Tracing magnetic fields with spectroscopic channel maps
  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.
