Extremely deep 150 MHz source counts from the LoTSS Deep Fields

Astronomy and Astrophysics, vol. 648, pp. A5

Abstract

With the advent of new generation low-frequency telescopes, such as the LOw Frequency ARray (LOFAR), and improved calibration techniques, we have now started to unveil the subgigahertz radio sky with unprecedented depth and sensitivity. The LOFAR Two Meter Sky Survey (LoTSS) is an ongoing project in which the whole northern radio sky will be observed at 150 MHz with a sensitivity better than 100 μJy beam−1 at a resolution of 6′′. Additionally, deeper observations are planned to cover smaller areas with higher sensitivity. The Lockman Hole, the Boötes, and the Elais-N1 regions are among the most well known northern extra-galactic fields and the deepest of the LoTSS Deep Fields so far. We exploited these deep observations to derive the deepest radio source counts at 150 MHz to date. Our counts are in broad agreement with those from the literature and show the well known upturn at ≤1 mJy, mainly associated with the emergence of the star-forming galaxy population. More interestingly, our counts show, for the first time a very pronounced drop around S ~ 2 mJy, which results in a prominent “bump” at sub-mJy flux densities. Such a feature was not observed in previous counts’ determinations (neither at 150 MHz nor at a higher frequency). While sample variance can play a role in explaining the observed discrepancies, we believe this is mostly the result of a careful analysis aimed at deblending confused sources and removing spurious sources and artifacts from the radio catalogs. This “drop and bump” feature cannot be reproduced by any of the existing state-of-the-art evolutionary models, and it appears to be associated with a deficiency of active galactic nuclei (AGN) at an intermediate redshift (1 < z < 2) and an excess of low-redshift (z < 1) galaxies and/or AGN.

Authors 11

1. Isabella Prandoni

   Istituto di Radioastronomia di Bologna · National Institute for Astrophysics

   Affiliation as printed

   INAF - IRA, Via P. Gobetti 101, 40129 Bologna, Italy

2. Huib T. Intema Aachen Leiden Observatory

   Leiden University · Curtin University · International Centre for Radio Astronomy Research · Leiden Observatory

   Affiliation as printed

   International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia

   Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

3. Cyril Tasse

   Centre National de la Recherche Scientifique · Rhodes University · Université Paris Sciences et Lettres · Observatoire de Paris · Galaxies, Etoiles, Physique et Instrumentation

   Affiliation as printed

   Department of Physics & Electronics, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa

   GEPI & USN, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France

4. Reinout J. van Weeren Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

5. K. L. Emig Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

6. Dominik J. Schwarz

   Bielefeld University

   Affiliation as printed

   Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany

7. Thilo M. Siewert

   Bielefeld University

   Affiliation as printed

   Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany

8. Matteo Bonato

   Istituto di Radioastronomia di Bologna · Osservatorio Astronomico di Padova · National Institute for Astrophysics

   Affiliation as printed

   INAF - IRA, Via P. Gobetti 101, 40129 Bologna, Italy

   INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

   Italian ALMA Regional Centre, Via Gobetti 101, 40129, Bologna, Italy

9. R. Kondapally

   Royal Observatory

   Affiliation as printed

   SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK

10. S. K. Leslie Aachen Leiden Observatory

    Leiden University · Leiden Observatory

    Affiliation as printed

    Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

11. J. Sabater

    Royal Observatory · UK Astronomy Technology Centre

    Affiliation as printed

    STFC UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK

    SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK