Astronomy and Astrophysics – Astrophysics
Scientific paper
1993-04-26
Astronomy and Astrophysics
Astrophysics
30 pages, Phyzzx file, figures not included. For hardcopy with figures contact BEST@IASSNS.BITNET. Inst. for Advanced Study no
Scientific paper
10.1086/173515
If massive compact halo objects (\ms) are detected in ongoing searches, then \tsmctlmc, the ratio of the optical depth toward the Small and Large Magellanic Clouds, will be a robust indicator of the flattening of the Galactic dark matter halo. For a spherical halo, \tsmctlmc\ is about 1.45, independent of details 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. For an E6 halo (axis ratio $c/a = 0.4$), the ratio of optical depths is \tsmctlmc\ $\sim 0.95$, again independent of assumptions about Galactic parameters. This ratio can be measured with a precision as good as $\sim 10\%$ depending on the typical mass of the MACHOs. If the halo is highly flattened (\eg E6) and closely truncated (\eg at twice the solar galactocentric radius), then the optical depth toward the LMC can be reduced by a factor of about two. For these extreme parameters, and the assumption of a heavy Galactic disk and spheroid, the upper limit of the \m\ mass range to which ongoing experiments are sensitive is reduced from ${\cal O}(10^6)\,M_\odot$ to ${\cal O}(10)\,M_\odot$.
Gould Andrew
Sackett Penny D.
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