The Hα luminosity function of the galaxy cluster A521 at z = 0.25

We present an optical multicolor-imaging study of the galaxy cluster A521 at z = 0.25, using Suprime-Cam on the Subaru Telescope, covering an area of 32′× 20′ (9.4 × 5.8 h₅₀^-2 Mpc ² at z = 0.25). Our imaging data taken with both a narrowband filter, NB816 (λ₀ = 8150 Å and Δλ. = 120 Å), and broadband filters, B, V, R_C, i′, and z′, allow us to find 165 Hα emitters. We obtain the Hα luminosity function (LF) for the cluster galaxies within 2 Mpc; the Schechter parameters are α = -0.75 ± 0.23, φ* = 10^-0.45 to 10^-0.05 Mpc^-3, and L* = 10^41.86 to 10^42.2 ergs s^-1. Although the faint-end slope α is consistent with that of the local cluster Hα LFs, the characteristic luminosity L* is about 6 times (or ≈2 mag) brighter. This strong evolution implies that A521 contains more active star-forming galaxies than the local clusters, being consistent with the observed Butcher-Oemler effect. However, the bright L* of A521 may be, at least in part, due to the dynamical condition of this cluster.