Speaker
Description
Axion-like particles are viable dark matter candidates that would fit naturally in the concordance $\Lambda$CDM cosmological model. As bosonic particles, they form Bose-Einstein condensates with scales determined by their thermal de Broglie wavelength. Due to their extremely small masses, ultra-light axions with $10^{-22}$ eV masses can form condensates on scales comparable to dark matter halos. If their mass is even smaller, around $10^{-33}$ eV, they exhibit a slow roll behavior and effectively behave like dark energy. Both the dark matter and dark energy regimes of axionlike particles can be constrained by using clusters of galaxies as tracers for the highest peaks in the late-time matter density field. With its 5259 securely detected and optically confirmed galaxy clusters, eROSITA on board the SRG Mission, which was launched in 2019, offers the possibility of precision cosmology by using the halo mass function. We use the observed cluster abundance to constrain a cosmology including an ultra-light axion species with a certain mass and dark matter abundance. Selection effects are fully accounted for, while the mass calibration is performed with the weak gravitational lensing data from the DES, KiDS, and HSC surveys. I will present the constraints on ultra-light axion mass and abundance as well as the cosmological parameters obtained using the first All-Sky Survey of eROSITA in the Western Galactic Hemisphere.
