The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1

IV. Photometric redshifts and stellar masses^★

¹ SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: duncan@strw.leidenuniv.nl
³ School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
⁴ INAF–Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁵ Italian ALMA Regional Centre, Via Gobetti 101, 40129 Bologna, Italy
⁶ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Astrophysics, Department of Physics, Keble Road, Oxford, OX1 3RH, UK
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA
⁹ CSIRO Astronomy and Space Science, PO Box 1130, Bentley WA 6102, Australia
¹⁰ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹¹ Department of Physics & Astronomy, University of the Western Cape, Private Bag X17, Bellville, Cape Town 7535, South Africa
¹² Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziadzka 5, 87-100 Toruń, Poland
¹³ National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warszawa, Poland
¹⁴ Aix-Marseille Univ. CNRS, CNES, LAM, Marseille, France
¹⁵ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham, DH1 3LE, UK
¹⁶ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin, D09 W6Y4, Ireland
¹⁷ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands
¹⁸ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, Groningen, The Netherlands
¹⁹ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
²⁰ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
²¹ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa

Received: 1 July 2020
Accepted: 9 October 2020

Abstract

The Low Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS) is a sensitive, high-resolution 120-168 MHz survey split across multiple tiers over the northern sky. The first LoTSS Deep Fields data release consists of deep radio continuum imaging at 150 MHz of the Boötes, European Large Area Infrared Space Observatory Survey-North 1, and Lockman Hole fields, down to rms sensitivities of ~32, 20, and 22 μJy beam⁻¹, respectively. In this paper we present consistent photometric redshift (photo-z) estimates for the optical source catalogues in all three fields – totalling over 7 million sources (~5 million after limiting to regions with the best photometric coverage). Our photo-z estimation uses a hybrid methodology that combines template fitting and machine learning and is optimised to produce the best possible performance for the radio continuum selected sources and the wider optical source population. Comparing our results with spectroscopic redshift samples, we find a robust scatter ranging from 1.6 to 2% for galaxies and 6.4 to 7% for identified optical, infrared, or X-ray selected active galactic nuclei. Our estimated outlier fractions (|z_phot−z_spec|/(1+z_spec)>0.15) for the corresponding subsets range from 1.5 to 1.8% and 18 to 22%, respectively. Replicating trends seen in analyses of previous wide-area radio surveys, we find no strong trend in photo-z quality as a function of radio luminosity for a fixed redshift. We exploit the broad wavelength coverage available within each field to produce galaxy stellar mass estimates for all optical sources at z < 1.5. Stellar mass functions derived for each field are used to validate our mass estimates, with the resulting estimates in good agreement between each field and with published results from the literature.