Observational Manifestations of the First Protogalaxies in the 21 cm Line

TL;DR

This paper investigates how the 21-cm line absorption features of early low-mass protogalaxies depend on their properties and evolution, providing insights into their detectability with future radio telescopes.

Contribution

It presents a detailed analysis of the absorption properties of first protogalaxies in the 21-cm line, highlighting their dependence on mass and evolutionary stage, and discusses observational prospects.

Findings

Optical depths reach 0.1-0.2 for small impact parameters.

Gas accretion causes non-monotonic optical depth frequency dependence.

Lines from low-mass protogalaxies are resolvable with upcoming interferometers.

Abstract

The absorption properties of the first low-mass protogalaxies (mini-halos) forming at high redshifts in the 21-cm line of atomic hydrogen are considered. The absorption properties of these protogalaxies are shown to depend strongly on both their mass and evolutionary status. The optical depths in the line reach $\sim$0.1-0.2 for small impact parameters of the line of sight. When a protogalaxy being compressed, the influence of gas accretion can be seen manifested in a non-monotonic frequency dependence of the optical depth. The absorption characteristics in the 21-cm line are determined by the thermal and dynamical evolution of the gas in protogalaxies. Since the theoretical line width in the observer's reference frame is 1-6 kHz and the expected separation between lines 8.4 kHz, the lines from low mass protogalaxies can be resolved using ongoing and future low frequency interferometers.