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

TL;DR

This study uses high-resolution simulations within the standard cosmological model to quantify how non-linear structure formation processes reduce the maximum 21 cm absorption signal during cosmic dawn, emphasizing the importance of these effects in interpreting observations.

Contribution

It provides the first detailed assessment of non-linear effects on the 21 cm absorption feature using hydrodynamic simulations, highlighting a 15% reduction in the maximum absorption level.

Findings

Non-linear effects reduce the absorption by ~15% at redshift 17.

Shock and Compton heating significantly impact the 21 cm signal.

Results are crucial for accurate interpretation of cosmic dawn observations.

Abstract

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.