Effects of dust absorption on spectroscopic studies of turbulence
  We study the effect of dust absorption on the recovery velocity and density
spectra as well as on the anisotropies of magnetohydrodynamic turbulence using
the Velocity Channel Analysis (VCA), Velocity Coordinate Spectrum (VCS) and
Velocity Centroids. The dust limits volume up to an optical depth of unity. We
show that in the case of the emissivity proportional to the density of
emitters, the effects of random density get suppressed for strong dust
absorption intensity variations arise from the velocity fluctuations only.
However, for the emissivity proportional to squared density, both density and
velocity fluctuations affect the observed intensities. We predict a new
asymptotic regime for the spectrum of fluctuations for large scales exceeding
the physical depths to unit optical depth. The spectrum gets shallower by unity
in this regime. In addition, the dust absorption removes the degeneracy
resulted in the universal $K^{-3}$ spectrum of intensity fluctuations of
self-absorbing medium reported by Lazarian \& Pogosyan. We show that the
predicted result is consistent with the available HII region emission data. We
find that for sub-Alfv\'enic and trans-Alfv\'enic turbulence one can get the
information about both the magnetic field direction and the fundamental
Alfv\'en, fast and slow modes that constitute MHD turbulence.
