The Lockman Hole project: LOFAR observations and spectral index properties of low-frequency radio sources
Date Issued
2016
Author(s)
Mahony, E. K.
•
Morganti, R.
•
•
van Bemmel, I. M.
•
Shimwell, T. W.
•
•
Best, P. N.
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Brüggen, M.
•
Calistro Rivera, G.
•
•
Hardcastle, M. J.
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Harwood, J. J.
•
Heald, G.
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Jarvis, M. J.
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Mandal, S.
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Miley, G. K.
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Retana-Montenegro, E.
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Röttgering, H. J. A.
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Sabater, J.
•
Tasse, C.
•
van Velzen, S.
•
van Weeren, R. J.
•
Williams, W. L.
•
White, G. J.
Abstract
The Lockman Hole is a well-studied extragalactic field with extensive multi-band ancillary data covering a wide range in frequency, essential for characterizing the physical and evolutionary properties of the various source populations detected in deep radio fields (mainly star-forming galaxies and AGNs). In this paper, we present new 150-MHz observations carried out with the LOw-Frequency ARray (LOFAR), allowing us to explore a new spectral window for the faint radio source population. This 150-MHz image covers an area of 34.7 square degrees with a resolution of 18.6 × 14.7 arcsec and reaches an rms of 160 μJy beam-1 at the centre of the field. As expected for a low-frequency selected sample, the vast majority of sources exhibit steep spectra, with a median spectral index of α _{150}^{1400}=-0.78± 0.015. The median spectral index becomes slightly flatter (increasing from α _{150}^{1400}=-0.84 to α _{150}^{1400}=-0.75) with decreasing flux density down to S150 ∼10 mJy before flattening out and remaining constant below this flux level. For a bright subset of the 150-MHz selected sample, we can trace the spectral properties down to lower frequencies using 60-MHz LOFAR observations, finding tentative evidence for sources to become flatter in spectrum between 60 and 150 MHz. Using the deep, multi-frequency data available in the Lockman Hole, we identify a sample of 100 ultra-steep-spectrum sources and 13 peaked-spectrum sources. We estimate that up to 21 per cent of these could have z > 4 and are candidate high-z radio galaxies, but further follow-up observations are required to confirm the physical nature of these objects.
Volume
463
Issue
3
Start page
2997
Issn Identifier
0035-8711
Ads BibCode
2016MNRAS.463.2997M
Rights
open.access
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