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J. S. Arabadjis, J. N. Bregman (University of Michigan Astronomy Department)
We have developed a technique to map the three-dimensional structure of the local interstellar medium ({\sc ism}) using a maximum entropy reconstruction ({\sc mer}) technique. A set of column densities N to stars of known distance can in principle be used to recover a three-dimensional density field n, since the two quantities are related by simple geometry through the equation N = {\sf C} \cdot n, where {\sf C} is a tensor characterizing the stellar spatial distribution. In practice, however, there is an infinte number of solutions to this equation. We use a {\sc mer} algorithm to find the density field containing the least information which is consistent with the observations. The solution obtained with this technique is, in some sense, the {\sc ism} model containing the minimum structure. We apply the algorithm to several simulated data sets to demonstrate its feasibility and success at recovering ``real'' density contrasts.
One application of this technique is to soft X-ray absorption columns. A large set of columns can be obtained by modelling the several hundred stellar spectra in the ROSAT data archives at the {\sc heasarc}. The absorption of soft X-rays by the {\sc ism} is independent of the molecular state of the absorbers (monatomic or molecular gas, or dust grains), as well as their (modest) ionization state, making it an ideal tracer of the total amount of (baryonic) material. By applying this method to column density sets derived from different tracers -- for example, dust columns derived from stellar color excesses -- one can determine the structures in, and relative fractions of, the various phases of the {\sc ism}.
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