J/MNRAS/519/5723 Faraday RM GRID of the LoTSS survey DR2 (O'Sullivan+, 2023)
The Faraday Rotation Measure Grid of the LOFAR Two-metre Sky Survey
Data Release 2.
O'Sullivan S.P., Shimwell T.W., Hardcastle M.J., Tasse C., Heald G.,
Carretti E., Bruggen M., Vacca V., Sobey C., Van Eck C.L., Horellou C.,
Beck R., Bilicki M., Bourke S., Botteon A., Croston J.H., Drabent A.,
Duncan K., Heesen V., Ideguchi S., Kirwan M., Lawlor L., Mingo B.,
Nikiel-Wroczynski B., Piotrowska J., Scaife A.M.M., Van Weeren R.J.
<Mon. Not. R. Astron. Soc. 519, 5723-5742 (2023)>
=2023MNRAS.519.5723O 2023MNRAS.519.5723O (SIMBAD/NED BibCode)
ADC_Keywords: Polarization ; Galaxies, photometry ; Galaxies, radio ;
Cross identifications ; Positional data ; Infrared ; Optical ;
Redshifts ; Observatory log
Keywords: polarization - catalogues - techniques: polarimetric -
galaxies: active - magnetic fields - radio continuum: galaxies
Abstract:
A Faraday rotation measure (RM) catalogue, or RM Grid, is a valuable
resource for the study of cosmic magnetism. Using the second data
release (DR2) from the LOFAR Two-metre Sky Survey (LoTSS), we have
produced a catalogue of 2461 extragalactic high-precision RM values
across 5720 deg2 of sky (corresponding to a polarized source areal
number density of ∼0.43 deg-2). The linear polarization and RM
properties were derived using RM synthesis from the Stokes Q and U
channel images at an angular resolution of 20 arcsec across a
frequency range of 120 to 168 MHz with a channel bandwidth of 97.6
kHz. The fraction of total intensity sources (>1 mJy/beam) found to be
polarized was ∼0.2 per cent. The median detection threshold was 0.6
mJy/beam (8σQU), with a median RM uncertainty of 0.06 rad/m2
(although a systematic uncertainty of up to 0.3 rad/m2 is possible,
after the ionosphere RM correction). The median degree of polarization
of the detected sources is 1.8 per cent, with a range of 0.05 per cent
to 31 per cent. Comparisons with cm-wavelength RMs indicate minimal
amounts of Faraday complexity in the LoTSS detections, making them
ideal sources for RM Grid studies. Host galaxy identifications were
obtained for 88 per cent of the sources, along with redshifts for 79
per cent (both photometric and spectroscopic), with the median
redshift being 0.6. The focus of the current catalogue was on
reliability rather than completeness, and we expect future versions of
the LoTSS RM Grid to have a higher areal number density. In addition,
25 pulsars were identified, mainly through their high degrees of
linear polarization.
Description:
The construction of large-area 'RM Grid' catalogues are a key goal for
current and future radio telescopes. The RM Grid is shorthand for a
collection of Faraday rotation measure (RM) values from linearly
polarized radio sources observed across a particular area of sky, and
it enables many different science goals in the study of magnetic
fields on different scales in the Universe. The combination of RM Grid
catalogues at metre and centimetre wavelengths provide an important
means to better understand the different contributions to the Faraday
rotation along the line of sight.
In this paper, we use the LoTSS polarization data at an angular
resolution of 20 arcsec of Q and U images and also the Stokes I source
catalogues and images at 6 arcsec. LoTSS is observing the northern sky
with the LOFAR High-Band Antennas (HBA) at Declinations greater than
0° with a frequency range of 120-168 MHz. As part of DR2 (DR2;
Shimwell et al. 2022A&A...659A...1S 2022A&A...659A...1S, Cat. J/A+A/659/A1), here we
present the LoTSS-DR2 RM Grid covering 5720 deg2 of the sky. This is
approximately a quarter of the final sky area expected from the full
LoTSS survey. In addition to the RM Grid catalogue, we also provide
access to a wide range of ancillary data products.
In order to find linearly polarized sources, the RM synthesis
technique was applied on the Q and U images using pyrmsynth. For each
field, we used the polarized intensity image (created as described
above) to identify candidate polarized source components. After
addressing the fake polarized source issue, 4280 sources remained, for
which some further automated cuts were made. Then, for each entry in
the final LoTSS RM Grid catalogue of 2461 unique polarized components,
we extracted the single-pixel Q and U versus frequency spectra at the
component location. Hereafter, we switched to using the rm-tools
package to analyse the final selected polarized sources more
comprehensively. Thus, we re-ran RM synthesis on the extracted
spectra. The catalogued RM and polarized intensity was obtained by
fitting a parabola to the main peak outside of the leakage range, and
correcting for polarization bias.
