The meaning of EROS/MACHO

Abstract

Most of the mass density in the Universe and in the halo of our own galaxy -- exists in the form of dark matter. Overall, the contribution of luminous matter (in stars) to the mass density of the Universe is less than 1%; primordial nucleosynthesis indicates that baryons contribute between 1% and 10% of the critical density (0.01h{sup {minus}2} {approx_lt} {Omega}{sub B} {approx_lt} 0.02h{sup {minus}2}; h = the Hubble constant in units of 100 kms{sup {minus}1}Mpc{sup {minus}1}); and other evidence indicates that the total mass density is at least 10% of critical density, and likely much greater. If the universal density is as low as 10% of the critical density there may be but one kind of dark matter. More likely, the universal density is greater than 10%. and there are two kinds of dark matter, and thus two dark matter problems: In what form does the baryonic dark matter exist? and In what form does the nonbaryonic dark matter exist? The MACHO and EROS collaborations have presented evidence for the microlensing of stars in the LMC by 10{sup {minus}1{plus_minus}1} M{sub {circle_dot}} dark objects in the halo of our own galaxy and may well have solved one of the dark matter puzzles by identifying the form of the baryonic dark matter. It is too early to make precise statements about the fraction of the mass density in the halo of our galaxy contributed by lensing objects (= f{sub m}), though the EROS/MACHO data suggest that f{sub m} is probably 0.1 or larger. Taking our galaxy to be typical and taking account a fraction f{sub m} of the mass in the portion of the halo that contributes most significantly to microlensing (within 20 kpc of the galactic center), I estimate that lensing objects contribute a fraction 0.008f{sub m}/h of the critical density, and clearly cannot account for the bulk of the dark matter if {Omega}{sub 0} {much_gt} 0.1.