Magellanic cloud gravitational microlensing results: What do they mean?

Recent results from gravitational microlensing surveys of the Large Magellanic Cloud are reviewed. The combined microlensing optical depth of the MACHO and EROS-1 surveys is t(LMC) = 2.1(-0.8)(+1.3) x 10(-7) which is substantially larger than the background of tau(back) less than or equal to 0.5 x 10(-7) from lensing by known stellar populations although it is below the expected microlensing optical depth of tau = 4.7 x 10(-7) for a halo composed entirely of Maches. The simplest interpretation of these results is that nearly half of the dark halo is composed of Maches with a typical mass of order 0.5 Mo. This could be explained if these Maches are old white dwarfs, but it is not obvious that the generation of stars that preceded these white dwarfs could have gone undetected. It is also possible that Maches are not made of baryons, but there is no compelling model for the formation of non-baryonic Maches. Therefore, a number of authors have been motivated to develop alternative models which attempt to explain the LMC microlensing results with non-halo populations. Many of these alternative models postulate previously unknown dark stellar populations which contribute significantly to the total mass of the Galaxy and are therefore simply variations of the dark matter solution. However, models which postulate an unknown dwarf galaxy along the line of site to the LMC or a distortion of the LMC which significantly enhances the LMC self-lensing optical depth can potentially explain the LMC lensing results with only a small amount of mass, so these can be regarded as true non-dark matter solutions to the Macho puzzle. All such models that have been proposed so far have serious problems, so there is, as yet, no compelling alternative to the dark matter interpretation. However, the problem can be solved observationally with a second generation gravitational microlensing survey that is significantly more sensitive than current microlensing surveys. (C) 1998 Published by Elsevier Science B.V. All rights reserved.