The catalogue output columns follow the RMTable standardized format.
More, our catalog is compared with NVSS RM and cross-matched with
optical, infrared and ROMA-BZCAT blazar counterparts. Finally,
physical properties of component are estimated (e.g. luminosities,
sizes and morphologies, host galaxy informations). All columns are
presented in rmcat.dat for the 2461 polarized components.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
rmcat.dat 1261 2461 *The final LoTSS RM Grid catalogue of polarized
components
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Note on rmcat.dat: It follows the RMTable standard as described in
Van Eck et al. 2023ApJS..267...28V 2023ApJS..267...28V, with some additional value-added columns
(e.g. LoTSS-DR2 total intensity source associations, host galaxy coordinates,
redshift, etc.). Each row in the catalogue corresponds to a single polarized
component. More details on https://github.com/CIRADA-Tools/RMTable/tree/master
/docs .
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See also:
J/MNRAS/515/256 : Comoving IGMF redshift evolution (Pomakov+, 2022)
J/MNRAS/495/2607 : IGMF limits with LOFAR (O'Sullivan+, 2020)
J/MNRAS/484/3646 : LOFAR PSR low-frequency Faraday rotation measures
(Sobey+ 2019)
J/MNRAS/430/3086 : CoNFIG AGN sample (Gendre+, 2013)
J/MNRAS/301/235 : Multifrequency polarimetry of 300 pulsars (Gould+, 1998)
J/ApJ/926/65 : Compilation of double radio sources (Xu+, 2022)
J/ApJ/836/174 : Extragalactic peaked-spectrum radio sources
(Callingham+, 2017)
J/ApJ/771/105 : MWA Stokes I 189MHz sources (Bernardi+, 2013)
J/ApJ/733/69 : Polarized DRAO sources (Banfield+, 2011)
J/ApJ/702/1230 : Rotation measure image of the sky (Taylor+, 2009)
J/A+A/665/A71 : JVLA Observations of Chandra Planck clusters (Osinga+, 2022)
J/A+A/664/A83 : Nearby galaxies in LoTSS-DR2 (Heesen+, 2022)
J/A+A/659/A1 : LOFAR Two-metre Sky Survey (LoTSS) DR2 (Shimwell+, 2022)
J/A+A/648/A4 : LoTSS Deep Fields DR1 photometric redshifts (Duncan+, 2021)
J/A+A/623/A71 : LOTSS HETDEX Faraday depth cube (Van Eck+, 2019)
J/A+A/622/A1 : LOFAR Two-metre Sky Survey DR1 source catalog
(Shimwell+, 2019)
J/A+A/617/A136 : LOFAR M51 field low-frequency polarized sources
(Horellou+, 2018)
J/A+A/613/A58 : Polarized point sources in LOTSS-HETDEX (Van Eck+, 2018)
J/A+A/609/A1 : FR0CAT. a FIRST catalog of FR 0 radio galaxies (Baldi+,2018)
J/A+A/601/A81 : FIRST catalog of FR II radio galaxies (Capetti+, 2017)
J/A+A/598/A104 : LOFAR Two-metre Sky Survey (Shimwell+, 2017)
J/A+A/598/A49 : FRICAT. FIRST catalog of FR I radio galaxies (Capetti+,2017)
J/A+A/583/A137 : LOFAR-HBA 3C196 field RM cubes (Jelic+, 2015)
J/A+A/568/A74 : Low-frequency (115-175MHz) image of M51 (Mulcahy+, 2014)
J/ApJS/212/15 : Polarized NVSS sources SEDs (Farnes+, 2014)
VII/274 : The Roma BZCAT - 5th edition (Massaro+, 2015)
Byte-by-byte Description of file: rmcat.dat
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Bytes Format Units Label Explanations
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1- 5 I5 --- ID The unique identifier for each polarized
component (cat_id)
7- 17 F11.7 deg RAdeg Right Ascension of the polarized
component (J2000) (ra)
19- 28 F10.7 deg DEdeg Declination of the polarized component
(J2000) (dec)
30- 37 F8.3 rad/m2 RM Faraday rotation measure value at peak of
Faraday dispersion function excluding
sources due to instrumental polarization
as seen in section 2.2 (rm)
39- 49 F11.9 rad/m2 e_RM Uncertainty in the peak RM including some
systematic errors (rm_err)
51- 71 F21.19 rad/m2 errRM Uncertainty in the peak RM value based
only on the signal-to-noise ratio
(rmerrsnr)
73- 84 E12.10 Jy/beam PIlin Linear polarization intensity at the
reference frequency after correction for
