subhalo

A subhalo refers to a smaller galaxy or structure that orbits within a larger galaxy or dark matter halo. These entities are significant in the study of cosmology and galaxy formation, particularly within the framework of the Lambda Cold Dark Matter (ΛCDM) model, which describes the large-scale structure of the universe.

Subhalos form through hierarchical clustering, where smaller structures merge and evolve over time within the gravitational potential wells of larger systems. In the ΛCDM model, dark matter dominates the mass distribution, and subhalos are embedded within dark matter halos. These substructures can host visible matter, including stars, gas, and dark matter, though many remain dark due to insufficient baryonic content to form observable galaxies.

Observationally, subhalos are challenging to detect directly, especially those without significant stellar populations. However, their presence can be inferred through gravitational lensing effects, stellar streams, or satellite galaxies orbiting within larger galaxies like the Milky Way. Simulations, such as those from the Millennium or Illustris projects, predict a vast population of subhalos, many of which may not correspond to visible galaxies.

Subhalos play a crucial role in understanding galaxy evolution, as they can influence star formation, tidal stripping, and mergers. Some subhalos may become dwarf galaxies, while others may be stripped of their gas and stars, leaving behind only dark matter remnants. The study of subhalos also helps refine models of dark matter distribution and the processes governing galaxy formation in the universe.