Velocity Structure of the Interstellar Medium as Seen by the Spectral Correlation Function

(Abridged) We use the statistical tool known as the ``Spectral CorrelationFunction" [SCF] to intercompare simulations and observations of the atomicinterstellar medium. The simulations considered mimic three distinct sets ofphysical conditions. One of them (run "ISM") is intended to represent a mixtureof cool and warm atomic gas, and includes self-gravity and magnetic fields. Foreach simulation, H I spectral-line maps are synthesized and intercompared, bothwith each other, and with observations, using the SCF. We find that, whenthermal broadening is large in comparison with fine-scale turbulent velocitystructure, it masks sub-thermal velocity sub-structure in the synthesizedspectra. The H I observations we use here for comparison are of the NorthCelestial Pole (NCP) Loop. None of the simulations match the NCP Loop data verywell. The most realistic sets of line profiles and SCF statistics comes fromartifically rescaling the velocity axis of run ISM. Without rescaling, almostall velocity structure is smeared out by thermal broadening. However, if thevelocity axis is expanded by a factor of 6, the SCF distributions of run ISM anthe NCP Loop match up fairly well. This means that the ratio of thermal toturbulent pressure in run ISM is much too large as it stands, and that thesimulation is deficient in turbulent energy. This is a consequence of run ISMnot including the effects of supernovae. We conclude that the SCF is a usefultool for understanding and fine-tuning simulations of interstellar gas, and inparticular that realistic simulations of the atomic ISM need to include theeffects of energetic stellar winds (e.g. supernovae) in order for the ratio ofthermal-to-turbulent pressure to give spectra representative of the observedinterstellar medium in our Galaxy.Comment: 25 pages, 24 figures. ApJ Accepted (May 20). Also available at: ftp://www.astrosmo.unam.mx/pub/j.ballesteros/Papers