Galaxies form and grow through gravitational collapse, mergers, and gas accretion. Dark matter plays a crucial role, providing the gravitational scaffolding for structure formation. These processes shape the cosmic web of filaments, clusters, and voids we observe today.

Elliptical and spiral galaxies follow different evolutionary paths. Ellipticals form through major mergers, while spirals grow more gradually. Environmental factors and ongoing processes like star formation and feedback continue to influence galaxy evolution over cosmic time.

Galaxy Formation and Evolution

Describe the main processes by which galaxies form and grow over cosmic time

• Gravitational collapse of matter in the early universe
  • Dark matter halos form first providing a gravitational well for baryonic matter (gas and dust) to fall into
  • Gas cools and condenses within dark matter halos forming stars and galaxies (Milky Way)
• Hierarchical merging
  • Smaller galaxies merge to form larger galaxies over time (Andromeda)
  • Major mergers can disrupt galaxy structure and trigger bursts of star formation
  • Minor mergers contribute to galaxy growth without significantly altering morphology (Sagittarius Dwarf Spheroidal Galaxy merging with the Milky Way)
  • Galactic cannibalism occurs when larger galaxies consume smaller ones
• Accretion of gas from the intergalactic medium
  • Cold gas flows along filaments of the cosmic web into galaxies (Magellanic Stream)
  • Accreted gas fuels ongoing star formation (Orion Nebula)
• Feedback processes
  • Stellar winds and supernovae can expel gas from galaxies regulating star formation (Crab Nebula)
  • Active galactic nuclei (AGN) feedback can heat and expel gas suppressing star formation in massive galaxies (M87)

Explain how dark matter and gravity shape the large-scale structure of the universe

• Cosmic web
  • Dark matter and gravity form a network of filaments and nodes on large scales (Sloan Great Wall)
  • Galaxies and galaxy clusters form along these filaments and at the nodes (Virgo Cluster)
• Dark matter halos
  • Invisible halos of dark matter surround galaxies and galaxy clusters
  • The gravitational influence of dark matter halos helps to hold galaxies and clusters together (Bullet Cluster)
  • The galactic halo extends far beyond the visible part of a galaxy
• Gravitational instability
  • Small density fluctuations in the early universe are amplified by gravity over time
  • Overdense regions collapse to form galaxies clusters and the cosmic web (Coma Cluster)
• Voids
  • Underdense regions expand and become largely empty of matter (Boötes Void)
  • Voids are surrounded by sheets filaments and nodes of the cosmic web (Local Void)

Compare the formation and evolution of elliptical versus spiral galaxies

• Elliptical galaxies
  • Form through major mergers of galaxies (Centaurus A)
  • Mergers disrupt the ordered motion of stars resulting in a smooth ellipsoidal shape
  • Contain mostly old red stars with little ongoing star formation (M87)
  • Gas is depleted or heated preventing further star formation
• Spiral galaxies
  • Form through more gradual processes such as gas accretion and minor mergers
  • Maintain a disk structure with spiral arms (Whirlpool Galaxy)
  • Contain a mix of young blue stars and old red stars (Andromeda)
  • Ongoing star formation occurs in the disk particularly in the spiral arms (Orion Arm)
• Environmental effects
  • Elliptical galaxies are more common in dense environments such as galaxy clusters (Coma Cluster)
  • Spiral galaxies are more common in less dense environments such as the outskirts of clusters or in the field (Local Group)
• Evolution
  • Spiral galaxies can transform into elliptical galaxies through major mergers (Mice Galaxies)
  • Elliptical galaxies can grow larger through subsequent mergers with other galaxies (Hercules A)

Cosmic Structure and Dark Matter

Explain how dark matter and gravity shape the large-scale structure of the universe

• Density fluctuations in the early universe
  • Quantum fluctuations in the early universe created small variations in density
  • These density fluctuations were amplified by inflation a period of rapid expansion
• Growth of structure
  1. Overdense regions attract more matter through gravity becoming even more dense (Galaxy clusters)
  2. Underdense regions become less dense as matter flows away from them (Voids)
• Role of dark matter
  • Dark matter interacts gravitationally but not electromagnetically
  • It forms the scaffolding for the large-scale structure of the universe (Cosmic web)
  • Baryonic matter follows the distribution of dark matter forming galaxies and clusters
• Cosmological simulations
  • Computer simulations model the evolution of the universe from early density fluctuations (Millennium Simulation)
  • These simulations reproduce the observed large-scale structure including the cosmic web
• Observational evidence
  • Galaxy surveys map the distribution of galaxies in the universe (2dF Galaxy Redshift Survey)
  • The observed large-scale structure matches the predictions of dark matter models (Sloan Digital Sky Survey)
  • Redshift measurements help determine the distances and velocities of galaxies

Cosmic Expansion and Structure Formation

• Cosmic inflation
  • Rapid expansion of the early universe that amplified quantum fluctuations
• Baryon acoustic oscillations
  • Sound waves in the early universe that left imprints on the large-scale structure
• Star formation
  • Process by which gas collapses to form new stars, driving galaxy evolution