The implications of the massive , X-ray selected cluster of galaxies MS1054–03 at z = 0.83 are discussed in light of the hypothesis that the primordial density fluctuations may be nongaussian .
We generalize the Press-Schechter ( PS ) formalism to the nongaussian case , and calculate the likelihood that a cluster as massive as MS1054 would have been found in the Einstein Medium Sensitvity Survey ( EMSS ) .
A flat universe ( \Omega _ { M } + \Omega _ { \Lambda } = 1 ) is assumed and the mass fluctuation amplitude is normalized to the the present-day cluster abundance .
The probability of finding an MS1054-like cluster then depends only on \Omega _ { M } and the extent of primordial nongaussianity .
We quantify the latter by adopting a specific functional form for the PDF , denoted \psi _ { \lambda } , which tends to Gaussianity for \lambda \gg 1 but is significantly nongaussian for \lambda \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 10 , and show how \lambda is related to the more familiar statistic T, the probability of \geq 3 \sigma fluctuations for a given PDF relative to a Gaussian .
Special attention is given to a careful calculation of the virial mass of MS1054 from the available X-ray temperature , galaxy velocity , and weak lensing data .

We find that Gaussian initial density fluctuations are consistent with the data on MS1054 only if \Omega _ { M } \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 0.2. For \Omega _ { M } \geq 0.25 a significant degree of nongaussianity is required , unless the mass of MS1054 has been substantially overestimated by X-ray and weak lensing data .
The required amount of nongaussianity is a rapidly increasing function of \Omega _ { M } for 0.25 \leq \Omega _ { M } \leq 0.45 , with \lambda \leq 1 ( T \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 7 ) at the upper end of this range .
For a fiducial \Omega _ { M } = 0.3 , \Omega _ { \Lambda } = 0.7 universe , favored by several lines of evidence ( Wang et al . 1999 ) , we obtain an upper limit \lambda \leq 10 , corresponding to a T \geq 3. This finding is consistent with the conclusions of Koyama , Soda , & Taruya ( 1999 ) , who applied the generalized PS formalism to low ( z \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 0.1 ) and intermediate ( z \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 0.6 ) redshift cluster data sets .