Faint polarised sources in the Lockman Hole field at 1.4 GHz

Abstract

We aim to study the nature of the faint, polarised radio source population whose source composition and redshift dependence contain information about the strength, morphology, and evolution of magnetic fields over cosmic timescales. We use a 15 pointing radio continuum L-band mosaic of the Lockman Hole, observed in full polarisation, generated from archival data of the WSRT. The data were analysed using the RM-Synthesis technique. We achieved a noise of 7 μJy/beam in polarised intensity, with a resolution of 15''. Using infrared and optical images and source catalogues, we were able to cross-identify and determine redshifts for one third of our detected polarised sources. We detected 150 polarised sources, most of which are weakly polarised with a mean fractional polarisation of 5.4 %. With a total area of 6.5 deg^2 and a detection threshold of 6.25 σ we find 23 polarised sources per deg^2. Based on our multi wavelength analysis, we find that our sample consists of AGN only. We find a discrepancy between archival number counts and those present in our data, which we attribute to sample variance. Considering the absolute radio luminosty, to distinguish weak and strong sources, we find a general trend of increased probability to detect weak sources at low redshift and strong sources at high redshift. Further, we find an anti-correlation between fractional polarisation and redshift for our strong sources sample at z{\geq}0.6. A decrease in the fractional polarisation of strong sources with increasing redshift cannot be explained by a constant magnetic field and electron density over cosmic scales, however the changing properties of cluster environments over the cosmic timemay play an important role. Disentangling these two effects requires deeper and wider polarisation observations, and better models of the morphology and strength of cosmic magnetic fields.

arXiv comment17 pages, 16 figures, to be published in A&A

References in corpus (44)

• The Infrared Array Camera (IRAC) for the Spitzer Space Telescope
• The Spitzer Space Telescope Mission
• The Galaxy Evolution Explorer: A Space Ultraviolet Survey Mission
• Relativistic Jets in Active Galactic Nuclei
• The Herschel Multi-tiered Extragalactic Survey: HerMES
• SWIRE: The SIRTF Wide-area InfraRed Extragalactic Survey
• Faraday Rotation Measure Synthesis
• Magnetic Fields in Spiral Galaxies
• A 20cm VLA Survey of Abell Clusters of Galaxies VI. Radio/Optical Luminosity Functions
• An Accurate Flux Density Scale from 50 MHz to 50 GHz
• The Far- and Mid-Infrared/Radio Correlations in the Spitzer Extragalactic First Look Survey
• Cosmological Magnetic Fields Limits in an Inhomogeneous Universe
• The European Large Area ISO Survey I: Goals, Definition and Observation
• Revisiting the Fanaroff-Riley dichotomy and radio-galaxy morphology with the LOFAR Two-Metre Sky Survey (LoTSS)
• New limits on extragalactic magnetic fields from rotation measures
• The Phoenix Deep Survey: The 1.4 GHz microJansky catalogue
• Faint high-redshift AGN in the Chandra Deep Field South: the evolution of the AGN luminosity function and black hole demography
• The Tiered Radio Extragalactic Continuum Simulation (T-RECS)
• The Lockman Hole project: LOFAR observations and spectral index properties of low-frequency radio sources
• Simulating the Spitzer mid-IR color-color diagrams
• ALMA Observations of Gas-Rich Galaxies in z~1.6 Galaxy Clusters: Evidence for Higher Gas Fractions in High-Density Environments
• Predictions on high frequency polarization properties of extragalactic radio sources and implications for CMB polarization measurements
• M82 - A radio continuum and polarisation study I. Data reduction and cosmic ray propagation
• New constraints on the magnetization of the cosmic web using LOFAR Faraday rotation observations
• Differences in Faraday Rotation Between Adjacent Extragalactic Radio Sources as a Probe of Cosmic Magnetic Fields
• The Lockman Hole Project: new constraints on the sub-mJy source counts from a wide-area 1.4 GHz mosaic
• A Broadband Polarization Catalog of Extragalactic Radio Sources
• Radio Polarimetry of the ELAIS N1 Field: Polarized Compact Sources
• Photometry and Photometric Redshift catalogs for the Lockman Hole Deep Field
• Degree of Polarization and Source Counts of Faint Radio Sources from Stacking Polarized Intensity
• Polarization Properties of Extragalactic Radio Sources and Their Contribution to Microwave Polarization Fluctuations
• The distribution of polarized radio sources $>$15$\mu$Jy in GOODS-N
• ATLAS 1.4 GHz Data Release 2 -- II. Properties of the faint polarized sky
• ATLAS 1.4 GHz Data Release 2 -- I. Observations of the CDF-S and ELAIS-S1 fields and methods for constructing differential number counts
• The DRAO Planck Deep Fields: the polarization properties of radio galaxies at 1.4 GHz
• ATLBS: the Australia Telescope Low-brightness Survey
• The radio properties of infrared-faint radio sources
• Magnetic field disorder and Faraday effects on the polarization of extragalactic radio sources
• Radio Polarization Properties of Quasars and Active Galaxies at High Redshifts
• Weak lensing magnification of SpARCS galaxy clusters
• The host galaxies of micro-Jansky radio sources
• VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz
• A Large Scale Structure at Redshift 1.71 in the Lockman Hole
• Starburst-AGN Connection: A Lesson from High-z Powerful Radio Galaxies

Cited by in corpus (5)

• Magnetic field strength in cosmic web filaments
• First release of Apertif imaging survey data
• The MeerKAT Fornax Survey IV. A close look at the cluster physics through the densest rotation measure grid
• The Apertif science verification campaign - Characteristics of polarised radio sources
• LOFAR Deep Fields: Probing the sub-mJy regime of polarized extragalactic sources in ELAIS-N1. II. Analysis