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**Session 37 - X-ray Clusters: Implications for Cosmology.**

*Display session, Tuesday, June 09*

*Atlas Ballroom, *

## [37.04] Determination of q_0 from the Gas Fraction of Distant Clusters

*K. Rines, W. Forman, C. Jones (SAO), U. Pen (Harvard University), R. Burg (Johns Hopkins University)*
Clusters of galaxies are the largest gravitationally bound objects in
the universe. We therefore expect the baryonic mass fraction in a
cluster to be a fair sample of the baryonic fraction of the universe,
a conclusion which is supported by
simulations. If the gas mass fraction is assumed to be constant, we
can use any apparent change in the measured gas fraction between low
and high redshift clusters to determine the value of the cosmological
deceleration
parameter q_0 (Pen 1997). We calculate the gas fraction for a
sample of luminous (L_x > 10^45erg s^-1), nearby and distant
(0.3 < z < 0.6) clusters using data from ASCA and ROSAT. We
also determine the gas fraction using the method of Evrard, Metzler,
amp; Navarro (1997) to find the total mass within the radius where the
mass overdensity is 500 times the critical density. We present our
determinations of q_0 from these two methods including both
observational and systematic errors. This approach to determining
q_0 depends on the apparent observed change in the gas fraction
and not the value of the gas fraction itself, and hence it should be
less susceptible to systematic effects.

**Program
listing for Tuesday**