2018A&A...617A.136N


C.D.S. - SIMBAD4 rel 1.7 - 2020.05.29CEST03:19:41

2018A&A...617A.136N - Astronomy and Astrophysics, volume 617A, 136-136 (2018/9-1)

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

NELD 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. and VAN ECK C.L.

Abstract (from CDS):


Context. 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''x15'' 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 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.
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.

Abstract Copyright: © ESO 2018

Journal keyword(s): polarization - radio continuum: galaxies - galaxies: magnetic fields - galaxies: individual: M51 - methods: data analysis - techniques: polarimetric

VizieR on-line data: <Available at CDS (J/A+A/617/A136): catalog.dat LOFAR150MHz.fit list.dat fits/*>

Simbad objects: 43

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Number of rows : 43

N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2020
#notes
1 M 31 G 00 42 44.330 +41 16 07.50 4.86 4.36 3.44     ~ 10850 1
2 IC 342 SBG 03 46 48.514 +68 05 45.98   10.5       ~ 1380 1
3 3C 196 Sy1 08 13 36.056090 +48 13 02.63597   18.36 17.79     ~ 480 2
4 NAME GOODS-N Field reg 12 36 55.0 +62 14 15           ~ 909 1
5 NVSS J132540+490955 rG 13 25 40.265 +49 09 55.56           ~ 3 1
6 NVSS J132546+472605 Rad 13 25 47.9 +47 26 06           ~ 3 1
7 NVSS J132626+473741 Rad 13 26 26.71 +47 37 41.8           ~ 2 0
8 6C 132457+472117 QSO 13 27 03.372 +47 05 43.55           ~ 4 1
9 NVSS J132818+464622 Rad 13 28 17.9 +46 46 17           ~ 4 1
10 7C 132703.50+481633.00 BLL 13 29 09.2467253311 +48 01 09.651298389   16.99 16.77     ~ 28 1
11 NVSS J132922+480239 Rad 13 29 22.3 +48 02 39           ~ 2 1
12 [MZF2015] J1329+4706 Rad 13 29 31.089 +47 06 27.37           ~ 2 0
13 [MZF2015] J1329+4658c Rad 13 29 39.153 +46 59 09.45           ~ 2 0
14 NVSS J132939+465909 Rad 13 29 39.4 +46 59 09           ~ 3 1
15 [MZF2015] J1329+4717 Rad 13 29 41.628 +47 17 35.46           ~ 2 0
16 M 51 GiP 13 29 52.698 +47 11 42.93   9.26 8.36 8.40   ~ 3764 4
17 [MZF2015] J1330+4710 Rad 13 30 15.978 +47 10 23.36           ~ 2 0
18 FIRST J133045.0+470309 Rad 13 30 45.0 +47 03 09           ~ 1 0
19 [MZF2015] J1330+4703a Rad 13 30 45.085 +47 03 08.76           ~ 2 0
20 FIRST J133045.1+470316 Rad 13 30 45.1 +47 03 16           ~ 1 0
21 SDSS J133045.13+470317.2 G 13 30 45.13320 +47 03 17.2440           ~ 1 0
22 [MZF2015] J1330+4703b Rad 13 30 45.286 +47 03 28.76           ~ 2 0
23 FIRST J133045.3+470324 Rad 13 30 45.3 +47 03 24           ~ 1 0
24 NVSS J133051+475928 Rad 13 30 52.2 +47 59 28           ~ 3 1
25 [MZF2015] J1331+4713a Rad 13 31 22.554 +47 13 21.32           ~ 2 0
26 NVSS J133124+471317 Rad 13 31 24.00 +47 13 17.5           ~ 2 1
27 [MZF2015] J1331+4713b Rad 13 31 27.452 +47 13 01.08           ~ 2 0
28 NVSS J133128+454002 Rad 13 31 28.20 +45 40 01.0           ~ 2 0
29 NVSS J133135+453851 Rad 13 31 29.05 +45 39 57.8           ~ 3 1
30 NVSS J133150+474557 Rad 13 31 50.7 +47 45 57           ~ 2 1
31 NVSS J133246+445336 Rad 13 32 46.90 +44 53 36.7           ~ 2 1
32 NVSS J133255+470046 Rad 13 32 55.6 +47 00 46           ~ 2 1
33 NVSS J133258+454201 Rad 13 32 58.87 +45 42 02.2           ~ 3 2
34 NVSS J133335+465807 Rad 13 33 35.4 +46 58 08           ~ 3 1
35 EQ J133358+462204 ? 13 33 59.0 +46 22 08           ~ 1 0
36 NVSS J133613+490037 Rad 13 36 13.99 +49 00 37.3           ~ 1 0
37 NVSS J133707+485801 Rad 13 37 07.69 +48 58 00.8           ~ 4 1
38 NVSS J133737+490439 Rad 13 37 37.50 +49 04 39.9           ~ 2 1
39 NGC 5256 PaG 13 38 17.8 +48 16 41   14.1 13.42     ~ 395 2
40 NVSS J133920+464115 Rad 13 39 20.32 +46 41 15.6           ~ 2 0
41 4C 47.38 QSO 13 41 44.898 +46 57 18.00   19.7       ~ 25 1
42 3C 295 Sy2 14 11 20.467 +52 12 09.52   22.34 20.80 18.54   ~ 893 5
43 ELAIS N1 reg 16 10 01 +54 30.6           ~ 305 0

    Equat.    Gal    SGal    Ecl

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2020.05.29-03:19:41

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