Low-frequency radio spectra of submillimetre galaxies in the Lockman Hole
Journal article, 2021

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 (L1.4 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).
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.

Galaxies: starburst

Galaxies: structure

Galaxies: star formation

Radio continuum: galaxies

Submillimeter: galaxies

Galaxies: high-redshift


J. Ramasawmy

University of Hertfordshire

J.E. Geach

University of Hertfordshire

M. J. Hardcastle

University of Hertfordshire

P. N. Best

Royal Observatory

M. Bonato

Osservatorio Astronomico di Padova

Italian ALMA Regional Centre

Istituto di Radioastronomia

M. Bondi

Istituto di Radioastronomia

G. Calistro Rivera

European Southern Observatory (ESO)

R. Cochrane

Harvard-Smithsonian Center for Astrophysics

John Conway

Chalmers, Space, Earth and Environment, Onsala Space Observatory

K. E. K. Coppin

University of Hertfordshire

K. J. Duncan

Leiden University

Royal Observatory

J. S. Dunlop

Royal Observatory

M. Franco

University of Hertfordshire

C. García-Vergara

Leiden University

M. J. Jarvis

University of the Western Cape

R. Kondapally

Royal Observatory

I. McCheyne

University of Sussex

I. Prandoni

Istituto di Radioastronomia

H. Rottgering

Leiden University

D. J.B. Smith

University of Hertfordshire

C. Tasse

Rhodes University

Observatoire de Paris-Meudon

Station de Radioastronomie de Nançay

L. Wang

University of Groningen

Netherlands Institute for Space Research (SRON)

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 648 A14

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics

Signal Processing



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