High star formation activity in the central region of a distant cluster at z=1.46
We present an unbiased deep [OII] emission survey of a cluster XMMXCS J2215.9-1738 at z=1.46, the most distant cluster to date with a detection of extended X-ray emission. With wide-field optical and near-infrared cameras (Suprime-Cam and MOIRCS, respectively) on Subaru telescope, we performed deep imaging with a narrow-band filter NB912 (lambda_c=9139A, Delta_lambda=134A) as well as broad-band filters (B, z', J and Ks). From the photometric catalogues, we have identified 44 [OII] emitters in the cluster central region of 6'x6' down to a dust-free star formation rate of 2.6 Msun/yr (3 sigma). Interestingly, it is found that there are many [OII] emitters even in the central high density region. In fact, the fraction of [OII] emitters to the cluster members as well as their star formation rates and equivalent widths stay almost constant with decreasing cluster-centric distance up to the cluster core. Unlike clusters at lower redshifts (z<1) where star formation activity is mostly quenched in their central regions, this higher redshift 2215 cluster shows its high star formation activity even at its centre, suggesting that we are beginning to enter the formation epoch of some galaxies in the cluster core eventually. Moreover, we find a deficit of galaxies on the red sequence at magnitudes fainter than ~M*+0.5 on the colour-magnitude diagram. This break magnitude is brighter than that of lower redshift clusters, and it is likely that we are seeing the formation phase of more massive red galaxies in the cluster core at z~1. These results may indicate inside-out and down-sizing propagation of star formation activity in the course of cluster evolution.