Analysis of spiral arms using anisotropic wavelets: gas, dust and magnetic fields in M 51

¹ Institute of Continuous Media Mechanics, Korolyov str. 1, 614061 Perm, Russia
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ School of Mathematics and Statistics, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK e-mail: andrew.fletcher@ncl.ac.uk
⁴ Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden

Received: 17 March 2006
Accepted: 1 August 2006

Abstract

Context.The origin of the spiral pattern of magnetic fields in disc galaxies is an open question.

Aims.Comparison of the regular magnetic field orientation with the gaseous spiral arm pitch angles can tell us whether spiral shock compression is responsible for the magnetic spirals. We also wish to see whether the ridges of different components of the ISM show the large-scale, systematic shifts expected from density wave theory.

Methods.We have developed a technique of isolating elongated structures in galactic images, such as spiral arms, using anisotropic wavelets and apply this to maps of the CO, infrared and radio continuum emission of the grand-design spiral galaxy M 51.

Results.Systematic shifts between the ridges of CO, infrared and radio continuum emission that are several long are identified, as well as large variations in pitch angle along spiral arms, of a few tens of degrees. We find two types of arms of polarized radio emission: one has a ridge close to the ridge of CO, with similar pitch angles for the CO and polarization spirals and the regular magnetic field; the other does not always coincide with the CO arm and its pitch angle differs from the orientation of its regular magnetic field.

Conclusions.The offsets between ridges of regular magnetic field, dense gas and warm dust are compatible with the sequence expected from spiral density wave triggered star formation, with a delay of a few tens of millions of years between gas entering the shock and the formation of giant molecular clouds and a similar interval between the formation of the clouds and the emergence of young star clusters. At the position of the CO arms the orientation of the regular magnetic field is the same as the pitch angle of the spiral arm, but away from the gaseous arms the orientation of the regular field varies significantly. Spiral shock compression can explain the generation of one type of arm of strong polarized radio emission but a different mechanism is probably responsible for a second type of polarization arm.