The work which has been done so far represents only a first essay at the problem of detecting the signs of biased galaxy formation. Only a limited range of possible effects have been studied, and in only a small volume of the universe. It is much too early to pronounce final judgement on the viability of this idea. Nevertheless, it is true that, at the present time, there is no observational evidence suggesting the existence of galaxy biasing, and there is some evidence against its occurrence. Similarly, the determinations of mass-to-light ratios on a variety of scales leave much to be desired. Many of the dynamical tests produce ambiguous results. Many depend on the assumption that light does trace mass, and their results may be systematically too low if it does not. Nevertheless, the data, taken at face value, suggest that mass and light have a constant ratio on scales larger than an individual galactic halo, and provide no support for the contrary view.
What is most striking about these many findings is their
consistency. An outside observer, studying the observational data in
blissful ignorance of cosmological theory, would, I think, be
astonished by the suggestion that the universe is dominated by some
exotic form of dark matter with a very different distribution than
that of the galaxies. The products of primordial nucleosysnthesis, the
observed dynamics of galactic systems, and the properties of the
galaxy population all imply that the universe is dominated by baryons,
of density sufficient to give
0
0.15 and the baryons
reside in galaxies and their halos.
One might object that the idea of rather smoothly distributed
non-baryonic dark matter is not an arbitrary fudge, because the dark
halos of galaxies show that at least some exists. However, this need
not be so. Since, as Fig. 2 shows,
the mass density in dark halos is
consistent with the mass density in baryons, there is no particular
need to invoke other forms of matter. In fact, if galaxy formation is
efficient, so that most baryons are incorporated into galaxies, then
at least some fraction of the dark halos must be baryonic. The
luminous matter in galaxies, whose M/L
4, provides a cosmic mass
density equivalent to
0 =
0.007. Primordial nucleosynthesis suggests
that 0
h2 > 0.01. Thus, unless H0 >
120 km s1 Mpc-1, there are more
baryons in the universe than can be accounted for by the luminous
parts of galaxies. Choices of H0 are a matter of
taste. If one
believes, as I do, that the ages of the globular clusters requires that
H0 50 km
s-1 Mpc-1, then the total baryonic
mass to light ratio
of the universe, M/L)b > 23. The dark halos are the logical
repository of these extra baryons.
Oemler and Tucker (1988)
have shown
that the mass-to-light ratios and gas contents of clusters of galaxies
are most consistent with a baryonic mass-to-light ratio of about
100. If we are correct, then the dark halos are entirely baryonic.
The universe is a complicated place, and things are not always what they appear. Nevertheless, Occam's razor remains a useful guide. To abandon the straightforward interpretation of observations for a much more convoluted explanation requires a very compelling justification. It is not clear that the current difficulties in understanding the universe are sufficiently severe to provide that. We do not, as yet, understand physics at the GUT scale and beyond, nor do we understand the early universe, or the process of galaxy formation. It is hardly surprising that current theory cannot explain everything.