Letter to the Editor

A2255: The first detection of filamentary polarized emission in a radio halo

¹ Istituto di Radioastronomia – CNR/INAF, via Gobetti 101, 40129 Bologna, Italy e-mail: fgovoni@ira.cnr.it
² Dipartimento di Astronomia, Univ. Bologna, Via Ranzani 1, 40127 Bologna, Italy
³ INAF - Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
⁴ National Radio Astronomy Observatory, Socorro, NM 87801, USA
⁵ Kavli Institute of Particle Astrophysics and Cosmology, Menlo Park, CA 94025, USA

Received: 4 November 2004
Accepted: 26 November 2004

Abstract

A deep radio observation of the A2255 cluster of galaxies has been carried out at 1.4 GHz with the Very Large Array synthesis telescope. Thanks to the excellent (u, v) coverage and sensitivity achieved by our observation, the low brightness diffuse extended sources in the cluster (radio halo and relic) have been imaged with unprecedented resolution and dynamic range. We find that the radio halo has filamentary structures that are strongly polarized. The fractional linear polarization reaches levels of 20–40% and the magnetic fields appear ordered on scales of ~400 kpc. This is the first successful attempt to detect polarized emission from a radio halo and provides strong evidence that in this cluster the magnetic field is ordered on large scales.