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, then 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 MACHO 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 baryon 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. However, the cumulative uncertainties in the density determinations only require 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 estimate for Omega(MACHO) is hard to reconcile with the current best estimates of the baryonic content of the intergalactic medium indicated by measurements of the Lyman-alpha forest; however, measurements of Omega(Ly alpha) are at present uncertain, so that such a comparison may be premature. If the MACHOs are white dwarfs resulting from a single burst of star formation (without recycling), then their main sequence progenitors 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. Indeed a MACHO population of white dwarfs and the gas ejected from their main sequence progenitors accounts for a significant fraction of all baryons. 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 budget with the accompanying carbon production in the case of white dwarfs. In the simplest picture, even if the excess carbon is ejected from the Galaxy by a Galactic wind, measurements of carbon abundances in Lyman a forest lines with values 10(-2) solar require that only about 10(-2) of all baryons can have passed through the white dwarf progenitors. Such a fraction can barely be accommodated by our estimates of Omega(MACHO) and would be in conflict with Omega(star). (C) 1998 Elsevier Science B. V. All rights reserved.