The Halpha Luminosity Function of the Galaxy Cluster Abell 521 at z = 0.25

We present an optical multicolor-imaging study of the galaxy cluster Abell 521 at $z=0.25$, using Suprime-Cam on the Subaru Telescope, covering an area of $32\times 20$ arcmin${}^2$ ($9.4\times 5.8h_{50}^{-2}$ Mpc${}^2$ at $z = 0.25$)\mathrm{.}Ourimagingdatatakenwithbothanarrow-bandfilter,$NB816$($\lambda_0 = 8150$\mathring{A}and$\Delta \lambda = 120$\mathring{A}),andbroad-bandfilters,$B,V,R_{\rm C}, i^\prime$,and$z^\prime$allowustofind165H$\alpha$emitters\mathrm{.}WeobtaintheH$\alpha$ luminosity function (LF) for the cluster galaxies within 2 Mpc; the Schechter parameters are $\alpha =-0.75\pm 0.23$, ${\varphi }^{\star }={10}^{-0.25\pm 0.20}$ Mpc${}^{-3}$, and $L^\star = 10^{42.03 \pm 0.17}$ergs$^{-1}$\mathrm{.}Althoughthefaintendslope,$\alpha$, is consistent with that of the local cluster H$\alpha$ LFs, the characteristic luminosity, $L^{\star }$, is about 6 times (or $\approx 2$ mag) brighter. This strong evolution implies that Abell 521 contains more active star-forming galaxies than the local clusters, being consistent with the observed Butcher-Oemler effect. However, the bright $L^{\star }$ of Abell 521 may be, at least in part, due to the dynamical condition of this cluster.