J/MNRAS/497/3097 Faraday depth profile disparity in the MW mid-plane (Ma+, 2020)
The complex large-scale magnetic fields in the first Galactic quadrant as
revealed by the Faraday depth profile disparity.
Ma Y.K., Mao S.A., Ordog A., Brown J.C.
<Mon. Not. R. Astron. Soc., 497, 3097-3117 (2020)>
=2020MNRAS.497.3097M 2020MNRAS.497.3097M (SIMBAD/NED BibCode)
ADC_Keywords: Radio sources ; Polarization ; Magnetic fields ; Positional data
Keywords: ISM: magnetic fields - Galaxy: structure
Abstract:
The Milky Way is one of the very few spiral galaxies known to host
large-scale magnetic field reversals. The existence of the field
reversal in the first Galactic quadrant near the Sagittarius spiral
arm has been well established, yet poorly characterized due to the
insufficient number of reliable Faraday depths (FDs) from
extragalactic radio sources (EGSs) through this reversal region. We
have therefore performed broad-band (1-2GHz) spectropolarimetric
observations with the Karl G. Jansky Very Large Array (VLA) to
determine the FD values of 194 EGSs in the Galactic longitude range of
20°-52° within ±5° from the Galactic mid-plane, covering
the Sagittarius arm tangent. This factor of five increase in the EGS
FD density has led to the discovery of a disparity in FD values across
the Galactic mid-plane in the Galactic longitude range of
40°-52°. Combined with existing pulsar FD measurements, we
suggest that the Sagittarius arm can host an odd-parity disc field. We
further compared our newly derived EGS FDs with the predictions of
three major Galactic magnetic field models, and concluded that none of
them can adequately reproduce our observational results. This has led
to our development of new, improved models of the Milky Way disc
magnetic field that will serve as an important step towards major
future improvements in Galactic magnetic field models.
Description:
In this study, we focus on the large-scale magnetic field near the
Galactic mid-plane (|b|=<5°) in the Galactic longitude range of
20°=<l=<52°. The target extragalactic radio sources (EGSs)
were selected using two criteria. First, we chose sources from the
original NVSS catalogue (Condon et al. 1998AJ....115.1693C 1998AJ....115.1693C, Cat.
VIII/65) based on their listed polarization properties: debiased
polarized intensity (PI; Wardle & Kronberg 1974ApJ...194..249W 1974ApJ...194..249W) of
more than 4σ, and polarization fraction (p) of more than 0.5 per
cent. Secondly, we further included all EGSs in the Taylor et al.
(2009ApJ...702.1230T 2009ApJ...702.1230T, Cat. J/ApJ/702/1230) catalogue that were not
already selected as NVSS targets.
We performed new broad-band spectropolarimetric observations of the
176 on-axis targets using the VLA in L band (1-2GHz) in D-array
configuration under project code 18A-332. The project was divided into
seven observing blocks executed on 2018 September 1-10. In all
observing blocks, 3C 286 was observed and used as the absolute flux,
bandpass, and PA calibrators, while J1407+2827 was chosen as the
unpolarized on-axis leakage calibrator. Depending on the observing
block, either J1822-0938, J1859+1259, J1941+1026, or J1942+1026 was
used as the phase calibrator. The observations totalled 15h, with an
integration time of 3-5min on each on-axis target.
The Common Astronomy Software Applications (CASA) package (version
5.3.0; McMullin et al. 2007ASPC..376..127M 2007ASPC..376..127M) was used for all data
reduction procedures.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 95 171 RM-Synthesis results for on-axis targets
table2.dat 63 33 RM-Synthesis results for off-axis targets
tableb1.dat 64 171 Total Intensities and Spectral Indices of
On-axis Targets
tableb2.dat 58 33 Total Intensities and Spectral Indices of
Off-axis Targets
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See also:
VIII/65 : 1.4GHz NRAO VLA Sky Survey (NVSS) (Condon+ 1998)
J/ApJ/702/1230 : Rotation measure image of the sky (Taylor+, 2009)
J/ApJ/728/97 : VLA rotation measures in the Galactic plane (Van Eck+, 2011)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 A14 --- Name Source NVSS name (JHHMMSS-DDMMSS)
16- 17 A2 --- f_Name [*+ ] Flag on Name (1)
19- 23 F5.2 deg GLON Galactic longitude
25- 29 F5.2 deg GLAT Galactic latitude
31- 36 F6.1 rad/m2 FDnew ? Faraday depth from this work
38- 40 F3.1 rad/m2 e_FDnew ? Error on FDnew
42- 45 I4 rad/m2 FDVE11 ? Faraday depth from Van Eck et al.
