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

Ramasawmy, Joanna; Hardcastle, M. J.; Best, P. N.; Bonato, M.; Bondì, M.; Rivera, G. Calistro; Cochrane, R. K.; Conway, J.; Coppin, K. E. K.; Duncan, K. J.; Dunlop, James; Franco, Maximilien; García-Vergara, Cristina; Jarvis, Matt J.; Kondapally, R.; McCheyne, I.; Prandoni, I.; Röttgering, H. J. A.; Smith, Daniel; Tasse, C.; Wang, L.

doi:10.1051/0004-6361/202039858

Cited by 7 publications

(3 citation statements)

References 106 publications

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“…The median spectral index for the SFGs is −0.4. This is consistent with a further flattening of the spectra of SFGs compared to that found by Calistro Rivera et al ( 2017) at higher frequencies where the average spectral index of SFGs between 150 and 325 MHz was found to be −0.6 while that between 325 and 1400 MHz was a steeper −0.78 due to free-free absorption at the lower frequencies (see also Ramasawmy et al 2021).…”

Section: Spectral Indicessupporting

confidence: 90%

The LOFAR LBA Sky Survey: Deep Fields

Williams

Gasperin

Hardcastle

et al. 2021

A&A

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We present the first sub-mJy (≈ 0.7 mJy beam −1 ) survey to be completed below 100 MHz, which is over an order of magnitude deeper than previously achieved for widefield imaging of any field at these low frequencies. The high-resolution (15 × 15 arcsec) image of the Boötes field at 34-75 MHz is made from 56 hours of observation with the LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) system. The observations and data reduction, including direction-dependent calibration, are described here. We present a radio source catalogue containing 1, 948 sources detected over an area of 23.6 deg 2 , with a peak flux density threshold of 5σ . Using existing datasets, we characterise the astrometric and flux density uncertainties, finding a positional uncertainty of ∼ 1.2 arcsec and a flux density scale uncertainty of about 5 per cent. Using the available deep 144-MHz data, we identified 144-MHz counterparts to all the 54-MHz sources, and produced a matched catalogue within the deep optical coverage area containing 829 sources. We calculate the Euclidean-normalised differential source counts and investigate the low-frequency radio source spectral indices between 54 and 144 MHz. Both show a general flattening in the radio spectral indices for lower flux density sources, from ∼ −0.75 at 144-MHz flux densities between 100 and 1000 mJy to ∼ −0.5 at 144-MHz flux densities between 5 and 10 mJy. Such flattening is attributable to a growing population of star forming galaxies and compact core-dominated AGN.

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Section: Spectral Indicessupporting

confidence: 90%

The LOFAR LBA Sky Survey: Deep Fields

Williams

Gasperin

Hardcastle

et al. 2021

A&A

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“…In addition, bright z ∼ 2 submillimeter-detected starbursts show typical radio spectra of α ∼ −0.80 between observed-frame frequencies of 610 MHz − 1.4 GHz (roughly probing rest-frame 2 − 5 GHz; Ibar et al 2010;Thomson et al 2014;Algera et al 2020a), and hence do not show any evidence for spectral flattening due to freefree absorption. Using sensitive 150 MHz observations of high-redshift starbursts, Ramasawmy et al (2021) further find that free-free absorption is typically limited to rest-frame frequencies ν 1 GHz. Given that we probe more modestly star-forming galaxies in this work (average SFR 1.4 GHz ≈ 3−30 M yr −1 ; Figure 7) at restframe frequencies ν ≥ 2 GHz, we conclude that free-free absorption is unlikely to significantly affect the frequencies sampled in this work, and as such the low-frequency radio spectra should be well-described by a combination of power-law free-free and synchrotron emission.…”

Section: A Lack Of Free-free Emissionmentioning

confidence: 96%

COLDz: Probing Cosmic Star Formation With Radio Free-free Emission

Algera,

Hodge,

Riechers

et al. 2021

Preprint

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Radio free-free emission is considered to be one of the most reliable tracers of star formation in galaxies. However, as it constitutes the faintest part of the radio spectrum -being roughly an order of magnitude less luminous than radio synchrotron emission at the GHz frequencies typically targeted in radio surveys -the usage of free-free emission as a star formation rate tracer has mostly remained limited to the local Universe. Here we perform a multi-frequency radio stacking analysis using deep Karl G. Jansky Very Large Array observations at 1.4, 3, 5, 10 and 34 GHz in the COSMOS and GOODS-North fields to probe free-free emission in typical galaxies at the peak of cosmic star formation. We find that z ∼ 0.5 − 3 star-forming galaxies exhibit radio emission at rest-frame frequencies of ∼ 65 − 90 GHz that is ∼ 1.5 − 2× fainter than would be expected from a simple combination of freefree and synchrotron emission, as in the prototypical starburst galaxy M82. We interpret this as a deficit in high-frequency synchrotron emission, while the level of free-free emission is as expected from M82. We additionally provide the first constraints on the cosmic star formation history using free-free emission at 0.5 z 3, which are in good agreement with more established tracers at high redshift. In the future, deep multi-frequency radio surveys will be crucial in order to accurately determine the shape of the radio spectrum of faint star-forming galaxies, and to further establish radio free-free emission as a tracer of high-redshift star formation.

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“…In addition there are only several thousand galaxies detected in the SCUBA-2 Cosmology Legacy Survey (S2CLS) (Geach et al 2017) survey in Lockman-Hole and these can be cross-matched to the LO-FAR catalogue by a simple nearest neighbour crossmatch. This was done in Ramasawmy et al (2021) in prep and found only tens of matches, and so it has not been included in the data release.…”

Section: Far Infraredmentioning

confidence: 99%