Dissecting the RELICS cluster SPT-CLJ0615–5746 through intracluster light: Confirmation of the multiple merging state of the cluster formation

¹ Instituto de Astrofísica de Andalucía–CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: yojite@iaa.es
² Observatório Nacional, Rua General José Cristino, 77 – Bairro Imperial de São Cristóvão, Rio de Janeiro 20921-400, Brazil
³ Eureka Scientific Inc., 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
⁴ Department of Astronomy, University of Michigan, 311 West Hall, 1085 South University Ave., Ann Arbor, MI 48109-1107, USA
⁵ Observatório do Valongo, Universidade Federal do Rio de Janeiro, Ladeira do Pedro Antônio 43, Rio de Janeiro, RJ 20080-090, Brazil

Received: 3 April 2023
Accepted: 5 June 2023

Abstract

The intracluster light (ICL) fraction, measured at certain specific wavelengths, has been shown to provide a good marker for determining the dynamical stage of galaxy clusters, that is, merging versus relaxed, for low to intermediate redshifts. We apply it for the first time to a high-redshift system, SPT-CLJ0615–5746 at z = 0.97, using its Reionization Lensing Cluster Survey (RELICS) observations in the optical and infrared. We find the ICL fraction signature of the merging, with values ranging from 16 to 37%. A careful reanalysis of the X-ray data available for this cluster indicates at least one current and plausibly a second merger. These two results contradict previous works based on X-ray data, which claimed a relaxed state for SPT-CLJ0615–5746, and the results confirm the evidence presented by kinematic analyses. We also found an abnormally high ICL fraction in the rest-frame near-ultraviolet wavelengths, which may be attributed to the combination of several phenomena such as an ICL injection during recent mergers of stars with average early-type spectra, the reversed star formation-density relation found at this high redshift in comparison with lower-redshift clusters, and projection effects.