Maximum Absorption of the Global 21 cm Spectrum in the Standard
  Cosmological Model
  The absorption feature in the global spectrum is likely the first 21 cm
observable from the cosmic dawn, which provides valuable insights into the
earliest history of structure formation. We run a set of high-resolution
hydrodynamic simulations of early structure formation to assess the effect of
non-linear structure formation on the maximum absorption level (i.e. assuming
the spin temperature coupling is saturated) of the global 21 cm spectrum in the
standard cosmological framework. We ignore the star formation and feedbacks,
which also tends to reduce the absorption signal, but take into account the
inevitable non-linear density fluctuations in the intergalactic medium (IGM),
shock heating, and Compton heating, which can reduce the absorption level. We
found that the combination of these reduced the maximum absorption signal by
$\sim 15\%$ at redshift 17, as compared with the homogeneous or linearly
fluctuating IGM. These effects have to be carefully accounted for when
interpreting the observational results, especially when considering the
necessity of introducing new physics.
