J/A+A/622/A209 B2 0258+35 polarisation images (Adebahr+, 2019)
Polarised structures in the radio lobes of B2 0258+35.
Evidence of magnetic draping?
Adebahr B., Brienza M., Morganti R.
<Astron. Astrophys. 622, A209 (2019)>
=2019A&A...622A.209A 2019A&A...622A.209A (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, radio ; Polarization ; Magnetic fields
Keywords: galaxies: magnetic fields - polarization - galaxies: jets -
galaxies: active - galaxies: individual: NGC 1167 -
galaxies: individual: B2 0258+35
Abstract:
The contribution of active galactic nuclei to the magnetisation of the
Universe can be constrained by knowing their duty cycles, jet and
magnetic field morphologies, and the physical processes dominating
their interaction with the surrounding environment.
The magnetic field morphology and strength of radio lobes of AGN has
an influence on the mechanisms for the propagation of cosmic rays into
intergalactic space. Using the source B2 0258+35 we want to
investigate the interaction of its radio lobes with the surrounding
environment and examine the underlying physical effects.
Published HI and radio continuum data at λ21cm were combined
with newly reduced archival Westerbork Radio Synthesis Telescope
polarisation data at the same wavelength to investigate the polarised
emission in the radio lobes of B2 0258+35. We assumed energy
equipartition between the cosmic rays and the magnetic field to
calculate their pressure and investigate the physical processes
leading to the detected emission.
We detected a unique S-shaped diffuse polarised structure. The lobes
have a pressure of p=1.95±0.4x10-14dyn/cm2. The calculated total
magnetic field strengths are low (Beq=1.21±0.12uG). We observe
depolarisation in the northern lobe, which might originate from the
HI-disc in the foreground. In addition we see an anti-correlation
between the pressure and the fractional polarisation along the
S-shaped structure. Therefore we consider magnetic draping and
magnetic field compression as possible effects that might have created
the observed S-shape.
Our results suggest that magnetic draping can be effectively used to
explain the observed polarised structures. This is likely due to the
combination of a relatively low magnetic field strength, enabling
super-Alfvenic motion of the rising lobes (with MA=2.47-3.50), and
the coherency of the surrounding magnetic field. Moreover, the draped
layer tends to suppress any mixing of the material between the radio
lobes and the surrounding environment, but can enhance the mixing and
re-acceleration efficiencies inside the lobes, providing an
explanation for the average flat spectral index observed in the lobes.
Description:
The polarisation study presented in this paper is based on unpublished
data of the radio galaxy B2 0258+35 from the WSRT archive. The data
consist of two full 12 h tracks pointing at RA: 03:02:17.8,
Dec: +35:14:36. The observations were undertaken between 4 and 6
November 2006. Each dataset was recorded over the full bandwidth of
160 MHz consisting of eight 20MHz sub-bands with 64 channels each and
with centre frequencies equal to 1451, 1433, 1411, 1393, 1371, 1351,
1331, and 1311 MHz. All four linear correlations (XX, XY, YX, YY) were
recorded, so that the data contained full polarisation information.
Sources 3C48 and 3C147 were used as calibrators and were observed
before and after the target field in each run, respectively.
Objects:
--------------------------------------------------
RA (2000) DE Designation(s)
--------------------------------------------------
03 01 42.36 +35 12 20.3 B2 0258+35 = NGC 1167
--------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 114 14 List of fits images
fits/* . 14 Individual fits images
--------------------------------------------------------------------------------
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 48 A21 "datime" Obs.date Observaiton date
50- 53 I4 Kibyte size Size of FITS file
55- 77 A23 --- FileName Name of FITS file, in subdirectory fits
79-114 A36 --- Title Title of the FITS file
--------------------------------------------------------------------------------
Acknowledgements:
Bjorn Adebahr, adebahr(at)astro.rub.de
(End) Patricia Vannier [CDS] 26-Feb-2019