Interpretation of the Spatial Power Spectra of Neutral Hydrogen in the Galaxy and in the Small Magellanic Cloud

Recent 21 cm radio observations of H$_I$ regions in the Small MagellanicCloud, have revealed spatial power spectra of the intensity, which are quitesimilar in shape to those previously deduced for the Galaxy. The similarity, inspite the differences in the physical parameters between the Galaxy and theSMC, suggests that the shape of the power spectra reflects some underlyingmechanism which is not too sensitive to the environmental specifics. In thispaper we present an interpretation for the observational power spectra in termsof a large scale turbulence in the interstellar medium, in which the emittingH$_I$ regions are embedded. The turbulence gives rise to density fluctuationswhich lead to the observed intensity fluctuations, in the H$_I$ regions. Theobservational power spectra are used to deduce the turbulence spectralfunction. In the SMC, the turbulence largest eddies are comparable in scale to the SMCitself. This implies that turbulent mixing should have smoothed out any largescale abundance gradients. Indeed, this seems to be the case, observationally. The turbulence is also expected to amplify and shape up the large scalemagnetic field. Indeed, the observational data indicate the existence of alarge scale disordered field of the strength expected from energy equilibriumwith the turbulent velocity field. The large scale turbulence is most probablygenerated by instabilities in the large scale flows induced by the tidal closeencounter with the LMC $ \sim 2\times 10^8{\rm yr}$ ago. The life-time of thelargest eddies is $\sim 4\times 10^8{\rm yr}$ so the turbulence had not yetenough time to decay and persists even though the energy source is no longerthere.Comment: 13 pages, 1 figure, to appear in Ap