Reliable detection and characterization of low-frequency polarized sources in the LOFAR M51 field
Journal article, 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.

Radio continuum: galaxies


Methods: data analysis

Galaxies: individual: M51

Techniques: polarimetric

Galaxies: magnetic fields


A. Neld

Chalmers, Space, Earth and Environment, Onsala Space Observatory

Cathy Horellou

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics

D. D. Mulcahy

Max Planck Society

University of Manchester

R. Beck

Max Planck Society

S. Bourke

Chalmers, Space, Earth and Environment, Onsala Space Observatory

Tobia Carozzi

Chalmers, Space, Earth and Environment, Onsala Space Observatory

K.T. Chyz̊y

Jagiellonian University in Kraków

John Conway

Chalmers, Space, Earth and Environment, Onsala Space Observatory

J.S. Farnes

Radboud University

A. Fletcher

Newcastle University

M. Haverkorn

Radboud University

G. Heald

University of Groningen

Commonwealth Scientific and Industrial Research Organisation (CSIRO)

A. Horneffer

Max Planck Society

B. Nikiel-Wroczyński

Jagiellonian University in Kraków

R. Paladino

Istituto nazionale di astrofisica (INAF)

S. S. Sridhar

University of Groningen

C.L. Van Eck

Radboud University

Astronomy and Astrophysics

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

Vol. 617 A136

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics




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