polarization bias (polint)
86- 98 E13.9 Jy/beam e_PIlin Uncertainty in the linear polarization
intensity (polint_err)
100- 111 E12.10 --- FracP Fractional linear polarization of the
polarized component (fracpol)
113- 125 E13.10 --- e_FracP Uncertainty in the fractional linear
polarization (fracpol_err)
127- 139 F13.10 Jy/beam I Total intensity at the position of the
polarized component (stokesI)
141- 153 E13.10 Jy/beam e_I Uncertainty in the total intensity
(stokesI_err)
155- 166 E12.9 Hz FPlin Reference frequency for the linear
polarization quantities (reffreq_pol)
168- 173 E6.3 Hz Ftot Reference frequency for the total
intensity values (reffreq_I)
175- 179 F5.3 rad/m2 FWHM Full width at half maximum of the RMSF
rotation measure spread function
(rmsf_fwhm)
181- 191 A11 --- Field LoTSS sky are field name (field)
193- 196 I4 pix Xpos The x-pixel coordinate within an
individual LoTSS field ranging from 0 to
3200 for a pixel width of 4.5 arcsec (x)
198- 201 I4 pix Ypos The y-pixel coordinate within an
individual LoTSS field, ranging from 0 to
3200 for a pixel height of 4.5 arcsec (y)
203- 213 F11.7 --- snrMAD SNR of polarization intensity computed
with MAD median absolute deviation of RM
values (snrRMtoolsMAD)
215- 219 F5.1 deg Ang Electric vector polarization angle (angle)
221- 240 F20.16 deg RANdeg ? Right Ascension from NVSS Condon et al.
1998AJ....115.1693C 1998AJ....115.1693C, Cat.VIII/65 (RA_NVSS)
(J2000) (RA_NVSS) (5)
242- 259 F18.15 deg DENdeg ? Declination from NVSS Condon et al.
1998AJ....115.1693C 1998AJ....115.1693C, Cat. VIII/65 of the
(J2000) (DEC_NVSS) (5)
261- 266 F6.1 rad/m2 NVSSRM ? Faraday rotation measure value taken
from NVSS Condon et al.
1998AJ....115.1693C 1998AJ....115.1693C, Cat.VIII/65 (NVSS_RM)
268- 271 F4.1 rad/m2 e_NVSSRM ? The 1σ error of NVSSRM
(NVSSRMerr)
273- 278 F6.1 mJy INVSS ? Integrated Stokes I parameter flux
density from NVSS Condon et al.
1998AJ....115.1693C 1998AJ....115.1693C, Cat. VIII/65 (I_NVSS)
280- 285 F6.2 mJy PkNVSS ? Average peak polarized intensity from
NVSS Condon et al. 1998AJ....115.1693C 1998AJ....115.1693C,
Cat. VIII/65 (P_NVSS)
287- 291 F5.2 mJy mNVSS ? Percent polarization as ratio of peak
polarized intensity to peak Stokes I from
NVSS Condon et al. 1998AJ....115.1693C 1998AJ....115.1693C,
Cat. VIII/65 (PI_NVSS)
293- 312 F20.17 arcsec Sep ? Angular separation between LOFAR and
NVSS positions (Separation_NVSS)
314- 333 F20.16 deg GLON Galactic Longitude of the polarized
component (l)
335- 353 F19.15 deg GLAT Galactic Latitude of the polarized
component (b)
355- 377 F23.19 rad/m2 RRM The residual RM after subtraction of the
Galactic RM as RM-GRM usefull when RM is
dominated by GRM (RRM)
379- 401 F23.18 rad/m2 GRM The Galactic RM (GRM)
403- 421 F19.16 rad/m2 e_GRM Error of GRM (GRMerr)
423- 442 F20.15 arcsec LGZ ? Largest angular size estimated from
visual inspection (LGZ_Size)
444- 453 F10.7 mag rmag ? Optical r band magnitude from Optical
identification with LEGACY optical (MAG_R)
455- 464 F10.7 mag W1mag ? The Wise W1 infrared magnitude (MAG_W1)
466- 475 F10.7 mag W2mag ? The Wise W2 infrared magnitude (MAG_W2)
477- 487 E11.8 --- zph ? Photometric redshift of the host
galaxies (zphot) (6)
489- 508 E20.18 --- e_zph ? Error of zph (zphot_err)
510- 522 E13.10 --- ErrPos Positional uncertainty (pos_err)
524- 525 I2 h RAgh ? Right ascension of the host galaxy
(J2000) (lgz_ra)
527- 528 I2 min RAgm ? Right ascension of the host galaxy
(J2000) (lgz_ra)
530- 534 F5.2 s RAgs ? Right Ascension of the host galaxy
(J2000) (lgz_ra)
536 A1 --- DEg- ? Declination sign of the host galaxy
(J2000) (lgz_ra)
537- 538 I2 deg DEgd ? Declination of the host galaxy (J2000)
(lgz_ra)
