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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 Omegab as predicted from light element abundances through the theory of big-bang nucleosynthesis (BBN) it yields the cosmic mass density [20]:

Equation 9 (9)

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. bullet 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. bullet The total mass of clusters may be underestimated as in the M/L method. bullet 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 Omegam. 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. bullet 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]. bullet The current BBN results suffer from uncertainty in the primordial Deuterium abundance. The traditional estimates are of low Omegab, e.g., of 0.009 leq Omegabh2 leq 0.02 based on all the light elements, and 0.006 leq Omegabh2 leq 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 Omegabh2 = 0.024-0.005+0.006 [25].

With fb in the middle of the range quoted above, the estimate is either Omegam appeq 0.3h65-1/2 (for low Omegab) or Omegam appeq 0.55h65-1/2 (for high Omegab), but in either case Omegam = 1 cannot be definitively excluded.

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