Deep Near‐Infrared Luminosity Function of a Cluster of Galaxies atz = 0.3

The deep near-infrared luminosity function of AC 118, a cluster of galaxies at z = 0.3, is presented. AC 118 is a bimodal cluster, as evidenced both by our near-infrared images of lensed galaxies, by public X-ray ROSAT images, and by the spatial distribution of bright galaxies. Taking advantage of the extension and depth of our data, which sample an almost unexplored region in the depth versus observed area diagram, we derive the luminosity function (LF), down to the dwarf regime ( M * + 5), computed in several cluster portions. The overall LF, computed on a 2.66 Mpc 2 areas ( H 0 = 50 km s -1 Mpc -1 ), has an intermediate slope (α = -1.2). However, the LF parameters depend on the surveyed cluster region: the central concentration has 2.6 times more bright galaxies and 5.3 times less dwarfs per typical galaxy than the outer region, which includes galaxies at an average projected distance of ~580 kpc (errors are quoted at the 99.9% confidence level). The LF in the secondary AC 118 clump is intermediate between the central and outer one. In other words, the near-infrared AC 118 LF steepens going from high to low-density regions. At an average clustercentric distance of ~580 kpc, the AC 118 LF is statistically indistinguishable from the LF of field galaxies at similar redshift, thus suggesting that the hostile cluster environment plays a minor role in shaping the LF at large clustercentric distances, while it strongly affects the LF at higher galaxy density.