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

TL;DR

This study examines the low-frequency radio spectra of submillimetre galaxies in the Lockman Hole, revealing spectral flattening likely due to free-free absorption and suggesting complex, clumpy star-forming regions within these galaxies.

Contribution

It provides the first detailed analysis of low-frequency radio spectra of submillimetre galaxies, identifying spectral flattening and its possible link to galaxy-scale gas distribution.

Findings

20% of sources show spectral flattening at low frequencies.

Spectral flattening attributed to free-free absorption in clumpy gas.

Two bright submm sources may be very high redshift galaxies.

Abstract

We investigate the radio properties of a sample of 53 sources selected at 850 $\mu$m from the SCUBA-2 Cosmology Legacy Survey using new deep, low-frequency radio imaging of the Lockman Hole field from the Low Frequency Array. Combining these data with additional radio observations from the GMRT and the JVLA, we find a variety of radio spectral shapes and luminosities within our sample despite their similarly bright submillimetre flux densities. 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. 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 $\lesssim$ 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) submm sources undetected at all other wavelengths. We speculate that these objects may be very high redshift sources, likely residing at z > 4.