This study proceeds with the development of the technique employing velocity gradients that were identified in Gonzalez-Casanova & Lazarian (2016) as a means of probing magnetized interstellar media. We demonstrate a few practical ways of improving the accuracy of tracing magnetic fields in diffuse interstellar media using velocity gradients. We show that the higher order velocity centroids are able to provide better tracing of magnetic fields compared with the first order velocity centroids that have been used so far. Another way of improving the tracing that we explore is based on removing from the analysis the regions of strong shocks, which we identify using both by the increase of the amplitude of the velocity and density gradients and the misalignment of the two gradient measures. Addressing the magnetic field tracing in superAlfvenic turbulence we introduce the procedure of filtering of low spatial frequencies that enables magnetic field tracing in the situations when the kinetic energy of turbulent plasmas dominate its magnetic energy. We provide theoretical and numerical arguments as to why we expect that the velocity gradients trace magnetic fields in diffuse media better than density gradients. We also demonstrate this by comparing the alignment of the velocity centroid gradients (VCGs) as well as intensity gradients (IGs) obtained with the GALFA HI survey and the Planck polarization data that traces magnetic field in the cold and warm diffuse atomic hydrogen. Finally, by using the simulations with self-gravity we demonstrate that in the regions of the gravitational collapse the alignment of the VCGs changes with respect to the magnetic field.
K. Yuen and A. Lazarian
Fri, 10 Mar 17