J/A+A/617/A136ivo://CDS.VizieR/J/A+A/617/A136doi:10.26093/cds/vizier.36170136CDSNeld A.Horellou C.Mulcahy D.D.Beck R.Bourke S.Carozzi T.D.,Chyzy K.T.Conway J.E.Farnes J.S.Fletcher A.Haverkorn M.Heald G.,Horneffer A.Nikiel-Wroczynski B.Paladino R.Sridhar S.S.Van Eck C.L.2018-11-05T10:52:51Z2018-10-02T08:57:31ZCDS support team

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

cds-question@unistra.frgalaxiespolarimetryThe 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. The aim of the paper is to develop a computationally efficient and rigorously defined source-finding algorithm for linearly polarized sources. We used a calibrated data set from the LOw Frequency ARray (LOFAR) at 150MHz centered on the nearby galaxy M51 to search for polarized background sources. With a new imaging software, we re-imaged the field at a resolution of 18''x15'' and cataloged a total of about 3000 continuum sources within 2.5{deg} of the center of M51. We made small Stokes Q and U images centered on each source brighter than 100mJy 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 100mJy but that were reported as polarized in the literature at at least another radio frequency. 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. 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.2018A&A...617A.136Nhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/617/A136CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/A+A/568/A74 : Low-frequency (115-175MHz) image of M51 (Mulcahy+, 2014)J/A+A/592/A123 : M51 total intensity image at 333MHz (Mulcahy+, 2016)J/ApJ/793/82 : LOFAR Bootes and 3C295 field sources (van Weeren+, 2014)J/A+A/593/A86 : Arp 220 LOFAR radio images at 150MHz (Varenius+, 2016)J/A+A/597/A98 : Faraday tomography of foreground towards IC342 (Van Eck+ 2017)J/A+A/598/A104 : LOFAR Two-metre Sky Survey (Shimwell+, 2017)J/A+A/613/A58 : Polarized point sources in LOTSS-HETDEX (Van Eck+, 2018)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/617/A136https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/617/A136http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/617/A136https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/617/A136https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/617/A136http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/617/A136GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/617/A136/catalog (LOFAR 150MHz catalog)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/617/A136/catalog?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/617/A136/catalog?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/617/A136/catalog?GETtext/xml+votable180.050000true204.8451161146.671561110.005555555555555556Cone search capability for table J/A+A/617/A136/list (List of fits files)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/617/A136/list?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/617/A136/list?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/617/A136/list?GETtext/xml+votable180.050000true202.4688947.194580.0055555555555555565/2755-2758 6/10682-10683 10686-10687 10730 11013-11015 11036-11038 11072 110749.939104750406538e-26 9.939104750406538e-26https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/617/A136?format=asciidefaultJ/A+A/617/A136/catalogLOFAR 150MHz catalogpvaluep value (p_value)stat.fit.goodnessdoublerecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSource[1/203] Source ID (Source ID)meta.idintRAJ2000Right ascension (J2000) (RA)degpos.eq.ra;meta.mainDEJ2000Declination (J2000) (DEC)degpos.eq.dec;meta.mainDistDistance from center of M51 (Distance)degpos.angDistancedoubleFtotTotal flux density at 150MHz (Total_flux)Jyphot.flux.density;em.radio.100-200MHzdoublee_Ftotrms uncertainty on Ftot (E_Total_flux)Jystat.errordoublephimaxValue of Faraday depth of peak in Faraday cube (phi_max)rad.m**-2phys.absorption.opticalDepthfloatFmaxPolarized flux density in Jy/beam/rmsf at phi_max (F_max)Jy/beamphot.flux.densitydoublealphaalpha (see paper) (alpha)phot.color;em.opt.V;em.line.Halphadoubleb_instpolLowest value of range of Faraday depths excluded because of instrumental polarization (instpol_min)rad.m**-2phys.absorption.opticalDepth;stat.minfloatB_instpolHighest value of range of Faraday depths excluded because of instrumental polarization (instpol_max)rad.m**-2phys.absorption.opticalDepth;stat.maxfloatIthresholdIntensity threshold set in the analysis (see paper) (I_threshold)Jy/beaminstr.sensitivitydoubleNcellsNumber of examined cells (see paper) (Examined_cells)meta.numberdoubleJ/A+A/617/A136/listList of fits filesbPol? Lower value of polarization intervalphys.absorption.opticalDepth;stat.minintnullablerecnoRecord 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.numberintNz? Number of pixels along Z-axismeta.numberintnullableObs.dateObservation date (YYY-MM-DDThh:mm:ss.s)time.epoch;obscharBPol? Upper value of polarization intervalphys.absorption.opticalDepth;stat.maxfloatnullabledPol[0.2]? Polarization resolutionspect.resolutionfloatnullablesizeSize of FITS fileKibytephys.size;meta.fitsintFileNameName of FITS file, in subdirectory fitsmeta.id;meta.fitscharTitleTitle of the FITS filemeta.title;meta.fitschar