http://arxiv.org/abs/2202.01610
Turbulence is expected to generate characteristic velocity and density fluctuations in the interstellar medium (ISM). Using HI survey data, we distinguish these contributions as the basis of comparisons with theoretical and magnetohydrodynamical (MHD) simulations. We used HI4PI multiphase observations and Gaussian components representing three HI phases, the cold, warm, and unstable lukewarm medium (CNM, WNM and LNM, respectively) to deduce characteristic fluctuations in turbulent density and velocity fields. We applied the velocity decomposition algorithm (VDA) for separating such fluctuations in the position-position-velocity (PPV) space. The VDA extends the velocity channel analysis (VCA) by Lazarian and Pogosyan and predicts that turbulent velocity and density fields are statistically uncorrelated. Applying the VDA decomposition to the observational data (here, HI4PI) yields – contrary to our expectations – a significant correlation between velocity and density fields. All HI phases contribute to this correlation. The correlations in the velocity wings suffer from attenuation caused by uncorrelated noise. Both VCA and VDA predict that dispersions from fluctuations in narrow velocity channels scale in proportion to the average intensity. Observed brightness temperature fluctuations follow, however, a square-root scaling – as expected for a sum of normally distributed random sources. Fluctuations in HI channel maps at high spatial frequencies are observed to be dominated by density structures, which is in opposition to the results from MHD simulations, where small-scale structures are predicted to have been generated by velocity caustics. The VCA predictions and VDA results on MHD simulations are not compatible with the HI observations. The observed turbulent velocity and density fields in the ISM are not statistically uncorrelated, but they do reveal significant interrelations.
P. Kalberla, J. Kerp and U. Haud
Fri, 4 Feb 22
46/65
Comments: 15 pages, 15 figures
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