An imaging K-band survey - II. The redshift survey and galaxy evolution in the infrared

We present further results from an imaging K-band survey of 552 arcmin^2, complete to a 5sigma limit of K~=17.3. This paper describes a redshift survey of 124 galaxies, and addresses the colours of faint galaxies and the evolution of the K-band luminosity function. The optical-to-infrared colours are consistent with the range expected from synthetic galaxy spectra, although there are some cases of very red nuclei. These may possibly be attributed to either extinction or metallicity gradients. Our data show no evidence for evolution of the K-band luminosity function at <e1>z<</e1>0.5, and the results are well described by a Schechter function with M^*_K=-22.75+/-0.13+5 log_10h and phi*=0.026+/-0.003h^3 Mpc^-3. This is a somewhat higher normalization than has been found by previous workers, and it removes much of the excess in faint K and b_J counts with respect to a no-evolution model. However, we do find evidence for evolution at z>0.5: M^*K is approximately 0.75 mag brighter at z=1. This luminosity evolution is balanced by a reduced normalization at high redshift: the total luminosity density is required to be approximately constant in order not to exceed the faint counts. The overall evolution is thus opposite to that expected in simple merger-dominated models; we briefly consider possible interpretations of this result.