Low-frequency radio spectra of submillimetre galaxies in the Lockman Hole
Journal article, 2021
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 (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).
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.
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).
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.
Galaxies: starburst
Galaxies: star formation
Galaxies: high-redshift
Galaxies: structure
Submillimeter: galaxies
Radio continuum: galaxies
Author
J. Ramasawmy
University of Hertfordshire
J.E. Geach
University of Hertfordshire
M. J. Hardcastle
University of Hertfordshire
P. N. Best
Royal Observatory
M. Bonato
Istituto di Radioastronomia
Italian ALMA Regional Centre
Istituto nazionale di astrofisica (INAF)
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
Observatoire de Paris-Meudon
Station de Radioastronomie de Nançay
Rhodes University
L. Wang
Netherlands Institute for Space Research (SRON)
University of Groningen
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 648 A14Subject Categories
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
Signal Processing
DOI
10.1051/0004-6361/202039858