Modified Cold Dark Matter Models in Light of 53W091, an Old Galaxy at High [CLC][ITAL]z[/ITAL][/CLC]

The epoch of galaxy formation provides an important additional test ofcosmological theories. Cold-dark-matter (CDM) models with cosmological constant($\Lambda$) are designed to account for the observed excess power in galaxydistribution, but at the same time suppress the power on small scales pushinggalaxy formation to recent epochs. We point out that the recently discoveredhigh redshift galaxy, 53W091, with accurate age measurements (Dunlop et al1996) provides a particularly important test of these models. In the flat$\Lambda$-dominated Universe, the redshift of formation of 53W091 decreaseswith decreasing $\Omega$. However, in the modified CDM models decreasing$\Omega$ suppresses the small scale power in the density field and this effectturns out to be dominant. We estimate the mass of the galaxy and show that itrepresents a very rare and unlikely event in the density field of such models.Similar problems would occur in other modifications of the CDM cosmogonies.Comment: in press ApJ Letter