3.2. Baryon Fraction and Nucleosynthesis
The baryon fraction in clusters, fb, is assumed to be equal to the cosmic value. Combined with the universal baryon density b as predicted from light element abundances through the theory of big-bang nucleosynthesis (BBN) it yields the cosmic mass density [20]:
New Developments:
The baryonic content is mostly in the form of X-ray
gas, and is estimated from ROSAT to large radii.
The reliability of the gas mass estimates is partly confirmed by
(a) direct spectral limits on gas inhomogeneity, and
(b) limits on the cluster magnetic fields from Faraday rotation of
background sources.
Independent cluster masses are obtained from gravitational lensing.
The Deuterium abundance is being measured from quasar absorption systems
that are assumed to be composed of unprocessed primordial material.
Pro: This method avoids assumptions about galaxy biasing and stellar
populations; it depends on the
relatively safe assumption that cluster formation proceeds by
collapse from a well mixed medium, and the assertion that only little
segregation occurred between the gas and the dark matter.
Con:
The baryonic mass could be overestimated if the X-ray gas is
locally inhomogeneous, or if large tangled magnetic fields
provide a significant part of the pressure in cluster centers.
The total mass of clusters may be underestimated as in the
M/L method.
The baryon fraction seems to increases with cluster mass and
sometimes with radius within a cluster.
Plausible physical processes eject gas from the inner regions of lower mass
systems, making fb an underestimate of the global baryon
fraction, and leading to an overestimate of
m.
The method assumes that the baryonic fraction in clusters is equal to
the universal value even though clusters only contain a few percent of the
galaxies in the universe.
The method relies on the uncertain interpretation of the
light element abundances and on the theory of big-bang nucleosynthesis.
Current Results:
The baryonic fraction is estimated to be in the range
fb = (0.03 - 0.08)h-3/2 by
[21], but has also
been estimated to have a factor of 5 range among galaxy groups and
clusters of similar mass
[22].
The larger values typically apply to the most massive
and best observed clusters, but preliminary ASCA results indicate
lower values in several rich clusters.
Low values are also indicated in groups
[23].
The current BBN results suffer from uncertainty in the primordial
Deuterium abundance. The traditional estimates are of low
b, e.g.,
of 0.009
bh2
0.02 based on all the light elements,
and
0.006
bh2
0.03 from Deuterium only
[24]
Recent, high-resolution spectra from the Keck telescope show a lower Deuterium
abundance in two quasar absorption systems that seem to consist of
unprocessed primordial material,
and correspondingly
bh2
= 0.024-0.005+0.006
[25].
With fb in the middle of the range quoted above,
the estimate is either
m
0.3h65-1/2 (for low
b)
or m
0.55h65-1/2
(for high b),
but in either case
m = 1 cannot be
definitively excluded.