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Star Formation in the XMMU J2235.3-2557 Galaxy Cluster at z=1.39

We present the first results of a narrow-band photometric study of the massive galaxy cluster XMMU J2235.3-2557 at z=1.39. We obtained deep $H$ narrow-band imaging with NIRI on Gemini North, corresponding to H-alpha emission at the cluster's redshift. Our sample consists of 82 galaxies within a radius of ~500 kpc, ten of which are spectroscopically confirmed cluster members. Sixteen galaxies are identified as excess line-emitters. Among just the excess line-emitting galaxies we find an average SFR of 3.6 +/- 1.3 Msun/yr. For spectroscopically confirmed cluster members we find a correlation between H broad-band magnitude and SFR such that brighter galaxies have lower SFRs. The probability that SFR and magnitude of confirmed members are uncorrelated is 0.7%. We also find a correlation between SFR and distance from the cluster centre for both confirmed and excess line-emitting candidate members, with a probability of 5% for there to be no correlation among confirmed members. All excess line-emitting candidate cluster members are located outside a radius of 200 kpc. We conclude that star formation is effectively shut off within the central 200 kpc radius (R_QUENCH ~ 200 kpc) of this massive galaxy cluster at z=1.39, when the universe was only 4.5 Gyr old.