Declining Rotation Curve and Brown Dwarf MACHOs

If the Galactic rotation speed at the Solar circle is $\sim 200$ km s$^{-1}$or smaller, which is supported by several recent studies, the rotation curve ofthe Galaxy could be declining in the outermost region. Motivated by this, weinvestigate the effect of such declining rotation curve on the estimate of theMACHO mass and the fractional contribution of the MACHOs to the Galactic darkhalo. Using Hernquist and Plummer halo models instead of the standard halomodel, we find that the MACHO mass could be significantly smaller than that forthe standard halo case. In particular, there exists a certain set of haloparameters for which the MACHO mass is 0.1$M_\odot$ or less and at the sametime the MACHO contribution to the total mass of the halo is almost 100 %. Thisresult indicates that a halo which consists solely of brown dwarfs can beconsistent with both of the observed microlensing properties and theconstraints from the rotation curve, provided the outer rotation curve isindeed declining.Comment: 8 pages and 4 figures, accepted for publication in ApJ Letter