Reliable detection and characterization of low-frequency polarized sources in the LOFAR M51 field
Artikel i vetenskaplig tidskrift, 2018

The new generation of broad-band radio continuum surveys will provide large data sets with polarization information. New algorithms need to be developed to extract reliable catalogs of linearly polarized sources that can be used to characterize those sources and produce a dense rotation measure (RM) grid to probe magneto-ionized structures along the line of sight via Faraday rotation. Aims. The aim of the paper is to develop a computationally efficient and rigorously defined source-finding algorithm for linearly polarized sources. Methods. We used a calibrated data set from the LOw Frequency ARray (LOFAR) at 150 MHz centered on the nearby galaxy M 51 to search for polarized background sources. With a new imaging software, we re-imaged the field at a resolution of 18″ × 15″ and cataloged a total of about 3000 continuum sources within 2.5° of the center of M 51. We made small Stokes Q and U images centered on each source brighter than 100 mJy in total intensity (201 sources) and used RM synthesis to create corresponding Faraday cubes that were analyzed individually. For each source, the noise distribution function was determined from a subset of the measurements at high Faraday depths where no polarization is expected; the peaks in polarized intensity in the Faraday spectrum were identified and the p-value of each source was calculated. Finally, the false discovery rate method was applied to the list of p-values to produce a list of polarized sources and quantify the reliability of the detections. We also analyzed sources fainter than 100 mJy but that were reported as polarized in the literature at at least another radio frequency. Results. Of the 201 sources that were searched for polarization, six polarized sources were detected confidently (with a false discovery rate of 5%). This corresponds to a number density of one polarized source per 3.3 square degrees, or 0.3 source per square degree. Increasing the false discovery rate to 50% yields 19 sources. A majority of the sources have a morphology that is indicative of them being double-lobed radio galaxies, and the ones with literature redshift measurements have 0.5 < z < 1.0. Conclusions. We find that this method is effective in identifying polarized sources, and is well suited for LOFAR observations. In the future, we intend to develop it further and apply it to larger data sets such as the LOFAR Two-meter Survey of the whole northern sky, LOTSS, and the ongoing deep LOFAR observations of the GOODS-North field.

Galaxies: magnetic fields


Radio continuum: galaxies

Galaxies: individual: M51

Methods: data analysis

Techniques: polarimetric


A. Neld

Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium

Cathy Horellou

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik, Extragalaktisk astrofysik

D. D. Mulcahy

Max Planck-institutet

University of Manchester

R. Beck

Max Planck-institutet

S. Bourke

Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium

Tobia Carozzi

Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium, Onsala rymdobservatorium, Observationssupport

K.T. Chyz̊y

Uniwersytet Jagiellonski w Krakowie

John Conway

Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium

J.S. Farnes

Radboud Universiteit

A. Fletcher

Newcastle University

M. Haverkorn

Radboud Universiteit

G. Heald

CSIRO Astronomy and Space Science

Rijksuniversiteit Groningen

A. Horneffer

Max Planck-institutet

B. Nikiel-Wroczyński

Uniwersytet Jagiellonski w Krakowie

R. Paladino

INAF Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna

S. S. Sridhar

Rijksuniversiteit Groningen

C.L. Van Eck

Radboud Universiteit

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 617 A136


Astronomi, astrofysik och kosmologi

Atom- och molekylfysik och optik




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