The Galaxy Population of Cl1601+42 at z=0.54

Abstract

Photometric redshifts are used to determine the rest frame luminosity function (LF) of both early-type and late-type galaxies to M_B ~ -17.6 for the cluster Cl1601+42 at z = 0.54. The total LF shows a steep faint-end slope alpha \~ -1.4, indicating the existence of a numerous population of dwarf galaxies. Luminous galaxies, with M_B < -19.5 are mostly red, early-type galaxies, with a LF best described by a Gaussian. Faint galaxies are predominantly blue, late-type galaxies, well fitted by a Schechter function with alpha ~ -1.7. Compared to clusters at lower redshift, the steepening of the faint end starts at brighter magnitudes for Cl1601+42, which may indicate a brightening of todays dwarf population relative to the giant population with increasing redshift. Early-type galaxies are centrally concentrated, and dominate the core region, implying that the radial gradient of early-type galaxies seen in local clusters is already established at z ~ 0.5. Bright, late-type galaxies are rare, consistent with a decrease in star formation in field galaxies as they are accreted on to the cluster, while faint, blue galaxies are evenly distributed across the cluster, except for a depletion in the core region. The blue fraction is f_B ~ 0.15, which is somewhat lower than the Butcher-Oemler average at z ~ 0.5. The value of f_B is found to increase with limiting magnitude and with radius from the centre.