Tracing interstellar magnetic field using the velocity gradient
  technique in shock and self-gravitating media
  This study proceeds with the development of the technique employing velocity
gradients that were identified in (\cite{GL17}, henceforth GL17) as a means of
probing magnetic field in interstellar media. We demonstrate a number of
practical ways on improving the accuracy of tracing magnetic fields in diffuse
interstellar media using velocity centroid gradients (VCGs). Addressing the
magnetic field tracing in super-Alfvenic turbulence we introduce the procedure
of filtering low spatial frequencies, that enables magnetic field tracing in
the situations when the kinetic energy of turbulent plasmas dominate its
magnetic energy. We propose the synergic way of of using VCGs together with
intensity gradients (IGs), synchrotron intensity gradients (SIGs) as well as
dust polarimetry. We show that while the IGs trace magnetic field worse than
the VCGs, the deviations of the angle between the IGs and VCGs trace the shocks
in diffuse media. Similarly the perpendicular orientation of the VCGs and the
SIGs or to the dust polarimetry data trace the regions of gravitational
collapse. We demonstrate the utility of combining the VCGs, IGs and polarimetry
using GALFA HI and Planck polarimetry data. We also provide an example of
synergy of the VCGs and the SIGs using the HI4PI full-sky HI survey together
with the Planck synchrotron intensity data.
