Lessons on early structure formation from a mature galaxy cluster
  observed at cosmic noon
  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 HST
photometry assuming no dust extinction. Here, we consider all low-mass haloes,
including minihaloes, that host the first stars and galaxies (5*10^5 Msun <
M_halo < 10^10 Msun). We also place new constraints on fuzzy dark matter models
of m_a < 5*10^-21 eV/c^2 for the ultra-light 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.
