Flattened Galactic Haloes and Baryonic Dark Matter

Abstract

We discuss the tight interconnection between microlensing optical depths, flattening of dark haloes and low-to-intermediate redshift baryonic census. By analysing plots of the microlensing optical depth as a function of galactic coordinates for different values of axis ratio $q$ of the galactic MACHO halo, we have shown that observations are best described by a flattened halo with $q\simeq 0.6$. There is no dynamical obstacle for such a choice of global halo shape. Both extremely flattened $q\simeq 0.2$ and spherical $q\simeq 1$ haloes have several difficulties, although not of equal severity. Consequences of such flattening for the cosmological density fraction contained in MACHOs are considered and comparison with mass in low and intermediate-redshift Ly$\alpha$ forest and other plausible reservoirs of gas is discussed in context of a unified description of the evolution of baryonic content of the universe.

14 pages, 15 figures, TeX, uses mn.tex and psfig.tex, MNRAS accepted

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