K-band luminosity function of the z = 1.237 cluster of galaxies RDCS J1252.9-2927 ^*

¹ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
² Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA e-mail: toft@astro.yale.edu
³ Max-Planck-Institut für extraterrestrische Physik, Postfach 1319, 85748 Garching, Germany
⁴ European Southern Observatory, Karl-Scwarzchild-Strasse 2, 85748 Garching, Germany
⁵ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
⁶ Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France
⁷ Instituto Nazionale di Astrofisica, Osservatorio Astronomico de Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
⁸ Department of Physics, University of California at Davis, 1 Shields Avenue, Davis, CA 95616, USA
⁹ Institute of Geophysics and Planetary Physics, LLNL, Livermore, CA 94551, USA

Received: 3 November 2003
Accepted: 14 April 2004

Abstract

Using deep VLT/ISAAC near-infrared imaging data, we derive the K_s-band luminosity function (LF) of the massive X-ray luminous cluster of galaxies RDCS J1252.9-2927. Photometric redshifts, derived from deep multi-wavelength 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 and faint end slope α of the LF without having to make an uncertain statistical background subtraction. The derived LF is well represented by the Schechter function with and . 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 mag. The derived α is similar to the value derived in the K_s-band in the cluster of galaxies MG2016+112 but is shallower (at the 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 , while the flattening of the faint end slope suggest that clusters at 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 () and subsequently evolved passively, and a population of lower mass galaxies which are gradually accreted from the field, primarily at lower redshift ().