Volume 648, April 2021
The LOFAR Two Meter Sky Survey
|Number of page(s)||19|
|Published online||07 April 2021|
Extremely deep 150 MHz source counts from the LoTSS Deep Fields
Leiden Observatory, Leiden University,
PO Box 9513,
Leiden, The Netherlands
2 INAF - IRA, Via P. Gobetti 101, 40129 Bologna, Italy
3 ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
4 Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
5 International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia
6 GEPI & USN, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
7 Department of Physics & Electronics, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa
8 Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
9 SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
10 Italian ALMA Regional Centre, Via Gobetti 101, 40129, Bologna, Italy
11 INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
12 Astrophysics, Department of Physics, Keble Road, Oxford, OX1 3RH, UK
13 University of the Western Cape, Private Bag X17, Bellville, Cape Town 7535, South Africa
14 Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
15 STFC UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
16 Astrophysics & Cosmology Research Unit, School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Durban 3690, South Africa
Accepted: 24 January 2021
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.
Key words: galaxies: evolution / surveys / radio continuum: general
© ESO 2021
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