J/A+A/687/A267 Polarized sources in ELAIS-N1 LOFAR deep field (Piras+, 2024)
LOFAR Deep Fields: Probing the sub-mJy regime of polarized extragalactic
sources in ELAIS-N1. I. The catalog.
Piras 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.
<Astron. Astrophys. 687, A267 (2024)>
=2024A&A...687A.267P 2024A&A...687A.267P (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, radio ; Polarization
Keywords: magnetic fields - polarization - methods: numerical -
methods: observational - techniques: polarimetric -
galaxies: individual (ELAIS-N1)
Abstract:
Quantifying 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 deg2 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, σQU, of 19uJy/beam was reached in the
central part of the field after stacking. 25 polarized sources were
detected above 8σQU, 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
6σQU 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.
Description:
Catalog of 33 polarized components in the ELAIS-N1 LOFAR deep field,
detected after stacking 8-hour observing epochs.
LOFAR fits datacubes for each components.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tableb1.dat 108 33 Catalog of polarized sources in the ELAIS-N1
field detected with LOFAR
list.dat 93 32 List of fits datacubes
fits/* . 32 Individual fits datacubes
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See also:
J/A+A/648/A2 : LOFAR Two-metre Sky Survey Deep Fields DR1 (Sabater+, 2021)
Byte-by-byte Description of file: tableb1.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- Source Source ID (NN, NN_A, NN_B)
6- 27 A22 --- LOFAR Name of associated total-intensity radio
component in the Sabater et al.
(2021A&A...648A...2S 2021A&A...648A...2S, Cat. J/A+A/648/A2)
29- 36 F8.4 deg RAdeg Right Ascension (J2000)
38- 44 F7.4 deg DEdeg Declination (J2000)
46- 54 E9.1 Jy/beam Ppeak Peak polarized intensity
56- 62 F7.3 rad/m2 RM Faraday rotation measure
64- 68 F5.3 rad/m2 e_RM Faraday rotation measure error
70- 76 F7.5 Jy/beam Ipeak Stokes I intensity in the image of
Sabater et al. (2021A&A...648A...2S 2021A&A...648A...2S) at
the pixel of the peak polarized intensity
78- 82 F5.2 % Pol Degree of polarization at the peak pixel
84- 88 A5 --- Polinfo Flag to indicate whether the source is
listed as polarized in other catalogs (1)
91-108 A18 --- FileName Name of the fits datacube in subdirectory
fits
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Note (1): Flags as follows:
1 = source is listed as polarized at 1.4GHz in the DRAO ELAIS-N1 catalog
(Grant et al., 2010ApJ...714.1689G 2010ApJ...714.1689G, cat. J/ApJ/714/1689)
2 = source is listed in the NVSS RM catalog (Taylor et al.,
2009ApJ...702.1230T 2009ApJ...702.1230T, Cat. J/ApJ/702/1230)
3 = source is listed as polarized at 150MHz and 20" resolution in the LOFAR
work of Herrera Ruiz et al. (2021A&A...648A..12H 2021A&A...648A..12H).
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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- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 31 I4 --- Nz Number of slices
33- 42 A10 "date" Obs.date Observation date
44- 49 I6 Kibyte size Size of FITS file
51- 68 A18 --- FileName Name of FITS file, in subdirectory fits
70- 93 A24 --- Title Title of the FITS file
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Acknowledgements:
Sara Piras, sara.piras(at)chalmers.se
(End) Patricia Vannier [CDS] 31-May-2024