Tracing magnetic fields with spectroscopic channel maps [GA]

We explore the ability of gradients of velocity channel map intensities to trace magnetic fields in turbulent diffuse media. This work capitalizes both on the modern theory of MHD turbulence that predicts the magnetic eddies tend to be aligned with the local direction of magnetic field, and the theory of Position-Position-Velocity (PPV) statistics that describes how the velocity and density fluctuations in real space are being mapped into the PPV space. We show that for steep, e.g. Kolmogorov-type density spectrum, the Velocity Channel Gradients (VChGs) in thin velocity channels are dominated by velocity contributions. While for the velocity channel thickness comparable to turbulent injection velocities, the VChGs are dominated by the properties of turbulent densities. As turbulent velocity structures are better aligned with magnetic fields, the tracing with thin channels has the ability of representing the magnetic field better. We decompose the results of 3D MHD simulations into Alfven, slow and fast modes and analyze synthetic maps produced with these modes. We show that Alfven and slow modes act in unison to trace magnetic field, while the velocity gradients produced by the fast mode are orthogonal to those produced by the first two modes. However, for thin channel maps the contributions from the Alfven and slow modes are shown to dominate which allows a reliable magnetic field tracing. We also introduce centroids that use only part of the spectral line rather the entire spectral line and apply them to GALFA 21 cm data. We compare the directions obtained with the gradients of these “reduced centroids” and the magnetic field directions as they are traced by the Planck polarization. We believe that the observed deviations can potentially reveal the variations of the magnetic field along the line of sight.

Read this paper on arXiv…

A. Lazarian and K. Yuen
Fri, 10 Mar 17

Comments: 13 pages, 14 figures