# University of Hertfordshire

## The Lockman Hole project: LOFAR observations and spectral index properties of low-frequency radio sources

Research output: Contribution to journalArticlepeer-review

### Documents

• E. K. Mahony
• R. Morganti
• I. Prandoni
• I. M. van Bemmel
• T. W. Shimwell
• M. Brienza
• P. N. Best
• M. Brüggen
• G. Calistro Rivera
• F. de Gasperin
• G. Heald
• M. J. Jarvis
• S. Mandal
• G. K. Miley
• E. Retana-Montenegro
• H. J. A. Röttgering
• J. Sabater
• C. Tasse
• S. van Velzen
• R. J. van Weeren
• W. L. Williams
• G. J. White
Original language English 2297-3020 24 Monthly Notices of the Royal Astronomical Society 463 3 5 Sep 2016 https://doi.org/10.1093/mnras/stw2225 Published - 11 Dec 2016

### Abstract

The Lockman Hole is a well-studied extragalactic field with extensive multi-band ancillary data covering a wide range in frequency, essential for characterising 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$\times$14.7 arcsec and reaches an rms of 160 $\mu$Jy 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 $\alpha_{150}^{1400}=-0.78\pm0.015$. The median spectral index becomes slightly flatter (increasing from $\alpha_{150}^{1400}=-0.84$ to $\alpha_{150}^{1400}=-0.75$) with decreasing flux density down to $S_{150} \sim$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 (USS) sources and 13 peaked spectrum sources. We estimate that up to 21 percent 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.

### Notes

E. K. Mahoy, et al., 'The Lockman Hole project: LOFAR observations and spectral index properties of low-frequency radio sources' , the version of record [MNRAS (December 11, 2016) Vol 463 (3): 2997-3020, first published on line September 5, 2016, is available on line at doi: 10.1093/mnras/stw2225 Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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