Using deep VLT/ISAAC near-infrared imaging data, we derive the K-s-band luminosity function (LF) of the z = 1.237 massive X-ray luminous cluster of galaxies RDCS J1252.9-2927. Photometric redshifts, derived from deep multiwavelength BVRIzJK(s) data, and calibrated using a large subset of galaxies with spectroscopic redshifts, are used to separate the cluster galaxy population from the foreground and background field galaxy population. This allows for a simultaneous determination of the characteristic magnitude K-s* and faint end slope alpha of the LF without having to make an uncertain statistical background subtraction. The derived LF is well represented by the Schechter function with K-s* = 18.54(-0.55)(+0.45) and alpha = -0.64(-0.25)(+0.27) The shape of the bright end of the derived LF is similar to that measured at similar restframe wavelengths (in the z-band) in local clusters, but the characteristic magnitude is brighter by DeltaM(z)* 1.40(-0.58)(+0.49) mag. The derived a is similar to the value derived in the K-s-band in the z = 1 cluster of galaxies MG2016+112 but is shallower (at the 2.2sigma level) than the value measured at similar restframe wavelength (in the z-band) in clusters in the local universe. The brightening of the characteristic magnitude and lack of evolution in the shape of the bright end of the LF suggests that the massive cluster ellipticals that dominate the bright end of the LF were already in place at z = 1.237, while the flattening of the faint end slope suggest that clusters at z similar to 1 contains relatively smaller fractions of low mass galaxies than clusters in the local universe. The results presented in this paper are a challenge for semi analytical hierarchical models of galaxy formation which predict the characteristic magnitude to grow fainter and the faint end slope to steepen with redshift as the massive galaxies break up into their progenitors. The observed evolution is consistent with a scenario in which clusters are composed of a population of massive galaxies which formed at high redshift (z much greater than 1) and subsequently evolved passively, and a population of lower mass galaxies which are gradually accreted from the field, primarily at lower redshift (z < 1).
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