© CAMBRIDGE UNIVERSITY PRESS 1999

1.5.2.3 MACHOs

Meanwhile, the MACHO (Alcock et al. 1996a) and EROS (Ansari et al. 1996, Renault et al. 1996) experiments have detected microlensing of stars in the Large Magellanic Cloud (LMC). While the number of such microlensing events is small (six fairly convincing ones from two years of MACHO data discussed in their latest conference presentations, and one from three years of EROS observations), it is several times more than would be expected just from microlensing by the known stars. The MACHO data suggests that objects with a mass of 0.5^+0.3_-0.2 M are probably responsible for this microlensing, with their total density equal to ~ 20-50 percent of the mass of the Milky Way halo around ~ 20 kpc radius (Gates, Gyuk, & Turner 1996). Neither the EROS nor the MACHO groups have seen short duration microlensing events, which implies strong upper limits on the possible contribution to the halo of compact objects weighing less than about 0.05 M. While the MACHO masses are in the range expected for white dwarfs, there are strong observational limits (Flynn, Gould, & Bahcall 1996) and theoretical arguments (Adams & Laughlin 1996) against white dwarfs being a significant fraction of the dark halo of our galaxy. Thus it remains mysterious what objects could be responsible for the observed microlensing toward the LMC. But the very large number of microlensing events observed toward the galactic bulge is probably explained by the presence of a bar aligned almost toward our position (Zhao, Rich, & Spergel 1996; cf. Bissantz et al. 1996 for a dissenting view). Possibly the relatively small number of microlensing events toward the LMC represent lensing by a tidal tail of stars stretching toward us from the main body of the LMC (Zhao 1997); there is even some data on the colors and luminosities of stars toward the LMC suggesting that this may actually be true (D. Zaritsky, private communication 1997).