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GR: Fachverband Gravitation, Relativistische Astrophysik und Kosmologie

GR 10: Poster

GR 10.8: Poster

Wednesday, March 18, 2026, 16:15–18:45, Redoutensaal

Probing Small-Scale Structure during Cosmic Dawn with the 21 cm Forest — •Monja Begau, Mihir Kulkarni, and Jens Niemeyer — Institute for Astrophysics and Geophysics, Göttingen, Germany

Observations of the 21 cm hyperfine transition of neutral hydrogen are expected to significantly improve our understanding of the thermal and structural evolution of the intergalactic medium during cosmic dawn. The 21 cm signal measures deviations of the hydrogen spin temperature from the cosmic microwave background, leading to absorption or emission. In addition to large-scale brightness-temperature fluctuations, neutral hydrogen along individual lines of sight can imprint narrow absorption features in the spectra of distant radio-loud sources. In analogy to the Lyman-α forest, this is referred to as the 21 cm forest. These absorption features probe small-scale density structure.

Modelling the 21 cm signal is challenging due to the nonlinear and nonlocal relation between matter density, star formation, and the resulting radiative backgrounds. In this work, we employ and extend the Zeus21 framework (arXiv:2302.08506v2, Julian B Munoz), an analytic model for the 21 cm signal during cosmic dawn.This framework is applied here to the description of the 21 cm forest. Since axion dark matter can modify structure formation on small scales, such effects may be reflected in the statistics of the 21 cm forest. We explore the potential of this approach to probe axion-like dark-matter scenarios through modifications of the predicted 21 cm forest power spectrum.

Keywords: 21cm signal; 21cm forest; axion dark matter; structure formation; cosmic dawn