The 2dF Galaxy Redshift Survey: the luminosity function of cluster galaxies

Abstract

We have determined the composite luminosity function (LF) for galaxies in 60 clusters from the 2dF Galaxy Redshift Survey. The LF spans the range −22.5 < M_(bj) < −15, and is well fitted by a Schechter function with M∗_(bJ) = −20.07 ± 0.07 and α = −1.28 ± 0.03 (H₀ = 100 km s⁻¹ Mpc⁻¹, Ω_M = 0.3, Ω_Λ = 0.7). It differs significantly from the field LF, having a characteristic magnitude that is approximately 0.3 mag brighter and a faint-end slope that is approximately 0.1 steeper. There is no evidence for variations in the LF across a wide range of cluster properties: the LF is similar for clusters with high and low velocity dispersions, for rich and poor clusters, for clusters with different Bautz–Morgan types, and for clusters with and without substructure. The core regions of clusters differ from the outer parts, however, in having an excess of very bright galaxies. We also construct the LFs for early (quiescent), intermediate and late (star-forming) spectral types. We find that, as in the field, the LFs of earlier-type galaxies have brighter characteristic magnitudes and shallower faint-end slopes. However, the LF of early-type galaxies in clusters is both brighter and steeper than its field counterpart, although the LF of late-type galaxies is very similar. The trend of faint-end slope with spectral type is therefore much less pronounced in clusters than in the field, explaining why variations in the mixture of types do not lead to significant differences in the cluster LFs. The differences between the field and cluster LFs for the various spectral types can be qualitatively explained by the suppression of star formation in the dense cluster environment, together with mergers to produce the brightest early-type galaxies.