Dwarf galaxy luminosity functions and cluster environments

We report an investigation of the luminosity function (LF) of dwarf galaxies (Mi <-15) inside 33 clusters at z ∼0.15-0.3, to address the crucial but unsettled problem of the faint-end slope (FS) in the cluster environments that may hold a key to the missing-satellite problem related to the standard cold dark matter formation scenario. By using a wide field imager mounted on the 8 m-class telescope, Subaru Suprime-Cam, and by selecting clusters in an optimal redshift range, we achieve a crucial balance between the depth and the spatial coverage of observation, that is essential for cluster dwarf studies. The deep and wide observations also provide us with image-by-image local background correction and tailor-made detection incompleteness correction on every cluster and background fields. These two essential but intertwined corrections were unfortunately often ignored in the previous studies. We find, when the accurate background and detection incompleteness corrections are made in a combined manner, that the faint end of the cluster LF shows a relatively flat slope (α ∼-1.2 to-1.4), comparable to the field LF. We also find: (1) FS becomes steeper at larger cluster-centric distance. (2) Blue dwarf galaxies tend to show steeper FS. (3) Galaxy LFs show steeper slopes inside clusters with high X-ray luminosity although cluster-to-cluster variations are significant. Investigations of the FS are sensitive to various systematics, and variations among individual clusters that we need to address carefully. With a slope of-1.2 to-1.4, our study does not show the excess of dwarfs previously reported in some observational studies. Our findings may indicate the missing-satellite problem remains in galaxy clusters.