Interstellar Turbulence. I. Retrieval of Velocity Field Statistics

We demonstrate the capability of principal component analysis (PCA) as applied by Heyer & Schloerb to extract the statistics of turbulent interstellar velocity fields as measured by the energy spectrum, E(k) ~ k-b. Turbulent velocity and density fields with known statistics are generated from fractional Brownian motion simulations. These fields are translated to the observational domain, T(x,y,v), considering the excitation of molecular rotational energy levels and radiative transfer. Using PCA and the characterization of velocity and spatial scales from the eigenvectors and eigenimages, respectively, a relationship is identified that describes the magnitude of line profile differences, dv, and the scale, L, over which these differences occur, dv ~ La. From a series of models with varying values of b, we find a = 0.33b - 0.05 for 1 < b < 3. This provides the basic calibration between the intrinsic velocity field statistics to observational measures and a diagnostic for turbulent flows in the interstellar medium. We also investigate the effects of noise, line opacity, and finite resolution on these results.