, but is
biased high when confusion comes into play. This needs to be accounted for in
the analysis of data from any relatively-low-resolution SKA pathfinders. For
the latter case, the observed radio source counts remain flat below the 5-sigma
level of 85 microJy as far as 40 microJy, then fall off earlier than the flux
hinted at by the SKADS simulations and a recent P(D) analysis (which is the
only other measurement from the literature at these flux-density levels, itself
extrapolated in frequency). Division into galaxy type via spectral-energy
distribution reveals that normal spiral galaxies dominate the counts at these
fluxes.
