Low-frequency radio spectra of submillimetre galaxies in the Lockman Hole

¹ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
e-mail: j.ramasawmy@herts.ac.uk
² SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
³ INAF-Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁴ Italian ALMA Regional Centre, Via Gobetti 101, 40129 Bologna, Italy
⁵ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2 85748 Garching bei München, Germany
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Sweden
⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁰ Astrophysics, Department of Physics, Keble Road, Oxford, OX1 3RH, UK
¹¹ Department of Physics & Astronomy, University of the Western Cape, Private Bag X17, Bellville, Cape Town, 7535, South Africa
¹² Astronomy Centre, Department of Physics & Astronomy, University of Sussex, Brighton, BN1 9QH, UK
¹³ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹⁴ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa
¹⁵ USN, Observatoire de Paris, CNRS, PSL, UO, Nançay, France
¹⁶ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, Groningen, The Netherlands
¹⁷ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands

Received: 5 November 2020
Accepted: 3 March 2021

Abstract

Aims. We investigate the radio properties of a sample of 850 μm-selected sources from the SCUBA-2 Cosmology Legacy Survey (S2CLS) using new deep, low-frequency radio imaging of the Lockman Hole field from the Low Frequency Array. This sample consists of 53 sources, 41 of which are detected at >5σ at 150 MHz.

Methods. Combining these data with additional observations at 324 MHz, 610 MHz, and 1.4 GHz from the Giant Metrewave Radio Telescope and the Jansky Very Large Array, we find a variety of radio spectral shapes and luminosities (L_{1.4 GHz} ranging from ~4 × 10²³−1 × 10²⁵) within our sample despite their similarly bright submillimetre flux densities (>4 mJy). We characterise their spectral shapes in terms of multi-band radio spectral indices. Finding strong spectral flattening at low frequencies in ~20% of sources, we investigate the differences between sources with extremely flat low-frequency spectra and those with ‘normal’ radio spectral indices (α > −0.25).

Results. As there are no other statistically significant differences between the two subgroups of our sample as split by the radio spectral index, we suggest that any differences are undetectable in galaxy-averaged properties that we can observe with our unresolved images, and likely relate to galaxy properties that we cannot resolve, on scales ≲1 kpc. We attribute the observed spectral flattening in the radio to free–free absorption, proposing that those sources with significant low-frequency spectral flattening have a clumpy distribution of star-forming gas. We estimate an average spatial extent of absorbing material of at most several hundred parsecs to produce the levels of absorption observed in the radio spectra. This estimate is consistent with the highest-resolution observations of submillimetre galaxies in the literature, which find examples of non-uniform dust distributions on scales of ~100 pc, with evidence for clumps and knots in the interstellar medium. Additionally, we find two bright (>6 mJy) S2CLS sources undetected at all other wavelengths. We speculate that these objects may be very high redshift sources, likely residing at z > 4.