J/A+A/705/A143 POSSUM RM grid of Shapley Supercluster core (Alonso-Lopez+, 2026)
Magnetic fields in the Shapley Supercluster core with POSSUM:
Challenging model predictions.
Alonso-Lopez D., O'Sullivan S.P., Bonafede A., Boess L.M., Stuardi C.,
Osinga E., Anderson C.S., Van Eck C.L., Carretti E., West J.L., Akahori T.,
Dolag K., Giacintucci S., Khadir A., Ma Y.K., Malik S.,
McClure-Griffiths N., Rudnick L., Seidel B.A., Tiwari S., Venturi T.
<Astron. Astrophys. 705, A143 (2026)>
=2026A&A...705A.143A 2026A&A...705A.143A (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, galaxy ; Polarization ; Magnetic fields ; Radio sources
Keywords: magnetic fields - polarization - galaxies: clusters: general -
galaxies: clusters: intracluster medium - galaxies: groups: general
Abstract:
Faraday rotation measure (RM) grids provide a sensitive means to trace
magnetized plasma across a wide range of cosmic environments.
We study the RM signal from the Shapley Supercluster core (SSC) in
order to constrain the magnetic field properties of its gas. The SSC
region consists of two galaxy clusters, A3558 and A3562, and two
galaxy groups between them, at z≃0.048.
We combined RM Grid data with thermal Sunyaev-Zeldovich effect data,
obtained from the POlarisation Sky Survey of the Universe's Magnetism
(POSSUM) pilot survey, and Planck, respectively. To robustly determine
the gas density, its magnetic field properties, and their correlation
|B|∝neeta, we studied the RM scatter in the SSC region
(GRM) and its behavior as a function of distance to the nearest
cluster and/or group (dnrst). We compared observational results with
semi-analytic Gaussian random field models and more realistic
cosmological magnetohydrodynamical (MHD) simulations.
With a sky-density of 36RMs/deg2, we detect an excess RM scatter of
30.5±4.6rad/m2 in the SSC region. When we compare with models, we
find an average magnetic field strength of ∼1-3 uG (in the groups and
clusters). The GRM(dnrst) profile, derived from data ranging from
∼0.3-1.8r500 for all objects, is systematically flatter than expected
compared to the models, with eta<0.5 being favored. Despite this
discrepancy, we find that cosmological MHD simulations matched to the
SSC structure most closely align with scenarios where the magnetic
field is amplified by the turbulent velocity (vturb) in the
intercluster regions BF∝ne1/2*vturb on scales
dnrst≲0.8.
The dense RM grid and precision provided by POSSUM allows us to probe
magnetized gas in the SSC clusters and groups on scales within and
beyond their r500. Flatter-than-expected RM scatter profiles reveal a
significant challenge in reconciling observations with even the most
realistic predictions from cosmological MHD simulations in the
outskirts of interacting clusters.
Description:
In this work, we study the magnetic field properties of the Shapley
Supercluster Core (SSC) with ASKAP-POSSUM data from the POSSUM Pilot
II survey.
We focus on the potential of these dense RM grids to estimate the
magnetic properties of the gas in the intercluster bridge region
between the clusters A3558 and A3562.
This catalog contains Rotation Measures (RM) derived from Band 1
(800-1088MHz) and Band 2 (1296-1440MHz) ASKAP-POSSUM Pilot II survey
observations of two fields: 'core' and 'south', of the SSC. This
catalog contains information about each background polarised source
from which an RM was derived by means of the RM synthesis technique
using the RM-Tools software.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
rm-ssc.dat 398 149 Polarised source catalog (RM Grid)
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See also:
J/AJ/110/463 : Kinematics of the Shapley supercluster (Quintana+ 1995)
J/AJ/130/2541 : Core of the Shapley supercluster at 1.4GHz (Miller+, 2005)
J/A+A/445/819 : Systems of galaxies in Shapley supercluster (Ragone+, 2006)
J/MNRAS/381/1035 : Spectroscopy in Shapley supercluster (Smith+, 2007)
J/A+A/638/A27 : Redshift database towards Shapley Supercluster
(Quintana+, 2020)
Byte-by-byte Description of file: rm-ssc.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 23 A23 --- SBID SBID identification
(idnumbersourcefinder)
25- 38 A14 --- Name Unique source identification,
JHHMMSS+DDMMSS (component_name)
40- 49 F10.6 deg RAdeg Right ascension (J2000) (ra)
51- 60 F10.6 deg DEdeg Declination (J2000) (dec)
62- 82 E21.17 deg e_RAdeg ?=0 Right ascension error (err_ra)
84-104 E21.17 deg e_DEdeg ?=0 Declination error (err_dec)
106-125 E20.18 Jy/beam StokesI Stokes I (stokesI)
127-138 E12.7 Jy/beam e_StokesI ? Stokes I error (err_stokesI)
140-160 F21.17 rad/m2 RM Rotation Measure (RM)
162-181 F20.17 rad/m2 e_RM Rotation Measure error (err_RM)
183-203 E21.19 Jy/beam PolInt Polarised intensity (polint)
205-225 E21.17 Jy/beam e_PolInt Polarised intensity error (err_polint)
227-246 F20.16 --- SNR-PI Signal to noise ratio in polarisation
(SNR_PI)
248-267 F20.16 deg PolAngle Polarisation angle (polangle)
269-287 F19.17 deg e_PolAngle Polarisation angle error (err_polangle)
289-308 F20.16 deg DerotPolAngle Polarisation angle (intrinsic)
(derot_polangle)
310-329 F20.17 deg e_DerotPolAngle Polarisation angle (intrinsic) error
(errderotpolangle)
331-350 F20.18 --- fracPol Fractional polarisation (fracpol)
352 I1 --- Band [1/2] ASKAP-POSSUM Band 1/Band 2
(askappossumBand) (1)
354-374 F21.17 rad/m2 RRM Residual Rotation Measure (RRM)
376-394 F19.16 rad/m2 e_RRM Residual Rotation Measure error
(err_RRM)
396-398 A3 --- Subsample Sub-sample of sources: on/off target
(subsample)
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Note (1): Bands as follows:
1 = Band 1 (800-1088MHz)
2 = Band 2 (1296-1440MHz)
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Acknowledgements:
David Alonso-Lopez, dalons07(at)ucm.es
(End) Patricia Vannier [CDS] 15-Jan-2026