Abstract
We present 4.5 {\mu}m luminosity functions for galaxies identified in 178 candidate galaxy clusters at 1.3 < z < 3.2. The clusters were identified as Spitzer/IRAC color-selected overdensities in the Clusters Around Radio-Loud AGN (CARLA) project, which imaged 421 powerful radio-loud AGN at z > 1.3. The luminosity functions are derived for different redshift and richness bins, and the IRAC imaging reaches depths of m*+2, allowing us to measure the faint end slopes of the luminosity functions. We find that {\alpha} = -1 describes the luminosity function very well in all redshifts bins and does not evolve significantly. This provides evidence that the rate at which the low mass galaxy population grows through star formation, gets quenched and is replenished by in-falling field galaxies does not have a major net effect on the shape of the luminosity function. Our measurements for m* are consistent with passive evolution models and high formation redshifts z_f ~ 3. We find a slight trend towards fainter m* for the richest clusters, implying that the most massive clusters in our sample could contain older stellar populations, yet another example of cosmic downsizing. Modelling shows that a contribution of a star-forming population of up to 40% cannot be ruled out. This value, found from our targeted survey, is significantly lower than the values found for slightly lower redshift, z ~ 1, clusters found in wide-field surveys. The results are consistent with cosmic downsizing, as the clusters studied here were all found in the vicinity of radio-loud AGNs -- which have proven to be preferentially located in massive dark matter halos in the richest environments at high redshift -- they may therefore be older and more evolved systems than the general protocluster population.
Highlights
Many attempts have been made to measure the formation epoch of galaxy clusters, generally finding high formation redshifts, zf ∼ 2–4
We study the evolution of the luminosity function of almost 200 galaxy cluster candidates at 1.3 < z < 3.2 discovered through the Clusters Around Radio Loud AGN (CARLA) project (Wylezalek et al 2013)
We assign the redshift of the targeted RLAGN to the Infrared Array Camera (IRAC)-selected sources in the cell, and we study the evolution of these galaxy cluster member candidates as a function of redshift
Summary
Many attempts have been made to measure the formation epoch of galaxy clusters, generally finding high formation redshifts, zf ∼ 2–4. As a few examples, Eisenhardt et al (2008) infer stellar formation redshifts of zf > 4 for cluster galaxies by comparing their I − [3.6] colors to passive galaxy evolution models. The CARLA fields cover an area of ∼5.2 × 5.2, roughly corresponding to a region with a radius of 1–1.5 Mpc for the typical redshift of the RLAGN This radius is in good agreement with sizes of typical mid-IR selected clusters (e.g., Brodwin et al 2011). We placed roughly 500 random, independent (i.e., non-overlapping) apertures with radius r = 1 arcmin onto the SpUDS survey to estimate the typical blank field density of IRAC-selected sources. To determine the mean background, we consider cells in the SpUDS survey with surface densities of IRAC-selected sources in the range 9.6 ± 2.1 arcmin−2 (see Figure 2)
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