MASSIVE COMPACT HALO OBJECTS VIEWED FROM A COSMOLOGICAL PERSPECTIVE -CONTRIBUTION TO THE BARYONIC MASS DENSITY OF THE UNIVERSE

We estimate the contribution of massive compact halo objects (MACHOs) and their stellar progenitors to the mass density of the Universe. If the MACHOs that have been detected reside in the halo of our Galaxy, t hen a simple extrapolation of the Galactic population (out to 50 kpc) of MACHOs to cosmic scales gives a cosmic density rho(MACHO) = (1- 5) x 10(9) h M-circle dot Mpc(-3), which in terms of the critical density corresponds to Omega(MACHO) = (0.0036 - 0.017) h(-1). Should the MACH O halo extend out to much further than 50 kpc, then Omega(MACHO) would only be larger. Such a mass density is comparable to the cosmic baryo n density implied by Big Bang nucleosynthesis. If we take the central values of the estimates, then MACHOs dominate the baryonic content of the Universe today, with Omega(MACHO)/Omega(B) similar to 0.7 h. Howev er, the cumulative uncertainties in the density determinations only re quire that Omega(MACHO)/Omega(B) greater than or equal to 1/6 hf(gal), where the fraction of galaxies that contain MACHOs f(gal) > 0.17 and h is the Hubble constant in units of 100 km s(-1) Mpc(-1). Our best es timate for Omega(MACHO) is hard to reconcile with the current best est imates of the baryonic content of the intergalactic medium indicated b y measurements of the Lyman-alpha forest; however, measurements of Ome ga(Ly alpha) are at present uncertain, so that such a comparison may b e premature. If the MACHOs are white dwarfs resulting from a single bu rst of star formation (without recycling), then their main sequence pr ogenitors would have been at least twice more massive: Omega(star) = 0 .007- 0.034) h(-1). Thus, far too much gaseous baryonic material would remain in the Galaxy unless there is a Galactic wind to eject it. Ind eed a MACHO population of white dwarfs and the gas ejected from their main sequence progenitors accounts for a significant fraction of all b aryons. This fact must be taken into account when attempting to dilute the chemical by- products of such a large population of intermediate mass stars. We stress the difficulty of reconciling the MACHO mass bud get with the accompanying carbon production in the case of white dwarf s. In the simplest picture, even if the excess carbon is ejected from the Galaxy by a Galactic wind, measurements of carbon abundances in Ly man a forest lines with values 10(-2) solar require that only about 10 (-2) of all baryons can have passed through the white dwarf progenitor s. Such a fraction can barely be accommodated by our estimates of Omeg a(MACHO) and would be in conflict with Omega(star). (C) 1998 Elsevier Science B. V. All rights reserved.