  A recent publication by Clark et.al (2019, CX19) uses both GALFA-HI
observational data and numerical simulations to address the nature of intensity
fluctuations in Position-Position-Velocity (PPV) space. The study questions the
validity and applicability of the statistical theory of PPV space fluctuations
formulated in Lazarian & Pogosyan (2000, LP00) to HI gas and concludes that
$"${\it a significant reassessment of many observational and theoretical
studies of turbulence in HI}$"$. This implies that dozens of papers that used
LP00 theory to explore interstellar turbulence as well as the ongoing research
based on LP00 theory are in error. This situation motivates the urgency of our
public response. In our Comment we explain why we believe the criticism in CX19
is based on the incorrect understanding of the LP00 theory. In particular, we
illustrate that the correlation between PPV slices and dust emissions in CX19
does not properly reveal the relative importance of velocity and density
fluctuations in velocity channel maps. While CX19 provides an explanation of
the change of the spectral index with respect to the thickness of PPV slice
based on the two-phase nature of H1 gas, we failed to see any observational
support for this idea. On the contrary, we show that the observations both in
two-phase HI and one phase CO show similar results. Moreover, the observed
change is in good agreement with LP00 predictions and spectral indexes of
velocity and density spectra that are obtained following LP00 procedures are in
good agreement with the numerically confirmed expectations of compressible MHD
turbulence theory. In short, we could not find any justification of the
criticism of LP00 theory that is provided in CX19. On the contrary, our
analysis testifies that both available observational and numerical data agree
well with the predictions of LP00 theory.
