J/A+A/617/A136 LOFAR M51 field low-frequency polarized sources (Horellou+, 2018)
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., Van Eck C.L.
<Astron. Astrophys. 617, A136 (2018)>
=2018A&A...617A.136N 2018A&A...617A.136N (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Polarization
Keywords: polarization - radio continuum: galaxies - galaxies: magnetic fields -
galaxies: individual: M51 - methods: data analysis -
techniques: polarimetric
Abstract:
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. 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° 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.
Description:
1) FITS image of Fig. 1 (LOFAR 150MHz total intensity image).
2) Catalog of 201 sources with flux density greater than 100mJy
at 150MHz and located within 2.5 degrees of the center of M51.
3) Faraday cubes of 19 sources discussed in Sect. 5.
Objects:
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RA (ICRS) DE Designation(s)
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13 29 52.70 +47 11 42.9 M51 = LEDA 47404
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
catalog.dat 119 201 LOFAR 150MHz catalog
LOFAR150MHz.fit 2880 12 Fits version of the catalog
list.dat 133 20 List of fits files
fits/* . 20 Individual fits files
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See also:
J/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)
Byte-by-byte Description of file: catalog.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- Source [1/203] Source ID (Source ID)
5- 14 F10.6 deg RAdeg Right ascension (J2000) (RA)
16- 25 F10.6 deg DEdeg Declination (J2000) (DEC)
27- 35 F9.7 deg Dist Distance from center of M51 (Distance)
37- 44 F8.6 Jy Ftot Total flux density at 150MHz (Total_flux)
46- 53 F8.6 Jy e_Ftot rms uncertainty on Ftot (ETotalflux)
55- 59 F5.1 rad/m2 phimax Value of Faraday depth of peak in
Faraday cube (phi_max)
61- 71 F11.9 Jy/beam Fmax Polarized flux density in Jy/beam/rmsf
at phi_max (F_max)
73- 81 F9.6 --- pvalue p value (p_value)
83- 91 F9.6 --- alpha alpha (see paper) (alpha)
93- 96 F4.1 rad/m2 b_instpol Lowest value of range of Faraday depths
excluded because of instrumental
polarization (instpol_min)
98-100 F3.1 rad/m2 B_instpol Highest value of range of Faraday depths
excluded because of instrumental
polarization (instpol_max)
102-110 F9.7 Jy/beam Ithreshold Intensity threshold set in the analysis
(see paper) (I_threshold)
112-119 F8.3 --- Ncells Number of examined cells (see paper)
(Examined_cells)
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 23 I4 --- Nx Number of pixels along X-axis
25- 28 I4 --- Ny Number of pixels along Y-axis
30- 33 I4 --- Nz ? Number of pixels along Z-axis
35- 55 A21 --- Obs.date Observation date (YYY-MM-DDThh:mm:ss.s)
57- 60 I4 --- bPol ? Lower value of polarization interval
62- 66 F5.1 --- BPol ? Upper value of polarization interval
68- 70 F3.1 --- dPol [0.2]? Polarization resolution
72- 77 I6 Kibyte size Size of FITS file
79- 98 A20 --- FileName Name of FITS file, in subdirectory fits
100-133 A34 --- Title Title of the FITS file
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Acknowledgements:
Cathy Horellou, cathy.horellou(at)chalmers.se
(End) Patricia Vannier [CDS] 03-Aug-2018