(2011ApJ...728...97V 2011ApJ...728...97V, Cat. J/ApJ/728/97)
47- 48 I2 rad/m2 e_FDVE11 ? Error on FDVE11
50- 55 F6.1 rad/m2 FDTSS09 ? Faraday depth from Taylor et al.
(2009ApJ...702.1230T 2009ApJ...702.1230T, Cat. J/ApJ/702/1230)
57- 60 F4.1 rad/m2 e_FDTSS09 ? Error on TSS09
62- 66 F5.2 % pnew Polarization fraction from this work
68- 71 F4.2 % e_pnew ? Error on pnew
73 A1 --- f_pnew [*] Flag on pnew (2)
75- 79 F5.2 % pTSS09 ? Polarization fraction from Taylor et al.
(2009ApJ...702.1230T 2009ApJ...702.1230T, Cat. J/ApJ/702/1230)
81- 84 F4.2 % e_pTSS09 ? Error on pTSS09
86- 90 F5.2 % pNVSS ? Polarization fraction from Condon et al.
(1998AJ....115.1693C 1998AJ....115.1693C, Cat. VIII/65)
92- 95 F4.2 % e_pNVSS ? Error on pNVSS
--------------------------------------------------------------------------------
Note (1): Flag as follows:
* = Source situated behind the prominent HII structure G26.5
+ = Source suffering from nπ-ambiguity in Taylor et al.
(2009ApJ...702.1230T 2009ApJ...702.1230T, Cat. J/ApJ/702/1230)
Note (2): Flag as follows:
* = Unpolarized source in our new observations (i.e. pnew below our
6σ cut-off). We give the 1σ value in pnew for this source.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 A14 --- Name Source NVSS name (JHHMMSS-DDMMSS)
16 A1 --- f_Name [*] Flag on Name (1)
18- 22 F5.2 deg GLON Galactic longitude
24- 28 F5.2 deg GLAT Galactic latitude
30- 35 F6.1 rad/m2 FDnew ? Faraday depth from this work
37- 39 F3.1 rad/m2 e_FDnew ? Error on FDnew
41- 45 F5.2 % pnew Polarization fraction from this work
47- 50 F4.2 % e_pnew ? Error on pnew
52 A1 --- f_pnew [*] Flag on pnew (2)
54- 58 F5.2 % pNVSS ? Polarization fraction from Condon et al.
(1998AJ....115.1693C 1998AJ....115.1693C, Cat. VIII/65)
60- 63 F4.2 % e_pNVSS ? Error on pNVSS
--------------------------------------------------------------------------------
Note (1): Flag as follows:
* = Source situated behind the prominent HII structure G26.5
Note (2): Flag as follows:
* = Unpolarized source in our new observations (i.e. pnew below our
6σ cut-off). We give the 1σ value in pnew for this source.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tableb1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 A14 --- Name Source NVSS name (JHHMMSS-DDMMSS)
16 A1 --- f_Name [*] Flag on Name (1)
18- 22 F5.2 deg GLON Galactic longitude
24- 28 F5.2 deg GLAT Galactic latitude
30- 34 F5.2 --- alpha Spectral index
36- 39 F4.2 --- e_alpha Error on alpha
41- 46 F6.1 mJy S1.4 Flux density at 1.4GHz from our observations
48- 51 F4.1 mJy e_S1.4 Error on S1.4
53- 58 F6.1 mJy SNVSS Flux density at 1.4GHz from NVSS (Condon et al.
1998AJ....115.1693C 1998AJ....115.1693C, Cat. VIII/65)
60- 64 F5.1 mJy e_SNVSS Error on SNVSS
--------------------------------------------------------------------------------
Note (1): Flag as follows:
* = Spatially blended source
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tableb2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 A14 --- Name Source NVSS name (JHHMMSS-DDMMSS)
16- 20 F5.2 deg GLON Galactic longitude
22- 26 F5.2 deg GLAT Galactic latitude
28- 32 F5.2 --- alpha Spectral index
34- 37 F4.2 --- e_alpha Error on alpha
39- 43 F5.1 mJy S1.4 Flux density at 1.4GHz from our observations
45- 47 F3.1 mJy e_S1.4 Error on S1.4
49- 53 F5.1 mJy SNVSS Flux density at 1.4GHz from NVSS (Condon et al.
1998AJ....115.1693C 1998AJ....115.1693C, Cat. VIII/65)
55- 58 F4.1 mJy e_SNVSS Error on SNVSS
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 26-Jul-2023