Far beyond stacking: Fully bayesian constraints on sub-microJy radio source populations over the XMM-LSS-VIDEO field

TL;DR

This paper introduces a fully Bayesian stacking method to accurately measure radio source counts at microJy levels, revealing that normal spiral galaxies dominate these faint populations and highlighting biases in previous approaches.

Contribution

It presents the first fully Bayesian stacking framework for radio source count estimation, addressing confusion bias and applying it to simulations and real data at unprecedented faint flux levels.

Findings

Bayesian stacking accurately recovers counts in simulations.

Observed counts remain flat down to 40 microJy before declining.

Normal spiral galaxies dominate the faint radio source population.

Abstract

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.