540- 541 I2 arcmin DEgm ? Declination of the host galaxy (J2000)
(lgz_ra)
543- 547 F5.2 arcsec DEgs ? Declination of the host galaxy (J2000)
(lgz_ra)
549- 550 I2 --- f_zbest ? A value of 0/1 corresponds to a
photometric/spectroscopic redshift in
zbest (photspecz_best)
552- 572 F21.17 deg RAgdeg ? Right ascension of the host galaxy
(lgzradeg)
574- 591 F18.15 deg DEgdeg ? Declination of the host galaxy
(lgzdecdeg)
593- 612 F20.18 --- zbest ? Redshift of host galaxy spectroscopic
if available otherwise photometric
(z_best)
614- 625 E12.8 Jy/beam PIori The original polarized intensity from
selection of candidate sources as
described in section 2.2 (p_orig)
627- 635 F9.5 --- SNRori The signal-to-noise ratio in polarization
from original selection of candidate
sources (snr_orig)
637- 642 F6.1 deg Phimax ? The maximum observed polarization angle
(phi_max)
644- 645 I2 --- Nbr The number of images used for RM measures
(num)
647- 653 F7.2 rad/m2 RMmed Median value of faraday rotation measure
(medRM)
655- 659 F5.2 rad/m2 RMmad The median absolute deviation of RM
values (madRM)
661- 664 F4.2 --- FrPmed Median of fractional linear polarization
values (medfpol)
666- 669 F4.2 --- FrPmad The median absolute deviation of
fractional linear polarization values
(madfpol)
671- 685 F15.11 deg RAcdeg Right ascension of the centre of the
field (J2000) (ra_centre)
687- 699 F13.10 deg DEcdeg Declination of the centre of the field
(J2000) (dec_centre)
701- 704 F4.2 deg BeamD ? Distance of the polarized component
from the centre of the field (beamdist)
706- 724 A19 --- BibCode The bibcode of this study (catalog_name)
726 A1 --- Stokes Stokes parameter complex U flag which is
always used (complex_flag)
728- 750 A23 --- Method The method used as RM synthesis technique
was applied on the Q and U images using
pyrmsynth which is always used (rm_method)
752- 761 A10 --- Iono Flag indicates RM-extract for ionosphere
RM correction is desirable since the error
are dominated by this residual errors
(ionosphere)
763 I1 --- Nc Number of components in source extraction
which is always one (Ncomp)
765- 783 A19 --- BibBias The BibCode for polarized intensity image
corrected of polarization bias as George
et al. 2012PASA...29..214G 2012PASA...29..214G (pol_bias)
785- 788 A4 --- FType Flux type which is always a peak
(flux_type)
790- 801 F12.10 deg Bmaj The beam major axis (beam_maj) (7)
803- 814 F12.10 deg Bmin The beam minor axis (beam_min) (7)
816- 819 F4.1 deg BPA The beam position angle axis (beam_pa) (7)
825- 829 E5.2 Hz MinFreq LoTSS-DR2 observational minimum frequency
is 120 MHz (minfreq)
831- 836 E6.3 Hz MaxFreq LoTSS-DR2 observational maximum frequency
is 168 MHz (maxfreq)
838- 844 F7.1 Hz BWidth The channel image bandwidth is 97.6 kHz
(channelwidth)
846- 848 I3 --- Nchan The number of Stokes Q and U image planes
or channels used in RM synthesis (Nchan)
850- 861 E12.10 Jy/beam Noise The median noise of the Stokes Q and U
image planes (noise_chan)
863- 867 A5 --- Inst Telescope used is LOFAR (telescope)
869- 875 F7.1 s Time Integrated time of observation is 28800
(int_time)
877- 883 F7.1 d MJD The MJD for the observation of the
corresponding LoTSS field (epoch) (1)
885- 894 F10.8 d dT Observational time interval (obs_interval)
(2)
896- 899 F4.2 --- Leak Instrumental leakage estimate (leakage)
(3)
901- 919 A19 --- Ref The BibCode of reference literature of
LOFAR two-metre sky survey V. DR2 from
Shimwell et al. 2022A&A...659A...1S 2022A&A...659A...1S,
Cat. J/A+A/659/A1 (dataref)
921- 942 A22 --- LDR2 The radio name of the source designation
as ILTJHHMMSS.ss+DDMMSS.s from LoTSS-DR2
of Shimwell et al. 2022A&A...659A...1S 2022A&A...659A...1S,
Cat. J/A+A/659/A1 (SourcenameDR2)
944- 963 F20.16 --- RADR2deg ? Right ascension from PyBDSF or
