Using MACHOs to Measure the Shape of the Galactic Halo

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Session 60 -- Galactic Structure
Oral presentation, Thursday, January 13, 10:15-11:45, Crystal Forum Room (Crystal City Marriott)

[60.05] Using MACHOs to Measure the Shape of the Galactic Halo

P.D. Sackett (Inst. for Advanced Study), A. Gould (Ohio State U.)

If recently reported gravitational lensing events represent the detection of Massive Compact Halo Objects (\ms) that make up the dark halo of the Milky Way, then \tsmctlmc, the ratio of the optical depth to lensing toward the Small and Large Magellanic Clouds, will be a robust indicator of the flattening of the Galactic dark matter halo. The optical depth to lensing along any single\/ line-of-sight does not provide a good measure of halo flattening because the dark halo structure parameters are insufficiently constrained by our current knowledge of the shape of the Galactic rotation curve, the assumed mass of the Galactic disk and spheroid, and the truncation distance (if any) of the dark halo. The ratio \tsmctlmc, however, is nearly independent of these Galactic parameters, but varies rather strongly with the vertical-to-radial axis ratio (c/a) of the density distribution of the MACHOs in the halo. For a spherical halo, the ratio of optical depths \tsmctlmc\ is about 1.45; for an E6 halo ($c/a = 0.4$), the ratio is \tsmctlmc\ $\sim 0.95$. The precision with which the ratio can be measured depends on the typical mass of the MACHOs.

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