Other meanings of Galaxy formation and evolution
Astrophysics
Galaxy formation and evolution is the subfield of astrophysics that studies how galaxies—vast systems of stars, gas, dust, and dark matter—originate and change over cosmic time. It seeks to explain the observed diversity of galaxy types, from spirals to ellipticals, and their properties such as mass, luminosity, and star formation rate. The field integrates observations across the electromagnetic spectrum with theoretical models and simulations to reconstruct the history of the universe from the Big Bang to the present day.
The study of galaxy formation relies on observations of galaxies at various cosmic distances, which correspond to different look-back times. Deep surveys such as the Hubble Ultra-Deep Field and the James Webb Space Telescope's (JWST) early release observations have revealed galaxies as they existed when the universe was less than a billion years old. These observations show that galaxies were smaller, more irregular, and had higher star formation rates than present-day galaxies, supporting the hierarchical assembly model.
Key observational probes include galaxy redshift surveys, which map the large-scale distribution of galaxies, and measurements of the cosmic microwave background, which provide initial conditions. Spectroscopy allows astronomers to measure galaxy distances, chemical abundances, and star formation rates, while imaging reveals morphology and structure.
The prevailing theoretical framework is the Lambda-Cold Dark Matter (ΛCDM) model, in which structure grows from tiny primordial density fluctuations seeded by inflation. Dark matter, which constitutes about 27% of the universe's energy density, forms gravitational wells that attract baryonic gas, leading to galaxy formation. Galaxy formation is thus intimately linked to the assembly of dark matter halos.
Numerical simulations, such as the IllustrisTNG and EAGLE projects, model the coupled evolution of dark matter and baryons, including gas cooling, star formation, feedback from supernovae and active galactic nuclei (AGN), and magnetic fields. These simulations reproduce many observed galaxy properties, but challenges remain in matching the details of galaxy scaling relations and the morphology-density relation.
Several physical processes drive galaxy evolution. Gas accretion from the intergalactic medium supplies fuel for star formation, while feedback from supernovae and AGN can expel gas and quench star formation. Galaxy mergers are particularly important: major mergers of equal-mass galaxies can transform spirals into ellipticals and trigger starbursts, while minor mergers add mass and can disturb disks1.
Secular processes, such as bar instabilities and disk heating, also reshape galaxies over long timescales. The interplay between these processes determines the galaxy's star formation history, metallicity, and morphology.
Galaxies are broadly classified into two main types: blue, star-forming spirals and red, quiescent ellipticals. Observations show a bimodal distribution in color and star formation rate, with a 'green valley' of transitioning galaxies. The evolution of the galaxy population is characterized by a decline in the cosmic star formation rate since a peak at redshift ~2, and a growth in the number of quiescent galaxies over time2.
Environment plays a crucial role: galaxies in dense clusters are more likely to be quenched and elliptical, due to processes such as ram-pressure stripping and harassment. The morphology-density relation is a well-established observational trend.
Beyond the mainstream narrative, several niche topics enrich the field. For instance, 'fossil galaxies' like M32, a compact elliptical satellite of Andromeda, may be the remnants of stripped larger galaxies. 'Ultra-diffuse galaxies' (UDGs) are extremely low-surface-brightness galaxies that challenge formation models; some UDGs contain surprisingly few stars but have massive dark matter halos, while others appear to lack dark matter entirely3.
Another edge case is the 'green pea' galaxies, small, compact, intensely star-forming galaxies discovered by citizen scientists in the Galaxy Zoo project; they are local analogs of high-redshift star-forming galaxies. Additionally, the role of magnetic fields in galaxy evolution is a relatively new area of study, with simulations showing they can affect gas dynamics and star formation.
Historical contributions include the pioneering work of Beatrice Tinsley, who in the 1970s developed models of galaxy evolution that laid the groundwork for modern understanding, and the 'tuning fork' classification by Edwin Hubble, which, despite being a morphological sequence, is not an evolutionary sequence as often misinterpreted.
This article focuses on the astrophysical study of galaxy formation and evolution, distinct from other uses of the term.
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