J/MNRAS/508/3921 Study of Fararday RM in Magellanic clouds (Jung+, 2021)
Distant probes of rotation measure structure: where is the Faraday rotation
towards the Magellanic Leading Arm?
Jung S.L., Mcclure-griffiths N.M., Hill A.S.
<Mon. Not. R. Astron. Soc. 508, 3921-3935 (2021)>
=2021MNRAS.508.3921J 2021MNRAS.508.3921J (SIMBAD/NED BibCode)
ADC_Keywords: Magellanic Clouds ; Diffuse clouds ; Milky Way ; Radio sources ;
Supernova remnants ; Magnetic fields ; Positional data ;
Photometry
Keywords: polarization - ISM: clouds - ISM: magnetic fields -
ISM: supernova remnants
Abstract:
Faraday rotation measures (RMs) should be interpreted with caution
because there could be multiple magneto-ionized medium components that
contribute to the net Faraday rotation along sightlines. We introduce
a simple test using Galactic diffuse polarized emission that evaluates
whether structures evident in RM observations are associated with
distant circumgalactic medium or foreground interstellar medium. We
focus on the Magellanic Leading Arm region where a clear excess of RM
was previously reported. There are two gaseous objects standing out in
this direction: the distant Magellanic Leading Arm and the nearby
Antlia supernova remnant (SNR). We recognized narrow depolarized
filaments in the 2.3 GHz S-band Polarization All Sky Survey image that
overlaps with the reported RM excess. We suggest that there is a steep
gradient in Faraday rotation in a foreground screen arising from the
Antlia SNR. The estimated strength of the line-of-sight component of
the magnetic field is B||∼5µG, assuming that the excess of RM is
entirely an outcome of the magnetized supernova shell. Our analysis
indicates that the overlap between the RM excess and the Magellanic
Leading Arm is only a remarkable coincidence. We suggest for future RM
grid studies that checking Galactic diffuse polarization maps is a
convenient way to identify local Faraday screens.
Description:
There have been attempts to observationally constrain the magnetic
field strength associated with the Miky Way high-velocity clouds using
the RM grid (McClure-Griffiths et al. 2010ApJ...725..275M 2010ApJ...725..275M; Hill et al.
2013ApJ...777...55H 2013ApJ...777...55H; Kaczmarek et al. 2017MNRAS.467.1776K 2017MNRAS.467.1776K, Cat.
J/MNRAS/467/1776; Betti et al. 2019ApJ...871..215B 2019ApJ...871..215B, Cat.
J/ApJ/871/215). In this paper, we present a high-density RM grid
towards the Magellanic Leading Arm (LA)/Antlia supernova remnant (SNR)
regions. We combine it with the Galactic diffuse polarized emission in
order to test whether the Magellanic HVC or the Antlia SNR is the
source of Faraday rotation.
For constructing the RM grid covering our field of interest, we use
two publicly available RM catalogues in addition to new radio
continuum observations with the Australia Telescope Compact Array
(ATCA). First, we have a RM catalogue from the National Radio
Astronomy Observatory Very Large Array Sky Survey ((NVSS; Condon et
al. 1998AJ....115.1693C 1998AJ....115.1693C, Cat. VIII/65) RM catalogue (Taylor et al.
2009ApJ...702.1230T 2009ApJ...702.1230T, Cat. J/ApJ/702/1230) that covers the sky above a
declination of -40 degrees with the RM source density of 1 deg-2 on
average. Although it has a significant role in the search for
magnetised HVCs (McG10; Hill et al. 2013ApJ...777...55H 2013ApJ...777...55H) it should be
noted that the RMs are derived from limited coverage in frequency
domain 42 MHz-wide bands centred at 1364.9 and 1465.1 MHz. However,
tests has shown that the NVSS RM catalogue is mostly reliable for
sources located out of the Galactic plane and in the φ range used
for this study.
For the southern sky below a declination of -1 degree, there is the
S-band Polarization All Sky Survey (S-PASS; Carretti et al.
