Beyond Stacking: A Maximum-Likelihood Method to Constrain Radio Source Counts Below the Detection Threshold

TL;DR

This paper introduces a maximum likelihood statistical method to estimate the counts of faint radio sources below survey detection limits, validated through simulations and application to the FIRST survey data.

Contribution

The paper presents a novel maximum likelihood approach to constrain radio source counts below detection thresholds, enabling analysis of sources ten times fainter than the survey noise level.

Findings

Accurately constrains source counts down to one-tenth of flux noise rms.

Demonstrates effectiveness with simulated data over various sky areas.

Successfully recovers differential number counts from FIRST survey data.

Abstract

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.