High-z objects and cold-dark-matter cosmogonies: the case of 53W091

The recently discovered high redshift galaxy, 53W091, with accurate age measurements (Dunlop et al 1996) provides a measure of the small-scale power of the primordial density field and, as we show, a crucial test of the inflation-inspired models. It allows $\Omega$=1 cosmologies only for low values of $H_0$, but then pushes formation of that galaxy to redshift much greater than allowed for by the cold-dark-matter density field. Cold-dark-matter (CDM) models with cosmological constant ($\Lambda$) and low $\Omega$ would decrease the redshift at which this galaxy has collapsed. However, in CDM models decreasing $\Omega$ suppresses the small scale power in the density field and this effect turns out to be dominant. We estimate the mass of the galaxy and show that it represents a very rare and unlikely event in the density field of such models. Similar problems would occur in other modifications of the CDM cosmogonies.