The dwarf galaxy population in Abell 2218

We present results from a deep photometric study of the rich galaxy cluster Abell 2218 (z = 0.18) based on archival Hubble Space Telescope WFPC2 F606W images. These have been used to derive the luminosity function to extremely faint limits (M_F606W ≈ -13.2 mag, μ₀ ∼ 24.7 mag arcsec^-2) over a wide field of view (1.3 h^-2 Mpc ²). We find the faint-end slope of the luminosity function to vary with environment within the cluster, going from α = -1.23 ± 0.13 within the projected central core of the cluster (100 < r < 300 h ^-1 kpc) to α = -1.49 ± 0.06 outside this radius (300 < r < 750 h^-1 kpc). We infer that the core is 'dwarf depleted', and further quantify this by studying the ratio of 'dwarf' to 'giant' galaxies and its dependency as a function of clustercentric radius and local galaxy density. We find that this ratio varies strongly with both quantities, and that the dwarf galaxy population in A2218 has a more extended distribution than the giant galaxy population.