Galaxy clusters at 0.9 < z < 1.7 in the AKARI NEP deep field

Abstract

There is a huge gap between the properties of red-sequence selected massive galaxy clusters at z < 1 and Lyman-break selected proto-clusters at z > 3. It is important to understand when and how the z > 3 proto-clusters evolve into passive clusters at z < 1. We aim to fill this cluster desert by using space-based N4 (4 μm) imaging with AKARI. The z′- N4 color is a powerful separator of cluster galaxies at z > 1, taking advantage of the 4000 Å break and the 1.6 μm bump. We carefully selected 16 promising cluster candidates at 0.9 < z < 1.7, which all show an obvious over-density of galaxies and a prominent red-sequence. At this redshift range, the mid-infrared S15μm/S9μm flux ratio is an extinction-free indicator of galaxy star-formation activity due to the redshifted PAH emission lines (6.2, 7.7, and 8.6 μm). We show statistically that the cluster galaxies have a lower S15μm/S9μm flux ratio than do field galaxies, i.e., cluster galaxies already have lower star-formation activity at 0.9 < z < 1.7, pushing the formation epoch of these galaxy clusters to a higher redshift. © 2008. Astronomical Society of Japan.