J/A+A/687/A267ivo://CDS.VizieR/J/A+A/687/A267doi:10.26093/cds/vizier.36870267CDSPiras S.Horellou C.Conway J.E.Thomasson M.del Palacio S.,Shimwell T.W.O'Sullivan S.P.Carretti E.Snidaric I.Jelic V.,Adebahr B.Berger A.Best P.N.Brueggen M.Herrera Ruiz N.,Paladino R.Prandoni I.Sabater J.Vacca V.2025-02-27T20:01:44Z2024-12-24T10:55:17ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frradio-galaxiespolarimetryQuantifying the number density and physical characteristics of extragalactic polarized sources is important for the successful planning of future studies based on Faraday Rotation Measure (RM) grids of polarized sources to probe foreground Galactic and intergalactic magnetic fields. However, it is proving very hard to detect polarized signal from the population of very faint (sub-mJy) polarized sources at low radio frequencies, and their properties are mostly unknown. LOFAR can play an important role in such studies thanks to its sensitivity and angular resolution, combined with the precision on the inferred RM values that can be achieved through low-frequency broad-band polarimetry. The aim of this study is to probe the sub-mJy polarized source population with LOFAR. In this first paper we present the method used to stack LOFAR polarization datasets, the resulting catalog of polarized sources and the derived polarized source counts. The European Large Area ISO Survey-North 1 (ELAIS-N1) field, one of the deepest of the LOFAR Two-Metre Sky Survey (LoTSS) Deep Fields so far, was selected for a polarimetric study at 114.9-177.4MHz. A total area of 25 deg^2^ was imaged at 6"-resolution in the Stokes Q and U parameters. Alignment of polarization angles was done both in frequency and in Faraday space before stacking datasets from 19 eight-hour-long epochs taken in two different LOFAR observing cycles. A search for polarized sources was carried out in the final, stacked dataset, and the properties of the detected sources were examined. The depolarization level of sources known to be polarized at 1.4GHz was quantified. A one-sigma noise level, {sigma}_QU_, of 19uJy/beam was reached in the central part of the field after stacking. 25 polarized sources were detected above 8{sigma}_QU_, five of which had not been detected in polarization at any other radio frequencies before. Seven additional polarized components were found by lowering the threshold to 6{sigma}_QU_ at positions corresponding to sources known to be polarized at 1.4GHz. In two radio galaxies, polarization was detected from both radio lobes, so the final number of associated radio continuum sources is 31. The detected sources are weakly polarized, with a median degree of polarization of 1.75% for the sample of sources detected in polarized emission. For the 10 polarized sources previously identified in a pilot LOFAR study of the ELAIS-N1 field at 20"-resolution, the RM values are consistent but the degrees of polarization are higher in the 6"-resolution data. The sources previously detected in polarization at 1.4GHz are significantly depolarized at 150MHz. The catalog is used to derive the polarized source counts at 150MHz. This is the deepest and highest-resolution polarization study at 150 MHz to date. A full characterization of the sources and an analysis of the catalog will be presented in Paper II.2024A&A...687A.267Phttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/687/A267CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/A+A/648/A2 : LOFAR Two-metre Sky Survey Deep Fields DR1 (Sabater+, 2021)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/687/A267https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/687/A267http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/687/A267https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/687/A267https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/687/A267http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/687/A267GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/687/A267/tableb1 (Catalog of polarized sources in the ELAIS-N1 field detected with LOFAR)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/687/A267/tableb1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/687/A267/tableb1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/687/A267/tableb1?GETtext/xml+votable180.050000true239.014955.01580.005555555555555556Cone search capability for table J/A+A/687/A267/list (List of fits datacubes)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/687/A267/list?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/687/A267/list?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/687/A267/list?GETtext/xml+votable180.050000true244.8887555.07750.0055555555555555565/2537-2539 2880 6/9981 10145-10147 10170 11349 11351 11524https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/687/A267?format=asciiRadiodefaultJ/A+A/687/A267/tableb1Catalog of polarized sources in the ELAIS-N1 field detected with LOFARrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource ID (NN, NN_A, NN_B)meta.idcharLOFARName of associated total-intensity radio component in the Sabater et al. (2021A&A...648A...2S, Cat. J/A+A/648/A2)meta.id;meta.maincharRAJ2000Right Ascension (J2000)degpos.eq.ra;meta.mainDEJ2000Declination (J2000)degpos.eq.dec;meta.mainPpeakPeak polarized intensityJy/beamphot.flux.density;em.radiofloatRMFaraday rotation measurerad.m**-2phys.polarization.rotMeasurefloate_RMFaraday rotation measure errorrad.m**-2stat.errorfloatIpeakStokes I intensity in the image of Sabater et al. (2021A&A...648A...2S) at the pixel of the peak polarized intensityJy/beamphot.flux.density;em.radiodoublePolDegree of polarization at the peak pixel%phys.polarizationfloatPolinfoFlag to indicate whether the source is listed as polarized in other catalogs (1)meta.codecharFileNameName of the fits datacube in subdirectory fitsmeta.id;meta.fitscharJ/A+A/687/A267/listList of fits datacubesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right Ascension of center (J2000)degpos.eq.ra;meta.mainDEJ2000Declination of center (J2000)degpos.eq.dec;meta.mainNxNumber of pixels along X-axismeta.numberintNyNumber of pixels along Y-axismeta.numberintNzNumber of slicesmeta.numberintObs.dateObservation datetime.epoch;obsnullablesizeSize of FITS fileKibytephys.size;meta.fitsintFileNameName of FITS file, in subdirectory fitsmeta.id;meta.fitscharTitleTitle of the FITS filemeta.title;meta.fitschar