The magnetic fields of large Virgo Cluster spirals^*

¹ Obserwatorium Astronomiczne Uniwersytetu Jagiellońskiego, ul. Orla 171, 30-244 Kraków, Poland e-mail: markmet@oa.uj.edu.pl
² CDS, Observatoire Astronomique de Strasbourg, UMR 7550, 11 rue de l'université, 67000 Strasbourg, France
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Observatoire de Paris, GEPI, CNRS, and Université Paris 7, 5 Place Jules Janssen, 92195 Meudon Cedex, France

Received: 19 December 2006
Accepted: 28 April 2007

Abstract

Context.Because of its proximity the Virgo Cluster is an excellent target for studying interactions of galaxies with the cluster environment. Both the high-velocity tidal interactions and effects of ram pressure stripping by the intracluster gas can be investigated.

Aims.Optical and/or observations do not always show the effects of weak interactions between galaxies and their encounters with the cluster medium. For this reason we searched for possible anomalies in the magnetic field structure in Virgo Cluster spirals that could be attributed to perturbations in their gas distribution and kinematics.

Methods.Five angularly large Virgo Cluster spiral galaxies (NGC 4501, NGC 4438, NGC 4535, NGC 4548, and NGC 4654) were the targets of a sensitive total power and polarization study using the 100-m radio telescope in Effelsberg at 4.85 GHz. For two objects, polarization data at higher frequencies were obtained allowing Faraday rotation analysis.

Results.Distorted magnetic field structures were identified in all galaxies. Interaction-induced magnetized outflows were found in NGC 4438 (due to nuclear activity) and NGC 4654 (a combination of tidal tails and ram pressure effects). Almost all objects (except the anaemic NGC 4548), exhibit distortions in polarized radio continuum attributable to the influence of the ambient gas. For some galaxies they agree with observations of other species, but the magnetic field is sometimes (NGC 4535) the only tracer of the interaction with the cluster environment.

Conclusions.The cluster environment clearly affects the evolution of the galaxies due to ram pressure and tidal effects. Magnetic fields provide a very long lasting memory of past interactions. Therefore, they are a good tracer of weak interactions that are difficult to detect by other observations. Information about motions of galaxies in the sky plane and their three-dimensional distribution can also be obtained.