Galaxy evolution

Galaxy evolution is the physical transformation of galaxies across cosmic time through star formation, mergers, gas flows, and interactions with their surroundings.

Star formation: The process by which dense regions of gas collapse to form stars. It turns a galaxy’s cold gas into stars and shapes its evolving stellar population.
Hubble sequence: A classification of galaxies by their visual forms, including ellipticals, spirals, and lenticulars. Its changing population mix records the emergence of different galaxy structures.
Stellar population: A group of stars considered together by properties such as age, chemical composition, or formation history. Stellar populations let astronomers infer when a galaxy formed its stars.
Galaxy morphology: The study and classification of galaxies by their shapes and structural features. Structural transformation is one visible outcome of galaxy evolution.
Galaxy downsizing: The observed tendency for the most massive galaxies to form their stars earlier than less massive galaxies. It challenges simple expectations that larger systems should assemble and grow latest.
Galaxy merger: An interaction in which two galaxies combine into one larger system. Mergers rearrange stars and gas, trigger star formation, or help end it.
Lyman-break galaxy: A distant, star-forming galaxy identified by a sharp drop in ultraviolet light shortward of the Lyman limit. These galaxies provide snapshots of vigorous star formation in the early universe.
Interstellar medium: The gas and dust occupying the space between stars within a galaxy. Its phases supply star-forming material and respond to feedback.
Morphological quenching: The suppression of star formation when a galaxy’s stellar structure stabilizes its gas against gravitational collapse. It offers a route to quenching without requiring gas removal.
Missing satellites problem: The mismatch between predicted and observed numbers of low-mass satellite galaxies around large galaxies. Its resolution bears on how small galaxies form and survive over time.

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