The SRG/eROSITA all-sky survey: Constraints on Ultra-light Axion Dark Matter through Galaxy Cluster Number Counts

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 $\hat{I}{\text{c◯}}_{\mathrm{a}}$ 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 $m_{\text{a}}=10^{-27}$ eV with $\hat{I}{\text{c◯}}_{\text{a}}<0.0035$ and $m_{\text{a}}=10^{-26}$ eV with $\hat{I}{\text{c◯}}_{\text{a}}<0.0079$ (95% C.L.). When combined with cosmic microwave background probes, these bounds are tightened to $\hat{I}{\text{c◯}}_{\text{a}}<0.0030$ in the $m_{\text{a}}=10^{-27}$ eV mass bin and $\hat{I}{\text{c◯}}_{\text{a}}<0.0058$ in the $m_{\mathrm{a}}=10^{-26}$ eV mass bin (95% C.L.). 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.

18 pages, 13 figures, Accepted for publication by A&A