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

2016/09/02 by E. K. Mahony, R. Morganti, I. Prandoni +23 · 97 citations
Physics and Astronomy · #Active galactic nucleus #Astronomy #Astrophysics #Astrophysics and Cosmic Phenomena #Flattening #Flux (metallurgy) #Galaxies: Formation, Evolution, Phenomena #Galaxy #LOFAR #Low frequency #Physics #Population #Radio Astronomy Observations and Technology #Radio spectrum #Radio telescope #Redshift #Source counts #Spectral index #Spectral line #astro-ph.CO #astro-ph.GA

paper · pdf · doi:10.1093/mnras/stw2225

published in Monthly Notices of the Royal Astronomical Society 463(3), 2997-3020 (Oxford University Press) · 26 pages. Accepted for publication in MNRAS

arxiv created 2016/09/02 · openalex publication_date 2016/09/05 · arxiv updated 2016/09/21 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06

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 ₁₅₀¹⁴⁰⁰=-0.78± 0.015|⁠. The median spectral index becomes slightly flatter (increasing from |α ₁₅₀¹⁴⁰⁰=-0.84| to |α ₁₅₀¹⁴⁰⁰=-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.

Citations

• Multiwavelength Observations of the Second Largest Known FR II Radio Galaxy, NVSS 2146+82
• The NRAO VLA Sky Survey
• The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and Goals*
• A low-frequency radio halo associated with a cluster of galaxies
• Distant Radio Galaxies and their Environments
• Four Extreme Relic Radio Sources in Clusters of Galaxies
• SWIRE: TheSIRTFWide‐Area Infrared Extragalactic Survey
• XMM-Newtonobservations of the Lockman Hole: X-ray source catalogue and number counts
• The Two Micron All Sky Survey (2MASS)
• A Combined EIS–NVSS Survey Of Radio Sources (CENSORS) – III. Spectroscopic observations
• The ATESP 5 GHz Radio Survey I. Source Counts and spectral index properties of the faint radio population

Cited by

• LOFAR discovery of an ultra-steep radio halo and giant head–tail radio galaxy in Abell 1132
• Remnant radio-loud AGN in the Herschel-ATLAS field