8. The baryon mass fraction in clusters of galaxies
Abell (1958)
made the first useful catalog of the rich clusters
considered here and in the next test. A typical value of the
Abell cluster mass within the Abell radius
ra = 1.5h-1 Mpc is
3 × 1014h-1
M. The
cluster masses are reliably
measured (within Newtonian gravity) from consistent results from
the velocities of the galaxies, the pressure of the intracluster
plasma, and the gravitational deflection of light from background galaxies.
White (1992) and
White et al. (1993)
point out that rich clusters likely are large enough to contain
a close to fair sample of baryons and dark matter, meaning the
ratio of baryonic to total mass in a cluster is a good measure of
B0 /
M0. With
B0 from
the model for light elements
(Eqs. [62]), this gives a measure of the mean mass
density. The baryon mass fraction in clusters is still under
discussion.(83)
We adopt as the most direct and so maybe most reliable approach
the measurement of the baryonic gas mass fraction of clusters,
fgas, through the Sunyaev-Zel'dovich microwave decrement
caused by Thomson-Compton scattering of cosmic microwave background
radiation by the intracluster plasma. The
Carlstrom et al. (2001)
value for fgas gives
M0 ~
0.25,
(84)
in the range of Eq. (59). This test does not directly constrain
K0,
0, or the
dynamics of the dark energy.
83 See Hradecky et al. (2000), Roussel, Sadat, and Blanchard (2000), Allen, Schmidt, and Fabian (2002), and references therein. Back.
84 This assumes
B0
h2 = 0.014 from Eqs. (62). For the full
range of values in Eqs. (6) and (62),
0.1
M0
0.4 at two
standard deviations. Back.