Galaxy formation and structure
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Galaxy Formation: Cold Dark Matter, Gas Accretion, and Mergers
The current understanding of galaxy formation is rooted in the cold dark matter (CDM) paradigm. In this framework, galaxies form as baryonic matter condenses within dark matter halos that arise from the gravitational collapse of density perturbations in the early universe. The process begins with the cooling and condensation of gas, followed by star formation, and is influenced by both internal processes (like feedback from stars and supernovae) and external events (such as mergers and accretion of smaller systems) 458. Simulations and theoretical models show that the structure of galaxies—such as their rotation curves and mass distributions—naturally emerge from these processes, with baryons playing a significant role in the central regions of galaxies but not dominating overall .
Two Phases of Galaxy Growth: In Situ and Ex Situ Star Formation
Galaxy assembly occurs in two main phases. The first, at high redshift (z > 2), is dominated by rapid in situ star formation from infalling cold gas. The second, extending to lower redshifts, is characterized by the accretion of stars formed in smaller satellite systems (ex situ growth). This two-phase model explains why massive galaxies tend to have older stellar populations: most of their stars formed early in smaller structures, which later merged to build up the galaxy . The importance of accretion increases with galaxy mass, and for the most massive galaxies, most of their stellar mass is assembled through mergers rather than new star formation after z = 1 .
Evolution of Galaxy Structure: Morphology, Size, and Internal Features
Galaxy structures have evolved significantly over cosmic time. Early galaxies (z ~ 6) were typically small, compact, and irregular, while the familiar Hubble sequence of spirals and ellipticals emerged later. Structural analysis methods—such as Sérsic profile fitting and nonparametric indices (concentration, asymmetry, clumpiness)—reveal that galaxy sizes at a given stellar mass were 2–5 times smaller at high redshift compared to today . Internal features like bulges, disks, bars, and large star-forming clumps are key to understanding how galaxies grow and change. Mergers play a crucial role in shaping these structures and in driving the transition from star-forming to quiescent galaxies .
Spiral Structure and Star Formation: The Role of Stochastic Processes
The spiral arms seen in many galaxies are linked to patterns of star formation. Models show that self-propagating star formation, influenced by the galaxy’s rotation curve, can produce stable, large-scale spiral features that persist over billions of years. The specific appearance of spiral arms—such as their pitch angle and density—depends on the galaxy’s rotation curve, and different morphological types arise from these dynamics rather than from evolutionary changes between types .
Mass, Star Formation, and Environment: Key Drivers of Structure
The structure of a galaxy is most strongly predicted by its stellar mass and its position relative to the star-forming main sequence (a relation between star formation rate and mass). Galaxies above the main sequence tend to be more disk-dominated, while those below are more bulge-dominated. Environmental factors, such as a galaxy’s location within a group or cluster, have a smaller effect on structure compared to mass and star formation activity 369. Gas content, especially atomic hydrogen (HI), is closely linked to star-forming, disk-dominated galaxies, supporting the idea that gas accretion fuels ongoing star formation primarily in galaxy disks .
Quenching and Structural Transformation
As galaxies evolve, some cease forming stars—a process known as quenching. Structural changes often accompany quenching, with galaxies becoming more bulge-dominated and exhibiting redder colors. In low-mass galaxies, quenching can occur rapidly with significant morphological change or more slowly without major structural transformation. In more massive galaxies, structural transformation often precedes or accompanies the decline in star formation 69.
Conclusion
Galaxy formation and structure are shaped by a combination of cosmological processes, gas dynamics, star formation, and mergers. The evolution from small, irregular systems to the diverse galaxies seen today is driven primarily by mass, star formation activity, and the hierarchical assembly of structures. Ongoing research continues to refine our understanding of how these factors interact to produce the rich variety of galaxy forms observed across the universe 1345+5 MORE.
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