J/A+A/649/A94 A universal pattern in halo magnetic fields (Myserlis+, 2021)
An underlying universal pattern in galaxy halo magnetic fields.
Myserlis I., Contopoulos I.
<Astron. Astrophys. 649, A94 (2021)>
=2021A&A...649A..94M 2021A&A...649A..94M (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Galaxies, radio ; Magnetic fields ;
Polarization ; Radio continuum ; Radio sources ; VLBI
Keywords: galaxies: halos - galaxies: magnetic fields -
radio continuum: galaxies - magnetic fields - polarization -
radiative transfer
Abstract:
Magnetic fields in galaxy halos are in general very difficult to
observe. Most recently, the Continuum HAlos in Nearby Galaxies - an
EVLA Survey (CHANG-ES) collaboration investigated the radio halos of
35 nearby edge-on spiral galaxies in detail and detected large-scale
magnetic fields in 16 of them. We used the CHANG-ES radio polarization
data to create rotation measure (RM) maps for all galaxies in the
sample and stack them with the aim of amplifying any underlying
universal toroidal magnetic field pattern in the halo above and below
the disk of the galaxy. We discovered a large-scale magnetic field in
the central region of the stacked galaxy profile, which is
attributable to an axial electric current that universally outflows
from the center, both above and below the plane of the disk. A similar
symmetry-breaking has also been observed in astrophysical jets, but
never before in galaxy halos. This is an indication that galaxy halo
magnetic fields are probably not generated by pure magnetohydrodynamic
(MHD) processes in the central regions of galaxies. One such promising
physical mechanism is the Cosmic Battery operating in the innermost
accretion disk around the central supermassive black hole. We
anticipate that our discovery will stimulate a more general discussion
on the origin of astrophysical magnetic fields.
Description:
This data set contains the raw data of the RM maps shown in Appendix A
of the manuscript. We provide the RM values we calculated for various
positions within each galaxy of the sample. These data can be used to
reproduce the RM maps as well as all other results and figures in the
manuscript.
The RM values were calculated using the the publicly available
CHANG-ES data (Wiegert et al., 2015AJ....150...81W 2015AJ....150...81W, Cat. J/AJ/150/81).
In particular, we made use of the polarization angle (EVPA) maps at
both the L and C bands, centered at 1.5GHz and 6GHz, respectively,
to calculate the RM between them. The corresponding FITS files were
obtained from the CHANG-ES data release website:
https://www.queensu.ca/changes
We used the images made with uniform UV-weighting (robust=0) and
corrected for the primary beam, labeled "Rob 0" and "PBcor" on
the data release website, respectively. Nevertheless, we repeated our
analysis with the gaussian UV-tapered version of the maps (labeled
"UVtap") and/or the non-primary beam-corrected maps ("no PBcor") and
found that the RM results do not change significantly.
To construct the RM maps, we compared the EVPA images in the L and C
bands (hereafter L-map and C-map) for each galaxy in the sample on a
pixel-by-pixel basis. Given the different pixel size (the L-maps have
2-5 times larger pixels than the C-maps), we first re-sampled the
C-maps at the resolution of the L-maps, for each galaxy. For each
pixel in the L-map for which the EVPA value was not flagged, we
selected all non-flagged pixels in the C-map that fit into its area
and calculated their average EVPA value. To avoid errors from
averaging EVPA values, we calculate the Stokes Q and U parameters that
correspond to the selected C-map pixels using the linear polarization
and polarization angle (EVPA) C-band maps. We found that this method
delivers more stable results between neighboring pixels than averaging
the EVPA measurements directly.
The result of the previous process is two maps for each source, both
at the resolution of its L-map, with each applicable pixel having a
pair of EVPA measurements at 1.5GHz and 6GHz, respectively, which
can be used to obtain an RM value, based on Eq. (5) of the manuscript.
As with any other polarization angle measurement, both EVPAs at the L
and C bands have an ambiguity of 180 degrees. This means that, in
principle, RM in Eq. (5) can have an infinite number of values
obtained by adding or subtracting integer values of 180 degrees from
either of the EVPA measurements. For consistency, we decided to
consider the value of RM in Eq. (5) that is minimum in absolute terms.
For the minimization, we used the C-band EVPA as pivot, since the
higher frequency C-band data are less affected by Faraday rotation.
We note that the limited frequency coverage of the input data, as well
as the RM minimization method described above, restrict the RM
calculation to a maximum absolute value of about 50rad/m2.
Therefore, our RM results may be underestimated, especially in regions
with high electron density or magnetic field strength. Nevertheless,
we decided to follow the RM minimization approach to avoid extreme
differences between neighboring pixels in the RM maps of individual
galaxies. Our results suggest that the RM minimization can mitigate
(extreme) differences in the range of 75-105rad/m2 between
neighboring pixels by about 60%. Nevertheless, we note that the
minimization was implemented only in about 16% of all pixels and we
found that our results do not change significantly even if these
pixels are completely excluded from the analysis.
Finally, the RM maps were corrected for the contribution from the
Galactic Faraday depth using the estimate of Figure 15 in Oppermann et
al. (2015A&A...575A.118O 2015A&A...575A.118O). The Galactic RM contribution was calculated
as the average of several pixels around the location of each galaxy in
the "Galactic foreground" map obtained from the publication
website: https://wwwmpa.mpa-garching.mpg.de/ift/faraday/
2014/index.html The resulting Galactic RM values were subtracted from
the corresponding RM maps of all galaxies that we analyzed, except NGC
2613, where the Galactic RM was found to be about 174 rad/m^2, which
is outside the range that can be probed using the RM calculation
methodology described above. We would also like to note that
polarization angles have been corrected for ionospheric effects by the
CHANG-ES team (Wiegert et al., 2015AJ....150...81W 2015AJ....150...81W, Cat. J/AJ/150/81).
More details of the analysis can be found in Section 2 of the
manuscript.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
figa1.dat 40 3568 Raw RM data of Fig. A1 of the Appendix
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See also:
J/AJ/150/81 : CHANG-ES. IV. VLA D-configuration observations (Wiegert+, 2015)
Byte-by-byte Description of file: figa1.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- Name Galaxy name
10- 11 I2 h RAh Right ascension (J2000) (1)
13- 14 I2 min RAm Right ascension (J2000) (1)
16- 20 F5.2 s RAs Right ascension (J2000) (1)
23 A1 --- DE- Declination sign (J2000) (1)
24- 25 I2 deg DEd Declination (J2000) (1)
27- 28 I2 arcmin DEm Declination (J2000) (1)
30- 33 F4.1 arcsec DEs Declination (J2000) (1)
36- 40 F5.1 rad/m+2 RM Rotation measure
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Note (1): Position of the bin for the RM value.
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Acknowledgements:
Ioannis Myserlis, imyserlis(at)iram.es
(End) Ioannis Myserlis [IRAM, Spain], Patricia Vannier [CDS] 15-Apr-2021