Baryonic solutions and challenges for cosmological models of dwarf galaxies

Galaxies and their dark-matter haloes have posed several challenges to the dark energy plus cold dark matter (?CDM) cosmological model. This Review discusses the most notable challenges to ?CDM regarding dwarf galaxies and the insights provided by recent cosmological numerical simulations. Galaxies and their dark-matter haloes have posed several challenges to the dark energy plus cold dark matter (?CDM) cosmological model. These discrepancies between observations and theory intensify for the lowest-mass ('dwarf') galaxies. ?CDM predictions for the number, spatial distribution and internal structure of low-mass dark-matter haloes have historically been at odds with observed dwarf galaxies, but this is partially expected, because many predictions modelled only the dark-matter component. Any robust ?CDM prediction must include, hand in hand, a model for galaxy formation to understand how baryonic matter populates and affects dark-matter haloes. In this Review, we consider the most notable challenges to ?CDM regarding dwarf galaxies, and we discuss how recent cosmological numerical simulations have pinpointed baryonic solutions to these challenges. We identify remaining tensions, including the diversity of the inner dark-matter content, planes of satellites, stellar morphologies and star-formation quenching. Their resolution, or validation as actual problems with ?CDM, will probably require both refining of galaxy-formation models and improving numerical accuracy in simulations.

Automated summary

This Review discusses the challenges that galaxies and their dark-matter haloes pose to the dark energy plus cold dark matter (CDM) cosmological model, particularly in relation to dwarf galaxies, and the insights provided by recent cosmological numerical simulations. The discrepancies between observations and CDM predictions for low-mass dark-matter haloes are partially expected due to the lack of a model for galaxy formation. Improved galaxy-formation models and numerical accuracy are needed to resolve the remaining tensions and validate any actual problems with the CDM model.