Strong evolution in the luminosity-velocity relation at z ≳ 1?

We present a method for constraining the evolution of the galaxy luminosity-velocity (LV) relation in hierarchical scenarios of structure formation. The comoving number density of dark matter halos with circular velocities of 200 km s^-1 is predicted in observationally favored cold dark matter (CDM) cosmologies to be nearly constant over the redshift range 0 ≲ z ≲ 5. Any observed evolution in the density of bright galaxies implies in turn a corresponding evolution in the LV relation. We consider several possible forms of evolution for the zero point of the LV relation and predict the corresponding evolution in galaxy number density. The Hubble Deep Field suggests a large deficit of bright (Mv < -19) galaxies at 1.4 ≲ z ≲ 2. If taken at face value, this implies a dimming of the LV zero point by roughly 2 mag. Deep, wide-field, near-IR selected surveys will provide more secure measurements to compare with our predictions.