Tracing magnetic fields with spectroscopic channel maps

Abstract

We identify velocity channel map intensities as a new way to trace magnetic fields in turbulent media. This work makes use of both of the modern theory of MHD turbulence predicting that the magnetic eddies are aligned with the local direction of magnetic field, and also the theory of the spectral line Position-Position-Velocity (PPV) statistics describing how the velocity and density fluctuations are mapped into the PPV space. In particular, we use the fact that, the fluctuations of intensity of thin channel maps are mostly affected by the turbulent velocity, while the thick maps are dominated by density variations. We study how contributions of the fundamental MHD modes affect the Velocity Channel Gradients (VChGs) and demonstrate that the VChGs arising from Alfven and slow modes are aligned perpendicular to the local direction of magnetic field, while the VChGs produced by the fast mode are aligned parallel to the magnetic field. The dominance of the Alfven and slow modes in the interstellar media will therefore allow a reliable magnetic field tracing using the VChGs. We explore ways of identifying self-gravitating regions that do not require polarimetric information. In addition, we also introduce a new measure termed "Reduced Velocity Centroids" (RVCGs) and compare its abilities with the VChGs. We employed both measures to the GALFA 21cm data and successfully compared thus magnetic field directions with the {\it PLANCK} polarization. The applications of the suggested techniques include both tracing magnetic field in diffuse interstellar media and star forming regions, as well as removing the galactic foreground in the framework of cosmological polarization studies.