A MACHO (Massive Astronomical Compact Halo Object) is a hypothetical compact object, large numbers of which might account for a significant fraction of the dark matter in the galactic halos of large galaxies, including that of our own. If MACHOs exist they could take the form of black holes, neutron stars, red dwarfs, brown dwarfs, large rogue planets, or some other type of non-luminous body. A signature of these objects would be the occasional amplification of light from more distant sources by microlensing. No conclusions have yet been drawn about the existence or nature of MACHOs; however, various microlensing events detected by search efforts such as OGLE and the MACHO Project support the idea that invisible compact objects do exist in the Galaxy's halo.

 

MACHO Project

The MACHO Project was a collaboration between American, Australian, and British astronomers to search for MACHOs using a special-purpose charged coupled device camera on the 1.3-meter (50-inch) telescope of the Mount Stromlo and Siding Spring Observatories. It ran from 1992 to 1999 and was led by Charles Alcock at the Lawrence Livermore National Laboratory, California. The MACHO team identified several dozen microlensing events in the Galactic Bulge and Large Magellanic Cloud, including a few which suggest the presence of planets in orbit around distant stars. Immediate notification of possible in-progress lensing events, detected by the MACHO Project, was made available to other interested groups through the Global Microlensing Alert Network (GMAN).

 

EROS

EROS (Experience pour la Recherche d'Objet Sombres) is a French program to detect and study dark objects in the Galaxy, including brown dwarfs and MACHOs, through their gravitational microlensing effects on stars in the Large Magellanic Cloud. It employs the Schmidt camera and a charge-coupled device at the European Southern Observatory.

 

OGLE

Optical Gravitational Lensing Experiment (OGLE) is a search for MACHOs being conducted by a joint Polish and American team of astronomers, using a charge-coupled device on the 1-meter telescope at the Las Campanas Observatory. The team is headed by Bohdan Paczynski, of Princeton University.