The formation and evolution of galaxies have long been a key focus of cosmology, ever since Hubble demonstrated that spiral nebulae are external stellar systems comparable in size to the Milky Way. Theories regarding galaxy formation have varied, from the monolithic collapse of giant gas clouds to the assembly through the merger of numerous protogalactic fragments.
In Λ Cold Dark Matter ( Λ CDM), galaxies form within dark matter halos originating from primordial density fluctuations that start small and grow over time. Massive halos, and the galaxies they host, form through the merger of smaller halos. This hierarchical structure formation is often illustrated as a “merger tree“. Modern giant galaxies are thus the result of the assembly of numerous protogalactic fragments.
Galaxy Formation Models
Modified gravity might have played a role. A theory known as MOND(Modified Newtonian Dynamics), predicted in 1998 that structure formation in the early Universe would have happened very quickly much faster than the theory of cold dark matter, known as lambda-CDM, said by Case Western Reserve’s Professor Stacy McGaugh.
It is essential to compare and contrast different hypotheses, so we examine both monolithic and hierarchical galaxy formation models. The passive evolution of an early-formed monolithic galaxy is inspired by traditional interpretations of the evolutionary history of giant ellipticals. In contrast, hierarchical galaxy formation aligns with Λ CDM, with testable predictions supported by semi-analytic galaxy formation models (SAMs) and hydrodynamical simulations. Though simplified, the monolithic model serves as a useful benchmark for predictions.
Hierarchical Λ CDM Models
In Λ CDM, hierarchical galaxy formation involves in situ star formation within the largest progenitors and ex-situ growth through mergers. The largest galaxies are predicted to form later, as their assembly takes longer. While it might appear contradictory that large galaxies contain the oldest stars, this can be explained by the early formation of many stars in protogalaxies, which later merged to form the final giant galaxy. Therefore, many small precursor galaxies are expected at high redshifts, preceding today’s giant galaxies.
Numerous papers address galaxy evolution, including theoretical works that predict the relationship between observable galaxies and their parent dark matter halos. Despite model-specific differences, the fundamental prediction of hierarchical mass accumulation remains consistent across models as a core aspect of the Λ CDM structure formation paradigm. This is demonstrated in various simulations, such as the IllustrisTNG suite of magnetohydrodynamic simulations.
Monolithic Models
MOND proposed that the mass forming a galaxy assembles quickly and initially expands outward along with the rest of the Universe.
A key concept in galaxy evolution is the passive evolution of a monolithic “island universe.” This hypothesis assumes that nearly all the mass currently in a galaxy has always been part of it, evolving as a closed system since a formation redshift z f. While this provides a straightforward framework, it is an unrealistic oversimplification. Galaxy-mass accumulations of gas cannot spontaneously form in the early universe; mass must be gathered from the nearly homogeneous initial conditions of the early universe.
The large and bright structures seen by the James Webb Space Telescope (JWST) and very early in the Universe were predicted by MOND over a quarter century ago, McGaugh said. JWST was designed to answer some of the biggest questions in the universe, such as how and when stars and galaxies form. Until it was launched in 2021, no telescope was able to see that deeply into the universe and far back in time.
The Future of Understanding Galaxy Formation
The study of galaxy formation continues to be a crucial area in cosmology, with hierarchical and monolithic models providing different perspectives on how galaxies have evolved. While monolithic models offer a simplified view of early, isolated formation, hierarchical models in Λ CDM provide a more dynamic and evidence-supported approach, aligning with observations of galaxies forming through the merging of smaller structures. The advent of advanced tools enables us to observe these processes in unprecedented detail, helping refine our understanding of galaxy formation and evolution.
