... sources

Appendices A-C are only available in electronic form at http://www.edpsciences.org

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... medium

Note that Eq. (31) reduces to the familiar result $I(\vec{r}) =u(\vec{r}) u^*(\vec{r}) = I_{\rm src}(\vec{r}) ~{\rm e}^{-\tau (\vec{r})}$.

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... fluctuations

As we refer to fluctuations in the plane of the sky, the argument of the power spectrum is a two-dimensional vector. This power spectrum depends on the manner in which the averaging over radial distance (or radial velocity) has been performed. The inversion from the full three-dimensional HI density field to the power spectrum of optical depth variations is discussed at length in Lazarian & Pogosyan (2000).

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