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

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 ${\alpha }_{150}^{1400}=-0.78\pm 0.015$. 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.

arXiv comment26 pages. Accepted for publication in MNRAS

References in corpus (62)

• Astropy: A Community Python Package for Astronomy
• The Sloan Digital Sky Survey: Technical Summary
• LOFAR: The LOw-Frequency ARray
• The UKIRT Infrared Deep Sky Survey (UKIDSS)
• The Murchison Widefield Array: the Square Kilometre Array Precursor at low radio frequencies
• SUMSS: A Wide-Field Radio Imaging Survey of the Southern Sky. II. The Source Catalogue
• The Herschel Multi-tiered Extragalactic Survey: HerMES
• SWIRE: The SIRTF Wide-area InfraRed Extragalactic Survey
• Clusters of galaxies : observational properties of the diffuse radio emission
• Distant Radio Galaxies and their Environments
• A morphological algorithm for improving radio-frequency interference detection
• Observations of extended radio emission in clusters
• Radio and Millimeter Continuum Surveys and their Astrophysical Implications
• A broadband flux scale for low frequency radio telescopes
• The Very Large Array Low-frequency Sky Survey Redux (VLSSr)
• A low-frequency radio halo associated with a cluster of galaxies
• LOFAR 150-MHz observations of the Boötes field: Catalogue and Source Counts
• Chandra and Spitzer unveil heavily obscured quasars in the SWIRE/Chandra Survey
• Four Extreme Relic Radio Sources in Clusters of Galaxies
• LOFAR, VLA, and Chandra observations of the Toothbrush galaxy cluster
• The Spitzer Extragalactic Representative Volume Survey (SERVS): survey definition and goals
• The Phoenix Deep Survey: The 1.4 GHz microJansky catalogue
• On the evolution of young radio-loud AGN
• Correcting direction-dependent gains in the deconvolution of radio interferometric images
• LOFAR/H-ATLAS: A deep low-frequency survey of the Herschel-ATLAS North Galactic Pole field
• Spectral ageing in the lobes of FR-II radio galaxies: New methods of analysis for broadband radio data
• Deep multi-frequency radio imaging in the Lockman Hole using the GMRT and VLA: I. The nature of the sub-mJy radio population
• The radio galaxy K-z relation: the 10^12 Msol mass limit; masses of galaxies from the L_K luminosity, up to z >4
• Applying full polarization A-Projection to very wide field of view instruments: An imager for LOFAR
• The LOFAR Multifrequency Snapshot Sky Survey (MSSS) I. Survey description and first results
• Deep multi-frequency radio imaging in the Lockman Hole using the GMRT and VLA: II. The spectral index of submillimetre galaxies
• XMM-Newton observations of the Lockman Hole: X-ray source catalogue and number counts
• The LOFAR radio environment
• Spectral ageing in the lobes of cluster-centre FR-II radio galaxies
• Broadband Spectral Modeling of the Extreme Gigahertz-Peaked Spectrum Radio Source PKS B0008-421
• The ATESP radio survey III. Source counts
• Eight-Dimensional Mid-Infrared/Optical Bayesian Quasar Selection
• A Combined EIS-NVSS Survey Of Radio Sources (CENSORS) III: Spectroscopic observations
• Deep Low-Frequency Radio Observations of the NOAO Bootes Field: I. Data Reduction and Catalog Construction
• The ATESP 5 GHz radio survey I. Source counts and spectral index properties of the faint radio population
• The HerMES SPIRE submillimeter local luminosity function
• A sample of 6C radio sources designed to find objects at redshift > 4: the radio data
• FR II radio galaxies at low frequencies I: morphology, magnetic field strength and energetics
• T-RaMiSu: The Two-meter Radio Mini Survey I. The Bootes Field
• Spectral index properties of milliJansky radio sources
• 10C Survey of Radio Sources at 15.7 GHz: II - First Results
• Photometry and Photometric Redshift catalogs for the Lockman Hole Deep Field
• VLBI observations of the CORALZ sample: young radio sources at low redshift
• The Spectral Variability of the GHz-Peaked Spectrum Radio Source PKS 1718-649 and a Comparison of Absorption Models
• Compact radio cores: from the first black holes to the last
• A 610-MHz survey of the Lockman Hole with the Giant Metrewave Radio Telescope - I. Observations, data reduction and source catalogue for the central 5 square degrees
• A search for distant radio galaxies from SUMSS and NVSS: I. Sample definition, radio and K-band imaging
• The ATESP Radio Survey I. Survey Description, Observations and Data Reduction
• Deep GMRT 150 MHz observations of the LBDS-Lynx region: Ultra-Steep Spectrum Radio Sources
• The faint source population at 15.7 GHz - I. The radio properties
• LOFAR low-band antenna observations of the 3C295 and Bootes fields: source counts and ultra-steep spectrum sources
• What are the megahertz peaked-spectrum sources?
• Ultra Steep Spectrum radio sources in the Lockman Hole: SERVS identifications and redshift distribution at the faintest radio fluxes
• HELP : The Herschel Extragalactic Legacy Project & The Coming of Age of Multi-Wavelength Astrophysics
• Multiwavelength Observations of the Second Largest Known FR II Radio Galaxy, NVSS 2146+82
• Further GMRT observations of the Lockman Hole at 610 MHz
• The Lockman Hole with LOFAR - Searching for GPS and CSS sources at low frequencies

