So are brown dwarfs ubiquitous, occurring in sufficient numbers to account for an appreciable amount of the ‘dark matter’ in the galaxy? Studies are ongoing, but the three major ones that have already been completed indicate that the answer is no. The MACHO Project (Massive Astrophysical Compact Halo Object, a term for large astronomical bodies that can explain what seems to be dark matter in galactic haloes), along with the EROS and OGLE collaborations, all involved studies of the Small and Large Magellanic Clouds, satellite galaxies to our own Milky Way.
Using stars in the LMC and SMC as light sources, the teams observed the stars for several years and looked for the kind of lensing events that would indicate the presence of a dark object between the star and the observer. The MACHO and EROS teams announced the detection of three MACHOs as far back as September of 1993. That was after looking at 1.8 million stars for one year (MACHO) and 3 million stars for three years (EROS). By the end of the decade, the teams had a combined score of about twenty microlensing events.
The final paper of the MACHO collaboration, published in 2000, concluded that a Galactic halo consisting entirely of MACHOs was now ruled out, and estimated that about 20% of the Galactic halo was in the form of MACHOs. The EROS team preferred to present its results as an upper limit on the number of MACHOs in the halo, with no more than about 8% of the halo in MACHOs having masses of about one-tenth to one times the mass of the Sun. A combined analysis of the two experiments showed that, within the uncertainties of each experiment, they are consistent with each other and that less than 20% of the halo is in the form of MACHOs.
MACHOs, the least exotic candidates for dark matter, have now been effectively ruled out as the main component of the dark matter, leaving WIMPs to dominate the Galaxy. Nevertheless, there seems to be evidence for some MACHOs in the Galactic halo, even if not enough to be interesting from a dark matter point of view.
