Searching for galaxy clusters in the Kilo-Degree Survey^⋆

¹ INAF–Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: mario.radovich@oapd.inaf.it
² INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
³ Dipartimento di Fisica e Astronomia, Alma Mater Studiorum – Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁴ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INFN – Sezione di Bologna, viale Berti-Pichat 6/2, 40127 Bologna, Italy
⁶ Zentrum für Astronomie, Universitatät Heidelberg, Philosophenweg 12, 69120 Heidelberg, Germany
⁷ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 20 July 2016
Accepted: 23 December 2016

Abstract

Aims. In this paper, we present the tools used to search for galaxy clusters in the Kilo Degree Survey (KiDS), and our first results.

Methods. The cluster detection is based on an implementation of the optimal filtering technique that enables us to identify clusters as over-densities in the distribution of galaxies using their positions on the sky, magnitudes, and photometric redshifts. The contamination and completeness of the cluster catalog are derived using mock catalogs based on the data themselves. The optimal signal to noise threshold for the cluster detection is obtained by randomizing the galaxy positions and selecting the value that produces a contamination of less than 20%. Starting from a subset of clusters detected with high significance at low redshifts, we shift them to higher redshifts to estimate the completeness as a function of redshift: the average completeness is ~85%. An estimate of the mass of the clusters is derived using the richness as a proxy.

Results. We obtained 1858 candidate clusters with redshift 0 <z_c< 0.7 and mass 10^13.5 < M₅₀₀ < 10¹⁵ M_⊙ in an area of 114 sq. degrees (KiDS ESO-DR2). A comparison with publicly available Sloan Digital Sky Survey (SDSS)-based cluster catalogs shows that we match more than 50% of the clusters (77% in the case of the redMaPPer catalog). We also cross-matched our cluster catalog with the Abell clusters, and clusters found by XMM and in the Planck-SZ survey; however, only a small number of them lie inside the KiDS area currently available.