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Today's Colloquium: Observing nearby spiral galaxies with a new generation of radio telescopes

Submitter: David Mulcahy
Description: Magnetic fields are an essential part of the interstellar medium, both within our own Galaxy and in other external star-forming galaxies. In spiral galaxies these fields are thought to arise due to differential and helical turbulence: the so-called ''dynamo model''. They govern the propagation of relativistic cosmic rays, controlling their density and distribution, and dominate the energy budget in the extended galactic disk.

There is increasing evidence that magnetic fields can extend deep into the intergalactic space; however as it is far from regions of acceleration, the synchrotron emission is weak due to lack of cosmic rays electrons which suffer from various loss processes. Fortunately, low energy electrons which emit emission at low frequencies suffer less from energy loss processes and are expected to travel further, especially in the presence of ordered magnetic fields.

The first part of this talk will present low frequency radio observations of the face-on galaxy, M51, from the LOFAR telescope. In combination with novel theoretical models of cosmic ray propagation, I will address multiple outstanding questions in galactic astrophysics: how far do galactic magnetic disks extend and what is their structure; what is the dominant mechanism of cosmic ray propagation in these regions? Using these theoretical models, the cosmic ray confinement time within the galaxy can be accurately calculated.

In the second part of this talk, I shall present new JVLA observations of the face-on spiral galaxy, NGC 628, at 2-4 GHz. The observations aim to increase our understanding of the disk-halo interaction which is vital to explain the evolution of spiral galaxies. Studying a face-on spiral galaxy like NGC 628 helps the separation of magnetic field components in 3D. Thus through the technique of Rotation Measure Synthesis, any Faraday rotation observed is due solely to the vertical magnetic field. This would help find turbulent magnetic fields being carried out from the disk and would provide an important mechanism for the dynamo to amplify the ordered magnetic field without quenching.
Copyright: David Mulcahy
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