What Could the Machos Be?

Abstract

If the Universe has a significant baryonic dark component in the form of compact objects in galaxy halos (machos), then there is a minute chance (about $10^{-7}$) that one of the Galactic machos passes sufficiently close to our line of sight to a star out of some $10^7$ monitored stars in the Magellanic Clouds (MCs) that it brightens by more than 0.3 magnitude due to gravitational focusing. After a brief discussion of the current controversy over the interpretation of the observed events, i.e., whether the lensing is caused by halo white dwarfs or machos in general or by stars in various observed or hypothesized structures of the Clouds and the Galaxy, I propose a few observations to put ideas of the pro-macho camp and the pro-star camp to test. In particular, I propose a radial velocity survey towards the MCs.