The Complexity of Calculating Exemplar

Traditional methods for estimating rearrangement distances between genomes assume that there is at most one copy of each gene in each genome. In the case that there are multiple genes from the same gene family in a genome, Sanko# (1999) proposes the estimation of true exemplars, a selection of one gene from each gene family in both genomes such that the distance between the resulting exemplar strings is minimized

The Galactic Halo Ionizing Field and

. There has been much debate in recent decades as to what fraction of ionizing photons from star forming regions in the Galactic disk escape into the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4m and the AAT 3.9m telescopes may now provide the strongest evidence that at least some of the radiation escapes the disk completely. While the distance to the Magellanic Stream is uncertain, the observed Hff emission is most plausibly explained by photoionization due to hot, young stars. Our model requires that the mean Lyman-limit opacity perpendicular to the disk is ¯ ø ß 3, assuming the covering fraction of the resolved clouds is close to unity. Within the context of this model, it now becomes possible to determine distances to high velocity clouds, and the 3D orientation of the Magellanic Stream. Here, we discuss complications of the model (e.g., porosity, topology), future tests, ongoing improvements, and the importance of Hff limb brightening ..