J/A+A/688/A200 Faraday tomography of LoTSS-DR2 - inner Galaxy (Erceg+ 2024)
Faraday tomography of LoTSS-DR2 data. III. Revealing the Local Bubble and the
complex of local interstellar clouds in the high-latitude inner Galaxy.
Erceg A., Jelic V., Haverkorn M., Gajovic L., Hardcastle M., Shimwell T.W.,
Tasse, C.
<Astron. Astrophys. 688, A200 (2024)>
=2024A&A...688A.200E 2024A&A...688A.200E (SIMBAD/NED BibCode)
ADC_Keywords: Interstellar medium ; Magnetic fields ; Polarization ;
Radio continuum
Keywords: techniques: interferometric - techniques: polarimetric -
ISM: general - ISM: magnetic fields - ISM: structure -
local interstellar matter
Abstract:
The LOw-Frequency ARray (LOFAR) provides a unique opportunity to probe
the magneto-ionised structure of our Galactic neighbourhood with great
resolution. In this work, we present a new mosaic created with the
second release of LOFAR Two-Metre Sky Survey data (LoTSS-DR2), which
probes polarised synchrotron emission in the high-latitude inner
Galaxy. This is the third paper in a series whose main goal is
understanding the LOFAR Faraday tomographic data at low radio
frequencies and utilising it to explore the intricate structure of the
local interstellar medium (ISM).
Our objective is to characterise the observed emission through
multi-tracer analysis to better understand the volume and the
structures that may be observed with LOFAR. Furthermore, we exploit
Faraday depth as a unique tool to probe the diffuse magnetised
structure in the local ISM.
We produce a mosaic Faraday cube of LoTSS-DR2 data by applying a
rotation measure synthesis algorithm. From the cube, we construct
Faraday moment maps to characterise the nature of spectra.
Additionally, we quantify the linear depolarisation canals using the
Rolling Hough transform and use them to search for alignment with
other data sets. Utilising LoTSS-DR2 observations alongside
complementary data sets including Planck polarisation data, HI
emission maps and starlight polarisation measurements, we explore
conditions along observed lines of sight and estimate the distance to
the Faraday structures.
The Faraday cube reveals a remarkably ordered structure across
two-thirds of the observed area, whose orientation aligns well with
that of both the HI filaments and the magnetic field. We estimate the
minimum distance to the Faraday structures to be between 40 and 80 pc,
which puts them in the vicinity of the Local Bubble wall. The emission
is organised in a large gradient in Faraday depth whose origin we
associate with the curved wall of the Local Bubble.
Comparing our data with a model of the Local Bubble wall, we conclude
that we might be probing a contribution of the medium inside the Local
Bubble cavity as well, corresponding to the complex of local
interstellar clouds. Moreover, we propose a toy model incorporating an
ionised front of finite thickness into the Local Bubble wall, as a
curved, cold neutral shell alone is insufficient to produce the
observed gradient. We explore possible magnetic field strengths, as
well as the possible distribution of the neutral and ionised medium
inside the wall, within the constraints of the observed Faraday depth.
Description:
Data products derived from a Faraday depth cube of the LoTSS-DR2 inner
Galaxy observations, including the maximum polarised intensity image
and images of statistical moments (M0, M1 and m2). The images cover
around 1400 square degrees, centred around 0h +28°. Angular
resolution of this dataset is 5.5 arcmin. The maximum polarised
intensity image was created by taking the highest intensity in each
spectrum in the Faraday cube. We searched for a peak in the range from
-50 to 50 rad/m^2. The moments were computed using equations found in
Section 3.3. of the paper. Faraday resolution of the cube used to
derive this data is 1 rad/m2 and the cube can be found at:
https://data.fulir.irb.hr/islandora/object/irb:405 .
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 98 4 List of fits files
fits/* . 4 Individual fits files
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See also:
J/A+A/687/A23 : Faraday tomography of LoTSS-DR2 - outer Galaxy (Erceg+ 2024)
Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 30 F11.9 deg/pix scale Scale of the image
32- 35 I4 --- Nx Number of pixels along X-axis
37- 40 I4 --- Ny Number of pixels along Y-axis
42- 44 I3 rad/m2 FDmin ? Lower value of Faraday depth range
46- 47 I2 rad/m2 FDmax ? Upper value of Faraday depth range
49- 53 I5 Kibyte size Size of FITS file
55- 65 A11 --- FileName Name of FITS file, in subdirectory fits
67- 98 A32 --- Title Title of the FITS file (1)
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Note (1): Intensity is given in units of Jy/PSF/RMSF, where PSF is Point Spread
Function and RMSF is Rotation Measure Spread Function.
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Acknowledgements:
Ana Erceg, aerceg(at)irb.hr
Vibor Jelic, vibor(at)irb.hr
References:
Erceg et al., Paper I 2022A&A...663A...7E 2022A&A...663A...7E
Erceg et al., Paper II 2024A&A...687A..23E 2024A&A...687A..23E, Cat. J/A+A/687/A23
(End) Patricia Vannier [CDS] 16-Jul-2024