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• The Lockman Hole Project: new constraints on the sub-mJy source counts from a wide-area 1.4 GHz mosaic
• LoTSS/HETDEX: Optical quasars I. Low-frequency radio properties of optically selected quasars
• The duty cycle of radio galaxies revealed by LOFAR: remnant and restarted radio source populations in the Lockman Hole
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• The best of both worlds: Combining LOFAR and Apertif to derive resolved radio spectral index images
• One- and Two-point Source Statistics from the LOFAR Two-metre Sky Survey First Data Release
• A Wide-area GMRT 610~MHz survey of ELAIS N1 field
• Multi-frequency characterisation of remnant radio galaxies in the Lockman Hole field
• A New Catalog of Head Tail Radio Galaxies from the VLA FIRST Survey
• Radio spectral index from NVSS and TGSS
• The Dynamics and Energetics of Remnant and Restarting RLAGN
• Characterizing EoR foregrounds: A study of the Lockman Hole Region at 325 MHz
• Deep LOFAR 150 MHz imaging of the Boötes field: Unveiling the faint low-frequency sky
• PRAiSE: Resolved spectral evolution in simulated radio sources
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• Investigating ionospheric calibration for LOFAR 2.0 with simulated observations
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• Tailed Radio Galaxies from the TIFR GMRT Sky Survey
• Search for hybrid morphology radio galaxies from FIRST survey at 1400 MHz
• A new catalogue of head-tail radio galaxies from LoTSS DR1
• Finding AGN remnant candidates based on radio morphology with machine learning
• Faint polarised sources in the Lockman Hole field at 1.4 GHz
• Filamentary Baryons and Where to Find Them: A forecast of synchrotron radiation from merger and accretion shocks in the local Cosmic Web
• Matching LOFAR sources across radio bands
• Apertif 1.4 GHz continuum observations of the Boötes field and their combined view with LOFAR
• A multi-band AGN-SFG classifier for extragalactic radio surveys using machine learning
• J0011+3217: A peculiar radio galaxy with a one-sided secondary lobe and misaligned giant primary lobes
• Study of RCR Catalogue Radio Source Integral Spectra
• What is the origin of the stacked radio emission in radio-undetected quasars?: Insights from a radio-infrared image stacking analysis
• Optical detection of a GMRT-detected candidate high-redshift radio galaxy with 3.6-m Devasthal optical telescope
• Properties of Compact Faint Radio Sources as a Function of Angular Size from Stacking
• A FLASH on Blazars: Capturing the Radio Realm of 4FGL Blazars with SKAO Pathfinders
• A first quantification of the effects of absorption for HI Intensity Mapping experiments
• Revealing the intricacies of radio galaxies and filaments in the merging galaxy cluster Abell 2255. II. Properties of filaments using multi-frequency radio data
• Spectral indices in active galactic nuclei as seen by Apertif and LOFAR
• A GMRT 610 MHz radio survey of the North Ecliptic Pole (NEP, ADF-N) / Euclid Deep Field North
• Combining LOFAR and Apertif Data for Understanding the Life Cycle of Radio Galaxies