Modified cold dark matter models in light of 53W091, an old galaxy at high z

The epoch of galaxy formation provides an important additional test of cosmological theories. Cold dark matter (CDM) models with cosmological constant (Lambda) are designed to account for the observed excess power in galaxy distribution. At the same time, such models suppress the power on small scales, pushing galaxy formation to recent epochs. We point out that the recently discovered high-redshift galaxy, 53W091, with accurate age measurements (Dunlop et al. 1996), provides a particularly important test of these models. In the flat, Lambda-dominated universe, the redshift of formation of 53W091 decreases with decreasing Omega. However, in the modified 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.