Examining the local Universe isotropy with galaxy cluster velocity dispersion scaling relations

¹ Argelander-Institut für Astronomie (AIfA), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
³ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, NL-2333 CA Leiden, The Netherlands
⁴ University of California, Davis, CA 95616, USA
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁶ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via A. Corti 12, 20133 Milano, Italy
⁷ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
⁸ Institut für Astronomie und Astrophysik Tübingen (IAAT), Sand 1, 72076 Tübingen, Germany
⁹ Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea
¹⁰ University of Science and Technology, Daejeon 34113, Republic of Korea

^⋆ Corresponding author; apandya@astro.uni-bonn.de

Received: 1 August 2024
Accepted: 27 September 2024

Abstract

Context. In standard cosmology, the Universe is assumed to be statistically homogeneous and isotropic. This assumption suggests that the expansion rate of the Universe, as measured by the Hubble parameter, should be the same in all directions. However, our recent study based on galaxy clusters finds an apparent angular variation of approximately 9% in the Hubble constant, H₀, across the sky. In the study, the authors utilised galaxy cluster scaling relations between various cosmology-dependent cluster properties and a cosmology-independent property, i.e. the temperature of the intracluster gas (T). A position-dependent systematic bias of T measurements can, in principle, result in an overestimation of apparent H₀ variations. Therefore, it is crucial to confirm or exclude this possibility.

Aims. In this work, we search for directional T measurement biases by examining the relationship between the member galaxy velocity dispersion and gas temperature (σ_v − T) of galaxy clusters. Both measurements are independent of any cosmological assumptions and do not suffer from the same potential systematic biases. Additionally, we search for apparent H₀ angular variations independently of T by analysing the relations between the X-ray luminosity and Sunyaev-Zeldovich signal with the velocity dispersion, L_X − σ_v and Y_SZ − σ_v.

Methods. To study the angular variation of scaling relation parameters, we determined the latter for different sky patches across the extra-galactic sky. We constrained the possible directional T bias using the σ_v − T relation, as well as the apparent H₀ variations using the L_X − σ_v and Y_SZ − σ_v relations. We utilised Monte Carlo simulations of isotropic cluster samples to quantify the statistical significance of any observed anisotropies. We calculated and rigorously took into account a correlation of L_X and Y_SZ residuals.

Results. No significant directional T measurement biases are found from the σ_v − T anisotropy study. The probability that the previously observed H₀ anisotropy is caused by a directional T bias is only 0.002%. On the other hand, from the joint analysis of the L_X − σ_v and Y_SZ − σ_v relations, the maximum variation of H₀ is found in the direction of (295 ° ±71 ° , − 30 ° ±71 ° ) with a statistical significance of 3.64σ, fully consistent with our previous results.

Conclusions. Our findings, based on the analysis of new scaling relations utilising a completely independent cluster property, σ_v, strongly corroborate the previously detected anisotropy of galaxy cluster scaling relations. The underlying cause, for example, H₀ angular variation or large-scale bulk flows of matter, remains to be identified.