eROSITA Survey Constrains Ultra-light Axion Dark Matter

This paper presents constraints on ultra-light axion dark matter using galaxy cluster number counts obtained from the SRG/eROSITA all-sky survey. The research focuses on the scenario where ultra-light axions constitute only a fraction of the dark matter, exploring axion masses in the range of $10^{-32}$ eV to $10^{-24}$ eV. By combining eROSITA data with gravitational weak lensing data from other surveys, the study places upper bounds on the ultra-light axion relic density $\Omega_\mathrm{a}$ in independent logarithmic axion mass bins. The tightest bounds are reported in the mass bins around $m_\text{a} = 10^{-27}$ eV and $m_\text{a} = 10^{-26}$ eV. These results represent the first constraints on ultra-light axions obtained using the growth of structure measured by galaxy cluster number counts. The findings pave the way for future large surveys to obtain tighter constraints on the mass and relic density of ultra-light axions, potentially leading to a better understanding of the nature of dark matter. However, improved theoretical modeling of the abundance of halos is crucial for maximizing the potential of these surveys.

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