Helium abundance in galaxy clusters and Sunyaev-Zeldovich effect
  It has long been suggested that helium nuclei in the intracluster plasma can
sediment in the cluster gravitational potential well. Some theoretical
estimates for the cores of relaxed clusters predict an excess of helium
abundance by up to a factor of a few over its primordial value. The
intracluster helium abundance cannot be measured directly. This presents a
significant source of uncertainty for cosmological tests based on the X-ray
derived cluster quantities, such as the gas mass, total mass, and gas mass
fraction, all of which depend on the assumed helium abundance. We point out
that cluster distances derived by combining the Sunyaev-Zeldovich (SZ) and
X-ray data also depend on the helium abundance. This dependence can be used to
measure the abundance, provided the distance is known independently. For
example, if one adopts the WMAP H_0 value, then the recent H_0 measurement by
Bonamente and collaborators, derived from SZ data on 38 clusters assuming a
primordial helium abundance, corresponds to an abundance excess by a factor of
1.9+-0.8 within r~1 Mpc (using only their statistical errors). This shows that
interesting accuracy is within reach. We also briefly discuss how the SZ and
X-ray cluster data can be combined to resolve the helium abundance dependence
for the d_a(z) cosmological test.
