ar X iv : a st ro - p h / 98 09 19 8 v 1 1 5 Se p 19 98 The Impact of AXAF & XMM on Measurements of Ω 0 from Cluster Abundances

Two major X-ray satellites, AXAF & XMM, will be launched in 1998/1999. Both satellites have spectral and imaging capabilities, and are several times more sensitive than either ROSAT or ASCA. AXAF & XMM will yield significant scientific returns in many fields of astrophysics. We concentrate here on how AXAF & XMM will improve constraints on the density parameter, Ω0, via measurements of the cluster abundance at high redshift. In a high Ω 0 universe, the density of the most massive clusters will be van-ishingly small at high redshift. This density rises rapidly as Ω 0 decreases, meaning that the discovery of only one or two massive clusters at a redshift of z∼1 has the potential to rule out Ω 0 =1 to high significance [1]. This sensitivity has meant that measurements of cluster abundances 2 have become a popular means by which to constrain Ω 0. However, one has to be extremely cautious when making cluster abundance measurements, since any observational bias which mimics an under (or over) abundance of massive clusters will lead to an over (or under) estimate of Ω 0. For example, one could mimic an under abundance of massive clusters in an X-ray selected survey, such as the EMSS [8] [10], if the optical follow-up was incomplete: Optical follow-up becomes increasingly difficult as redshift increases and so there is a finite probability that some high z clusters will be missed and that other clusters will be assigned incorrect redshifts. The optical follow-up of the EMSS cluster sample is now complete [9], but in an earlier version of the catalog [10], one high redshift (z>0.5) cluster was missed (MS1610.4) and two more had underestimated redshifts (MS1054.4 & MS1137.5). Small errors like these have a significant effect on measured parameters, such as the significance of measured luminosity evolution [14] and on the value of Ω 0 [16]. To date, the analysis of cluster abundances has been made in the context of the Press-Schechter formalism, which gives an analytical relation for the number density of virialized dark matter halos as a function of mass and redshift 3 [15]. When even a small fraction of the high redshift clusters in a given catalog have artificially high masses, then the Press-Schechter derived value of Ω 0 will 2 By " abundances " we mean the number density of clusters as a function of mass and z. 3 This formulism has been …