The universality of turbulence in the molecular interstellar medium and its exploitation as a distance estimator

The turbulent energy spectrum of molecular clouds in a variety of environments is measured via principal component analysis ( PCA) of spectral line imaging observations at millimeter wavelengths. Molecular clouds with known distances have been previously shown to accurately obey a universal scale dependence of turbulent velocity dispersion over spatial scales of 1-50 pc, via both standard object-based analysis and, more recently, PCA. The PCA-based spectrum is accurately obeyed such that it may be used as a distance estimator for molecular clouds with similar to30% accuracy, where the error budget is contributed to strongly by input H II region distances used for the calibration. The use of (CO)-C-13 spectral line data for distance estimation is examined and compared to the distance calibration established for (CO)-C-12 observations. We show that distances estimated using (CO)-C-13 are in good agreement with those obtained using (CO)-C-12, with a possible similar to10% distance overestimation for (CO)-C-13 relative to the (CO)-C-12 calibration. Several molecular clouds with known distances are subjected to PCA, and we demonstrate that the universal spectrum is closely respected by all clouds; PCA-based distances estimated under the assumption of exact adherence to the universal spectrum are derived and are shown to be in excellent agreement with optically estimated distances. We examine the possibility that the PCA distance estimation method may be used to solve the kinematic distance ambiguity in the inner Galaxy. We establish how PCA may be used to diagnose severe blending of near/far emission and, in cases of little or no blending, to accurately establish the near or far distance. The inner Galaxy results provide initial support for the global validity of the universal PCA spectrum previously demonstrated for the outer Galaxy only. In conjunction with the accurate velocity information provided by millimeter wavelength spectral line data, PCA can provide useful information for studies of Galactic structure and kinematics.