The SRG/eROSITA All-Sky Survey

Constraints on ultralight axion dark matter through galaxy cluster number counts

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
² Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
³ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, I-40129 Bologna, Italy
⁴ Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794, USA
⁵ Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁶ Department of Physics, National Cheng Kung University, 70101 Tainan, Taiwan
⁷ IRAP, Université de Toulouse, CNRS, UPS, CNES, F-31028 Toulouse, France
⁸ University of Cambridge, Cavendish Laboratory and DAMTP, Cambridge CB3 0WA, United Kingdom
⁹ Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Nagoya University, Nagoya 464-8602, Japan
¹⁰ Institute for Advanced Research, Nagoya University, Nagoya 464-8601, Japan
¹¹ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study (UTIAS), The University of Tokyo, Chiba 277-8583, Japan
¹² Subaru Telescope, National Astronomical Observatory of Japan, 650 N Aohoku Place, Hilo, HI 96720, USA
¹³ Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
¹⁴ Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, CH-1290 Versoix, Switzerland
¹⁵ Universitäts-Sternwarte, Faculty of Physics, LMU Munich, Scheinerstr. 1, 81679 München, Germany

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Received: 5 February 2025
Accepted: 5 October 2025

Abstract

Ultralight axions are hypothetical scalar particles that influence the evolution of large-scale structures of the Universe. Depending on their mass, they can potentially be part of the dark matter component of the Universe as candidates commonly referred to as fuzzy dark matter. While strong constraints have been established for pure fuzzy dark matter models, the more general scenario where ultralight axions constitute only a fraction of the dark matter has been limited to only a few observational probes. In this work, we use the galaxy cluster number counts obtained from the first All-Sky Survey (eRASS1) of the SRG/eROSITA mission together with gravitational weak lensing data from the Dark Energy Survey, the Kilo-Degree Survey, and the Hyper Suprime-Cam to constrain the fraction of ultralight axions in the mass range 10⁻³² eV to 10⁻²⁴ eV. We put upper bounds on the ultralight axion relic density Ω_a in independent logarithmic axion mass bins by performing a full cosmological parameter inference. We find an exclusion region in the intermediate ultralight axion mass regime with the tightest bounds reported so far in the mass bins around m_a = 10⁻²⁷ eV with Ω_a < 0.0035 and m_a = 10⁻²⁶ eV with Ω_a < 0.0079; both are at a 95% confidence level. When combined with cosmic microwave background probes, these bounds are tightened to Ω_a < 0.0030 in the m_a = 10⁻²⁷ eV mass bin and Ω_a < 0.0058 in the m_a = 10⁻²⁶ eV mass bin, with both at a 95% confidence level. This is the first time that constraints on ultralight axions have been obtained using the growth of structure measured by galaxy cluster number counts. These results pave the way for large surveys, which can be utilized to obtain tight constraints on the mass and relic density of ultralight axions with better theoretical modeling of the abundance of halos.