Galaxy Clusters at 0.9 < z < 1.7 in the AKARI NEP Deep Field
Goto, Tomotsugu; Inami, Hanae; Nakagawa, Takao; Oyabu, Shinki; Ohyama, Youichi; Im, Myungshin; Matsuhara, Hideo; Lee, Hyung Mok; Takagi, Toshinobu; Wada, Takehiko; Hanami, Hitoshi; Imai, Koji; Pearson, Chris P.; Ishigaki, Tsuyoshi
United States, Japan, South Korea, Taiwan, Spain
Abstract
There is ahuge 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 apowerful separator of cluster galaxies at z > 1, taking advantage of the 4000Å 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 aprominent 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 alower 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.