A fundamental challenge in modern physics is identifying the nature of dark matter (DM). Nearly a century after this particle's existence was first inferred from its gravitational influence on large cosmic structures, it has yet to be directly detected on Earth.
The leading model for interpreting observations is Λ-Cold Dark Matter (ΛCDM), which has been instrumental for over 40 years in explaining how structures and galaxies form. In this model, "dark energy" (Λ) drives the universe's accelerated expansion, while "cold" dark matter (CDM) governs the gravitational collapse of structures. This model is testable, as it predicts the distribution, structure, and abundance of collapsed halos, the sites where galaxies form. However, these predictions are largely influenced by the complex physics governing galaxy formation and evolution. These issues pose a significant challenge and hamper the steady progress that has defined cosmology over the past decades.
While the comparison between simulation results and observations continues to face scrutiny, it is clear that simulations allow a high degree of freedom to "accommodate" their outcomes to observations. Given these issues, a natural question arises: can competing DM models be tested, constrained, or ruled out through comparisons between observations and simulations of galaxies? The flexibility in galaxy formation models makes it difficult to argue in favour of this.
Our project explored observational probes at scales where galaxies do not form. On these scales, DM-dominated halos’ properties are robust. The existence of these systems is well justified. Observationally, reconciling ΛCDM with the abundance of galaxies requires galaxies to form predominantly in halos above a characteristic mass of about 5 billion solar masses. Theoretically, efficient gas cooling and cosmic reionization dictate a similar mass scale value. ΛCDM predicts myriads of halos below this mass, which, while devoid of stars, should contain neutral hydrogen. The gas in these systems should be in hydrostatic equilibrium with their underlying DM halo and in thermal equilibrium with the external ultraviolet background radiation field. By studying the gas distribution of these systems one can probe the clustering properties of DM, and thus constrain its nature. We shall refer to these systems as REionization-Limited HI Clouds (RELHICs).