2019MNRAS.489.2330C 2019MNRAS.489.2330C). S-PASS/ATCA (Schnitzeler et al.
2019MNRAS.485.1293S 2019MNRAS.485.1293S) catalogue is derived from a follow-up
observations of sources selected from S-PASS using ATCA. It provides
the first wide-band (1.3-3.1 GHz) polarimetry data of compact sources
with the average polarized source number density of 0.2 deg-2. In
order to increase the polarized source density, we performed follow-up
observations on 737 fields in the region 10h 00min 00s < α < 13h
30min 00s and -52.3 degrees < δ < -32 degrees with ATCA. The
frequency range used for our analysis is 2 GHz-wide continuum band
from 1.1 to 3.1 GHz, and the and the spectral resolution is ∼ 1 MHz.
The total number of sources detected in the field is about 3000,
including both polarized and unpolarized sources.
Hereafter, we used the miriad software package (Sault et al.
1995ASPC...77..433S 1995ASPC...77..433S) provided by Australia Telescope National Facility
for data reduction and imaging. The software is particularly designed
for processing radio interferometry data observed with ATCA. For the
catalogue of RM sources used for our analysis, we adopted thresholds
as explained in the section 2.1.2 ATCA observations. After analysis,
210 sources that match the above criteria are presented in the
tablea1.dat inluding positional data, flux densities and Faraday depth.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
tablea1.dat 75 210 Our sample of polarized sources observed using
ATCA matching our criteria
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See also:
J/MNRAS/467/1776 : Faraday rotation in Magellanic Bridge (Kaczmarek+, 2017)
J/ApJ/871/215 : JVLA rotation measures of Smith cloud bckg sources
(Betti+, 2019)
VIII/65 : 1.4GHz NRAO VLA Sky Survey (NVSS) (Condon+ 1998)
J/ApJ/702/1230 : Rotation measure image of the sky (Taylor+, 2009)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Right ascension (J2000)
4- 5 I2 min RAm Right ascension (J2000)
7- 10 F4.1 s RAs Right ascension (J2000)
12 A1 --- DE- Sign of declination (J2000)
13- 14 I2 deg DEd Declination (J2000)
16- 17 I2 arcmin DEm Declination (J2000)
19- 22 F4.1 arcsec DEs Declination (J2000)
24- 31 F8.4 deg GLON Galactic longitude (l)
33- 39 F7.4 deg GLAT Galactic latitude (b)
41- 45 F5.2 Jy/beam SI Flux density per beam area in the S-band
polarized in Stokes I (SI) (1)
47- 50 F4.2 Jy/beam e_SI Mean error on SI (SIerr)
52- 55 F4.2 Jy/beam p Polarized surface brightness
per beam area (p) (2)
57- 61 F5.3 Jy/beam e_p Mean error on p (perr)
63- 69 F7.2 rad/m2 phi The Faraday depth φ (φ) (3)
71- 75 F5.2 rad/m2 e_phi Mean error on φ (φerr)
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Note (1): For S-band, the frequency range used for our analysis is 2 GHz-wide
continuum band from 1.1 to 3.1 GHz and the spectral resolution is
1 MHz (More details in the section 2.1.2 ATCA observations).
Also refers to the section 2.2 S-PASS Galactic diffuse polarization
to understand the observational wave-band and polarization channel
choices.
Note (2): As explained in the section 2.1.2 ATCA observations, the idea of RM
synthesis is to bring the complex polarized surface brightness p
defined in the equation (4) using Stokes Q and U, to the Faraday
depth φ domain so that one can interpret the changes in φ
along the line of sight.
Note (3): The Faraday depth φ is a parameter that describes the Faraday
rotation measure RM at individual Faraday screens (Burn
1966MNRAS.133...67B 1966MNRAS.133...67B) as the equation (1) and (2) in the section 1
Introduction.
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History:
From electronic version of the journal
(End) Luc Trabelsi [CDS] 22-Aug-2024