2020MNRAS.495.1700L

We demonstrate a new approach of indirectly constraining both early star and structure formation via mature galaxy clusters at cosmic noon (z ∼ 2), using the cluster XLSSC 122 as an example. With the standard Press Schechter formalism, we infer a rapid evolution of the star formation efficiency (the ratio of stellar to halo mass) from 10^–4 to 0.01 during z ∼ 20-13, based on the age distribution of stars in post-starburst galaxies of XLSSC 122, measured by Hubble Space Telescope photometry assuming no dust extinction. Here, we consider all low-mass haloes, including minihaloes, that host the first stars and galaxies (5×10⁵M_☉≲M_ halo_≲10¹⁰ M_☉). We also place new constraints on fuzzy dark matter models of m_a <= 5 x 10^–21 eV/c² for the ultralight boson mass, from the abundance of galaxies with star formation at z >= 13 in XLSSC 122. Our exploratory results are consistent with existing constraints. More comprehensive results will be obtained if our approach is extended to a large sample of clusters or field post-starburst galaxies at cosmic noon, with improved modelling of halo and stellar populations.