2.6.2. MACHOs

The massive compact halo objects (MACHOs) detected as gravitational microlensing events could be stellar remnants, that is, baryons. The nature of the MACHOs is not known, and there is no secure estimate of their global abundance. However, they do seem to comprise a new population not otherwise accounted for.

Current results from the MACHO collaboration (Alcock et al. 1997) indicate objects with mass comparable to that of the Sun (in one model, 0.5^+0.3_-0.2 M) may account for 20% to 100% of the dark mass in a standard spherical halo between the Milky Way and the Large Magellanic Cloud. This result must be taken with caution, however, because the experiment measures the MACHO mass column in just one direction in one halo and the extrapolation to a global density is subject to many uncalibrated assumptions. The Galactic MACHO population may have asymmetries or the MACHOs may be concentrated relative to the global dark matter in the halo. For example, the actual mass of MACHOs inferred within 50kpc (based on a spherical model, but without extrapolating to larger radii) is estimated at 68% confidence to be 13 to 32 x 10¹⁰ M. This can be compared to the mass of the disk (about 6 x 10¹⁰ M). A reasonable estimate of the minimum global density of MACHOs, viewing them as a new Galactic stellar population, is to assume that all disk galaxies have the same ratio of MACHO to disk mass, taking the low end of the estimated range; this yields

(34)

a minor entry in the budget. On the other hand the data are consistent with all of the Galactic dark matter being in MACHOs, so a reasonable upper limit derives from assuming that MACHOs comprise 100% of the density parameter in gravitation mass in equation (28), giving _{MACHO, max} = 0.25, and making MACHOs the dominant entry in the baryon budget. (At this level one expects detectable effects due to microlensing of quasar continuum emission regions, e.g. Dalcanton et al. 1994).

Assessments of this serious uncertainty will be guided by advances in observational constraints on the nature and amount of the MACHOs, and perhaps also by advances in understanding processes of star formation and death that could produce a substantial mass in baryonic MACHOs without leaving an unacceptably large amount of debris.