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

Abstract

Recent 21 cm radio observations of H${}_I$ regions in the Small Magellanic Cloud, have revealed spatial power spectra of the intensity, which are quite similar in shape to those previously deduced for the Galaxy. The similarity, in spite the differences in the physical parameters between the Galaxy and the SMC, suggests that the shape of the power spectra reflects some underlying mechanism which is not too sensitive to the environmental specifics. In this paper we present an interpretation for the observational power spectra in terms of a large scale turbulence in the interstellar medium, in which the emitting H${}_I$ regions are embedded. The turbulence gives rise to density fluctuations which lead to the observed intensity fluctuations, in the H${}_I$ regions. The observational power spectra are used to deduce the turbulence spectral function. In the SMC, the turbulence largest eddies are comparable in scale to the SMC itself. This implies that turbulent mixing should have smoothed out any large scale abundance gradients. Indeed, this seems to be the case, observationally. The turbulence is also expected to amplify and shape up the large scale magnetic field. Indeed, the observational data indicate the existence of a large scale disordered field of the strength expected from energy equilibrium with the turbulent velocity field. The large scale turbulence is most probably generated by instabilities in the large scale flows induced by the tidal close encounter with the LMC $\sim 2\times 10^8\mathrm{yr}$ ago. The life-time of the largest eddies is $\sim 4\times 10^8\mathrm{yr}$ so the turbulence had not yet enough time to decay and persists even though the energy source is no longer there.

13 pages, 1 figure, to appear in ApJ

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