THE K-BAND LUMINOSITY FUNCTION AT Z=1 - A POWERFUL CONSTRAINT ON GALAXY FORMATION THEORY

There are two major approaches to modelling galaxy evolution. The 'tra ditional' view is that the most massive galaxies were assembled early and have evolved with steeply declining star formation rates since a r edshift of 2 or higher. According to hierarchical theories, massive ga laxies were assembled much more recently from mergers of smaller subun its. Here we present a simple observational test designed to different iate between the two. The observed K-band flux from a galaxy is a good measure of its stellar mass even at high redshift. It is likely only weakly affected by dust extinction. We compute the evolution of the ob served K-band luminosity function for traditional, pure luminosity evo lution (PLE) models and for hierarchical models. At z = 0, both models can fit the observed local K-band luminosity function. By redshift 1, they differ greatly in the predicted abundance of bright galaxies. We calculate the redshift distributions of K-band selected galaxies and compare these with available data, We show that the number of K < 19 g alaxies with redshifts greater than 1 is well below the numbers predic ted by the PLE models. In the Songaila et al. (1994) sample of 118 gal axies with 16 < K < 18, 33 galaxies are predicted to lie at z > 1. Onl y two are observed. In the Cowie et al. sample of 52 galaxies with 18 < K < 19, 28 galaxies are predicted to lie at z > 1, Only five are obs erved. Both of these samples are more than 90 per cent complete. We co nclude that there is already strong evidence that the abundance of mas sive galaxies at z similar to 1 is far below the local value. This is inconsistent with the traditional model (unless most massive galaxies are extremely heavily obscured by dust at redshift 1), but similar to the expectations of hierarchical models.