One- and Two-point Source Statistics from the LOFAR Two-metre Sky Survey First Data Release

TL;DR

This paper analyzes the statistical distribution and clustering of radio sources in the LOFAR Two-metre Sky Survey DR1, confirming consistency with cosmological models and previous deep field results, and identifying systematic calibration issues at low flux densities.

Contribution

It provides the first detailed statistical analysis of LOFAR DR1 radio sources, including counts-in-cell, differential counts, and angular two-point correlation functions, with insights into survey completeness and calibration issues.

Findings

Point-source completeness exceeds 99% above 0.8 mJy.

Radio source distribution fits a compound Poisson model, not a simple Poisson.

Measured angular two-point correlation is consistent with cosmological expectations.

Abstract

The LOFAR Two-metre Sky Survey (LoTSS) will map the complete Northern sky and provide an excellent opportunity to study the distribution and evolution of the large-scale structure of the Universe. We study the completeness of the LoTSS first data release (DR1) and find a point-source completeness of 99 % above flux densities of 0.8 mJy and define a suite of quality cuts. We determine the count-in-cell statistics and differential source counts statistic and measure the angular two-point correlation function of the LoTSS radio sources. The counts-in-cell statistic reveals that the distribution of radio sources cannot be described by a spatial Poisson process. Instead, a good fit is provided by a compound Poisson distribution. The differential source counts are in good agreement with previous findings in deep fields at low radio frequencies and with simulated catalogues from the SKA design study sky and the Tiered Radio Extragalactic Continuum Simulation. The angular two-point correlation is $<10^{-2}$ at angular scales $> 1$ deg. Restricting the value added source catalogue to low-noise regions and a flux density threshold of 2 mJy provides our most reliable estimate of the angular two-point correlation. For smaller flux density thresholds systematic issues are identified, most likely related to the flux density calibration of the individual pointings. Based on the distribution of photometric redshifts of LoTSS sources and the Planck 2018 best-fit cosmological model, the theoretically predicted angular two-point correlation between 0.1 deg and 6 deg agrees with the measured clustering for a subsample of radio sources with redshift information. We find agreement with the expectation of large-scale statistical isotropy of the radio sky at the per cent level. The angular two-point correlation agrees well with the expectation of the cosmological standard model. (abbreviated)