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The absorption feature in the global spectrum is likely the first 21 cmobservable from the cosmic dawn, which provides valuable insights into theearliest history of structure formation. We run a set of high-resolutionhydrodynamic simulations of early structure formation to assess the effect ofnon-linear structure formation on the maximum absorption level (i.e. assumingthe spin temperature coupling is saturated) of the global 21 cm spectrum in thestandard cosmological framework. We ignore the star formation and feedbacks,which also tends to reduce the absorption signal, but take into account theinevitable non-linear density fluctuations in the intergalactic medium (IGM),shock heating, and Compton heating, which can reduce the absorption level. Wefound that the combination of these reduced the maximum absorption signal by$\sim 15\%$ at redshift 17, as compared with the homogeneous or linearlyfluctuating IGM. These effects have to be carefully accounted for wheninterpreting the observational results, especially when considering thenecessity of introducing new physics.

Maximum Absorption of the Global 21 cm Spectrum in the Standard Cosmological Model