A pair of early- and late-forming galaxy cluster samples: A novel way of studying halo assembly bias assisted by a constrained simulation^⋆

¹ Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), Taipei 10617, Taiwan
e-mail: ytl@asiaa.sinica.edu.tw
² Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Nagoya University, Nagoya 464-8602, Japan
³ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Chiba 277-8583, Japan
⁴ Shanghai Astronomical Observatory, Shanghai 200030, PR China
⁵ Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁶ Graduate Institute of Astrophysics and Department of Physics, National Taiwan University, Taipei 10617, Taiwan
⁷ Department of Physics, National Chung Hsing University, Taichung 402, Taiwan
⁸ Institute for Advanced Research, Nagoya University, Nagoya 464-8601, Japan

Received: 3 July 2022
Accepted: 1 August 2022

Abstract

The halo assembly bias, a phenomenon referring to dependencies of the large-scale bias of a dark matter halo other than its mass, is a fundamental property of the standard cosmological model. First discovered in 2005 from the Millennium Run simulation, it has been proven very difficult to be detected observationally, with only a few convincing claims of detection so far. The main obstacle lies in finding an accurate proxy of the halo formation time. In this study, by utilizing a constrained simulation that can faithfully reproduce the observed structures larger than 2 Mpc in the local universe, for a sample of 634 massive clusters at z ≤ 0.12, we found their counterpart halos in the simulation and used the mass growth history of the matched halos to estimate the formation time of the observed clusters. This allowed us to construct a pair of early- and late-forming clusters, with a similar mass as measured via weak gravitational lensing, and large-scale biases differing at the ≈3σ level, suggestive of the signature of assembly bias, which is further corroborated by the properties of cluster galaxies, including the brightest cluster galaxy and the spatial distribution and number of member galaxies. Our study paves a way to further detect assembly bias based on cluster samples constructed purely on observed quantities.