Old galaxies at high redshift and the cosmological constant

In a recent striking discovery, Dunlop et al. (1996) observed a galaxy at redshift z = 1.55 with an estimated age of 3.5 Gyr. This is incompatible with age estimates for a flat matter-dominated universe unless the Hubble constant is less than 45 km s^-1 Mpc^-1. While both an open universe and a universe with a cosmological constant alleviate this problem, I argue here that this result favors a nonzero cosmological constant, especially when it is considered in light of other cosmological constraints: (1) for the favored range of matter densities, the constraint is more stringent than the globular cluster age constraint, which already favors a nonzero cosmological constant; (2) the age-redshift relation for redshifts of order unity implies that the ratio between the age associated with redshift 1.55 and the present age is also generally larger for a cosmological constant-dominated universe than for an open universe; (3) structure formation is generally suppressed in low-density cosmologies, arguing against early galaxy formation. The additional constraints imposed by the new observation on the parameter space of h versus Ω_matter (where H = 100 h km s^-1 Mpc^-1) are derived for both cosmologies.