J/A+A/633/A150 Galactic Faraday sky (Hutschenreuter+, 2020)
The Galactic Faraday depth sky revisited.
Hutschenreuter S., Ensslin T.A.
<Astron. Astrophys. 633, A150 (2020)>
=2020A&A...633A.150H 2020A&A...633A.150H (SIMBAD/NED BibCode)
ADC_Keywords: Combined data ; Milky Way ; Magnetic fields
Keywords: ISM: magnetic fields - ISM: structure
Abstract:
The Galactic Faraday depth sky is a tracer for both the Galactic
magnetic field and the thermal electron distribution. It was
previously reconstructed from polarimetric measurements of
extra-Galactic point sources. Here we improve on these works by using
an updated inference algorithm and by taking into account the electron
emission measure as traced by free-free emission measured by the
Planck survey. In the future the data situation will improve
drastically thanks to the next generation Faraday rotation
measurements from the SKA and its pathfinders. Anticipating this, a
further aim of this paper is to update the map reconstruction method
with some of the latest developments in Bayesian imaging.
To this end we made use of information field theory, an inference
scheme that is particularly powerful in cases of noisy and incomplete
data. We demonstrate the validity of the new algorithm by applying it
to an existing data compilation. Even though we used exactly the same
data set, a number of novel findings are made; for example, a
non-parametric reconstruction of an overall amplitude field resembles
the free-free emission measure map of the Galaxy. Folding this
emission measure map into the analysis provides more detailed
predictions. The joint inference enables us to identify regions with
deviations from the assumed correlations between the emission measure
and Faraday data, thereby pointing us to Galactic structures with
distinguishably different physics. We find evidence for an alignment
of the magnetic field within the lines of sight along both directions
of the Orion arm.
Description:
Posterior mean and uncertainties of the Galactic Faraday depth sky and
the underlying components of the respective models are presented.
The original data was obtained combining the NVSS rotation measure
catalog of Taylor et al. (2009, Cat. J/ApJ/702/1230) with several
other catalogs of Faraday rotation data of polarized extragalactic
radio sources, increasing the spatial coverage and additionally
constraining the signal also in regions where several data sets
overlap.
Two versions are given, one is additionally constrained by Galactic
free-free emission (faradayskyw_ff.fits, componentswff.fits) while
the other (faradayskywo_ff.fits, componentswoff.fits) is not. Both
mean and uncertainties calculated from 50 samples of a variationally
approximated posterior. For further details we refer the reader to
Knollmueller et al. (2019, 2019arXiv190111033K 2019arXiv190111033K). The results were
inferred using the NIFTy package (The NIFTy Team (2019,
https://gitlab.mpcdf.mpg.de/ift/nifty)). The RM data this inference is
based on and the corresponding noise correction factors are available
at https://wwwmpa.mpa-garching.mpg.de/~ensslin/research/
data/faraday_revisited.html
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 156 4 List of fits files
fits/* . 4 Individual fits files
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See also:
J/ApJ/702/1230 : Rotation measure image of the sky (Taylor+, 2009)
Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- Nx Number of parameters
4- 9 I6 --- Ny Number of lines
11- 15 I5 Kibyte size Size of FITS file
17- 38 A22 --- FileName Name of FITS file, in subdirectory fits
40-156 A117 --- Title Title of the FITS file
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Acknowledgements:
Sebastian Hutschenreuter, hutsch(at)mpa-garching.mpg.de
References:
Knollmueller & Ensslin, 2019arXiv190111033K 2019arXiv190111033K,
Metric Gaussian Variational Inference
The NIFTy5 team, ascl:1903.008,
https://gitlab.mpcdf.mpg.de/ift/nifty
(End) Sebastian Hutschenreuter [MPA, Germany] Patricia Vannier [CDS] 17-Oct-2019