Galaxy formation and evolution

Galaxy formation and evolution describes how galaxies originate and change over cosmic time through gas accretion, star formation, interactions, and mergers.

Cold dark matter: A form of matter that interacts primarily through gravity and moves slowly compared with light. Its gravitational structures provide the scaffolding in which galaxies assemble.
Primordial nucleosynthesis: The production of light atomic nuclei during the first minutes after the Big Bang. Its primordial element abundances set the initial chemical composition of galaxy-forming gas.
Hubble sequence: A classification of galaxies by visual form, including ellipticals, lenticulars, and spirals. Its changing population mix records structural outcomes of galaxy evolution.
Monolithic collapse: A model in which a galaxy forms rapidly through the collapse of a single large gas cloud. It contrasts with hierarchical assembly from many smaller structures.
Missing satellites problem: The discrepancy between predicted and observed numbers of small dark matter halos around galaxies. It tests how efficiently small halos turn gas into observable galaxies.
Dark matter halo: A gravitationally bound concentration of dark matter surrounding a galaxy or galaxy group. Gas falls into these halos, setting the mass scales for galaxy growth.
Reionization: The cosmic era when ultraviolet radiation ionized most intergalactic hydrogen. Heating during reionization suppressed gas supply to the smallest early galaxies.
Starburst galaxy: A galaxy forming stars at a rate far above the typical rate for its mass. Starbursts show how mergers and gas inflows can briefly accelerate growth.
Hierarchical structure formation: The growth of cosmic structures through the gravitational merging of smaller systems. It provides the broad framework for galaxies assembling through successive mergers.
Too-big-to-fail problem: A mismatch between the dense subhalos predicted around the Milky Way and the observed dwarf galaxies. It challenges models of how dwarf galaxies populate dark matter subhalos.

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