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

TL;DR

This study identifies and analyzes galaxy clusters at redshifts 0.9 to 1.7 using AKARI infrared imaging, revealing that cluster galaxies exhibit lower star formation activity than field galaxies, indicating earlier formation epochs.

Contribution

First to use AKARI N4 imaging to select and confirm galaxy clusters at 0.9<z<1.7, demonstrating lower star formation activity in cluster galaxies at these redshifts.

Findings

Cluster galaxies have lower S15um/S9um flux ratios than field galaxies.

Cluster galaxies show reduced star formation activity at 0.9<z<1.7.

The formation epoch of these clusters is at higher redshift than previously thought.

Abstract

There is a huge gap between 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 the space-based N4(4um) imaging with the AKARI. The z'-N4 color is a powerful separator of cluster galaxies at z>1, taking advantage of the 4000A break and the 1.6um bump. We carefully selected 16 promising cluster candidates at 0.9<z<1.7, which all show obvious over-density of galaxies and a prominent red-sequence. At this redshift range, the mid-infrared S15um/S9um flux ratio is an extinction-free indicator of galaxy star formation activity due to the redshifted PAH emission lines (6.2,7.7 and 8.6um). We show statistically that the cluster galaxies have a lower S15um/S9um flux ratio than 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.