Discovery of a Rich Cluster at z = 1.63 using the Rest-Frame 1.6um "Stellar Bump Sequence" Method

TL;DR

The paper introduces the Stellar Bump Sequence (SBS) method, a new two-color algorithm for identifying high-redshift galaxy clusters using optical and mid-infrared data, successfully discovering a cluster at z=1.63.

Contribution

The paper presents the SBS algorithm, combining optical and MIR colors to robustly detect high-redshift clusters and demonstrates its effectiveness with new cluster discovery and validation.

Findings

SBS effectively identifies high-redshift clusters in wide-field data.

The stellar bump sequence color correlates with redshift, enabling accurate photometric redshifts.

Discovery of a new cluster at z=1.63 confirmed with spectroscopic redshifts.

Abstract

We present a new two-color algorithm, the "Stellar Bump Sequence" (SBS), that is optimized for robustly identifying candidate high-redshift galaxy clusters in combined wide-field optical and mid-infrared (MIR) data. The SBS algorithm is a fusion of the well-tested cluster red-sequence method of Gladders & Yee (2000) with the MIR 3.6um - 4.5um cluster detection method developed by Papovich (2008). As with the cluster red-sequence method, the SBS identifies candidate overdensities within 3.6um - 4.5um color slices, which are the equivalent of a rest-frame 1.6um stellar bump "red-sequence". In addition to employing the MIR colors of galaxies, the SBS algorithm incorporates an optical/MIR (z' - 3.6um) color cut. This cut effectively eliminates foreground 0.2 < z < 0.4 galaxies which have 3.6um - 4.5um colors that are similarly red as z > 1.0 galaxies and add noise when searching for high-redshift galaxy overdensities. We demonstrate using the z ~ 1 GCLASS cluster sample that similar to the red sequence, the stellar bump sequence appears to be a ubiquitous feature of high-redshift clusters, and that within that sample the color of the stellar bump sequence increases monotonically with redshift and provides photometric redshifts accurate to dz = 0.05. We apply the SBS method in the XMM-LSS SWIRE field and show that it robustly recovers the majority of confirmed optical, MIR, and X-ray-selected clusters at z > 1.0 in that field. Lastly, we present confirmation of SpARCS J022427-032354 at z = 1.63, a new cluster detected with the method and confirmed with 12 high-confidence spectroscopic redshifts obtained using FORS2 on the VLT. We conclude with a discussion of future prospects for using the algorithm.