Beyond Stacking: A Maximum-Likelihood Method to Constrain Radio Source
  Counts Below the Detection Threshold
  We present a statistical method based on a maximum likelihood approach to
constrain the number counts of extragalactic sources below the nominal
flux-density limit of continuum imaging surveys. We extract flux densities from
a radio map using positional information from an auxiliary catalogue and show
that we can model the number counts of this undetected population down to flux
density levels well below the detection threshold of the radio survey. We
demonstrate the capabilities that our method will have with future generation
wide-area radio surveys by performing simulations over various sky areas. We
show that it is possible to accurately constrain the number counts of the
simulated distribution down to one tenth of the flux noise rms with just a sky
area of 100 deg$^2$. We then test the application of our method using data from
the Faint Images of the Radio Sky at Twenty-Centimeters survey (FIRST). We
extract flux densities from the FIRST map, sensitive to 150 $\mu$Jy/beam (1
$\sigma$), using the positional information from a catalogue in the same field,
also acquired at the same frequency, sensitive to 12 $\mu$Jy/beam (1 $\sigma$).
Implementing our method, with known source positions, we are able to recover
the right differential number counts of the noise-dominated FIRST map fluxes
down to a flux density level which is one-tenth the FIRST detection threshold.