combination of PyBDSF components of
LoTSS-DR2 (J2000) (RA_DR2)
965- 982 F18.15 --- DEDR2deg ? Declination from PyBDSF or combination
of PyBDSF components of LoTSS-DR2 (J2000)
(DEC_DR2)
984-1004 E21.19 --- e_RADR2deg ? RMS uncertainty on RADR2deg errors
estimated by PyBDSF where only fitting
uncertainties are included (ERADR2)
1006-1026 E21.19 --- e_DEDR2deg ? RMS uncertainty on DEDR2deg errors
estimated by PyBDSF where only fitting
uncertainties are included (EDECDR2)
1028-1049 F22.19 Jy SpeakTot ? The total integrated Stokes I flux
density of the source at the reference
frequency from LoTSS-DR2 (TotalfluxDR2)
1051-1071 E21.19 Jy e_SpeakTot ? The 1-sigma error on SpeakTot
(ETotalflux_DR2)
1073-1092 F20.16 arcsec MajDR2 ? Major axis size of the source from
LoTSS-DR2 (Maj_DR2)
1094-1112 F19.16 arcsec MinDR2 ? Minor axis size of the source from
LoTSS-DR2 (Min_DR2)
1114-1134 F21.17 deg PADR2 ? Position angle of the source from
LoTSS-DR2 (PA_DR2)
1136-1156 E21.17 W/Hz L144 ? Estimate of the luminosity at 144 MHz
(L144)
1158-1178 F21.16 kpc Size ? Estimate of the projected largest
linear size (linearsize_kpc)
1180-1202 F23.19 --- RRM1d The RRM after subtraction of the average
GRM with a disc of diameter 1°
(RRM2022_1deg) (4)
1204-1225 F22.17 --- GRM1d The average GRM with a disc of diameter
1° (GRM2022_1deg)
1227-1245 F19.17 --- e_GRM1d The uncertainty in the average GRM1d
(GRMerr2022_1deg)
1247-1261 A15 --- Blazar Name designation as 5BZU/Q/G/B JHHMM+DDMM
from ROMA-BZCAT of Massaro et al.
2015Ap&SS.357...75M 2015Ap&SS.357...75M, Cat. VII/274
(bzcat_name)
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Note (1): The median epoch is the midpoint of time between the first and last
observations used to determine the RM. If a single observation was
used, it should be the time at which the observation was
half-complete. This time is stored as the modified Julian date
(MJD, JD-2400000.5). This column allows RMs to be used in analysis
of the evolution of RM over time.
Note (2): The interval of observation is the span of time between the beginning
of the first observation and the end of the final observation used to
determine the RM, in days. If only a single observation was used,
this is the difference between the start and end times of that
observation. This column allows RMs to be used in analysis
of the evolution of RM over time.
Note (3): An estimate of the degree of instrumental leakage present in Stokes Q
and U, expressed as a fraction of Stokes I. If a leakage correction
has been applied, this should be an estimate of the residual leakage
after correction. This information can be useful to assess the
significance of a detection (i.e., the risk of a reported RM being due
to instrumental leakage rather than the astrophysical source), as well
as the possible degree of systematic error introduced by leakage
(which is distinct from the random error which is usually reported for
quantities like Stokes Q and U or polarized intensity).
Note (4): RRM1d computed using v2 of the Galactic Faraday rotation sky at
https://wwwmpa.mpa-garching.mpg.de/~ensslin/research/data
/faraday2020.html .
Note (5): The centroid of the fitted radio source for the epoch of observation
1995±2.
Note (6): Were obtained from a hybrid template fitting and machine learning
approach from Duncan et al. 2021A&A...648A...4D 2021A&A...648A...4D, Cat. J/A+A/648/A4.
Note (7): Following a FWHM of the Gaussian beam model which describes the shape
of the synthesized beam at the reference frequency.
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History:
From electronic version of the journal,
https://github.com/CIRADA-Tools/RMTable?tab=readme-ov-file
(End) Luc Trabelsi [CDS] 19-Feb-2026