9.4 High-Redshift Galaxies and Cosmic Evolution

High-redshift galaxies offer a window into the early universe. By studying Lyman-break galaxies, Lyman-alpha emitters, submillimeter galaxies, and quasars, astronomers can piece together the story of galaxy formation and evolution.

These ancient cosmic structures reveal crucial information about star formation rates, chemical composition, and the growth of supermassive black holes. Their study helps us understand how galaxies have changed over billions of years.

High-Redshift Galaxy Types

Lyman-Break and Lyman-Alpha Galaxies

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• Lyman-break galaxies identified by distinctive break in ultraviolet spectrum
  • Break occurs due to absorption of light by neutral hydrogen in galaxy and intergalactic medium
  • Technique allows detection of galaxies at redshifts z > 2.5
• Lyman-alpha emitters characterized by strong emission line at 1216 Å
  • Emission results from recombination of ionized hydrogen in star-forming regions
  • Useful for studying galaxy formation and evolution in early universe
• Both types provide insights into star formation rates and chemical composition of early galaxies
• Detection methods involve specialized filters and spectroscopic observations
• Populations of these galaxies help constrain models of galaxy formation and evolution

Submillimeter Galaxies and Quasars

• Submillimeter galaxies detected at wavelengths around 850 μm
  • Highly dust-obscured, undergoing intense bursts of star formation
  • Typically found at redshifts between 2 and 5
  • Contribute significantly to cosmic star formation history
• Quasars represent extremely luminous active galactic nuclei
  • Powered by supermassive black holes accreting matter at high rates
  • Serve as cosmic beacons, visible across vast distances
  • Spectra provide information about intergalactic medium along line of sight
• Both types offer unique perspectives on galaxy evolution and cosmic environment
  • Submillimeter galaxies trace dust-enshrouded star formation
  • Quasars probe early black hole growth and cosmic structure

Galaxy Evolution and Assembly

Galaxy Luminosity Function and Downsizing

• Galaxy luminosity function describes distribution of galaxy luminosities in a given volume
  • Typically modeled using Schechter function
  • $\phi(L) = \phi^* (L/L^*)^\alpha e^{-L/L^*}$
  • $\phi^*$ represents normalization, $L^*$ characteristic luminosity, $\alpha$ faint-end slope
• Evolution of luminosity function provides insights into galaxy growth over cosmic time
• Downsizing refers to observed trend in galaxy evolution
  • Massive galaxies formed stars earlier and more rapidly than less massive galaxies
  • Contradicts simple hierarchical models of galaxy formation
  • Suggests complex interplay between galaxy mass, environment, and star formation history

Cosmic Star Formation History and Galaxy Assembly

• Cosmic star formation history traces evolution of star formation rate density over time
  • Peaks around redshift z ~ 2-3, known as "cosmic noon"
  • Declines steadily from z ~ 2 to present day
  • Measured using various tracers (UV luminosity, infrared emission, radio continuum)
• Galaxy assembly involves processes of mass growth and structural evolution
  • Includes mergers, accretion of gas from intergalactic medium, and in-situ star formation
  • Major mergers can trigger starbursts and morphological transformations
  • Minor mergers contribute to gradual mass growth and build-up of stellar halos
• Feedback processes (stellar winds, supernovae, active galactic nuclei) regulate galaxy growth
  • Can expel gas from galaxies, temporarily quenching star formation
  • Play crucial role in shaping galaxy mass-metallicity relation

Cosmic Structure and Reionization

Reionization and the Cosmic Web

• Reionization marks transition of intergalactic medium from neutral to ionized state
  • Occurred between redshifts z ~ 6-20
  • Driven by ionizing radiation from first stars and galaxies
  • Progress of reionization can be traced using quasar absorption spectra and 21-cm observations
• Cosmic web represents large-scale structure of matter distribution in universe
  • Consists of filaments, sheets, and voids
  • Filaments contain majority of galaxies and intergalactic gas
  • Voids represent underdense regions with few galaxies
• Formation of cosmic web driven by gravitational instability in early universe
  • Initial density fluctuations amplified by cosmic expansion
  • Dark matter forms backbone of structure, with baryonic matter following

Large-Scale Structure and Cosmological Implications

• Large-scale structure provides powerful probe of cosmological models
  • Baryon acoustic oscillations serve as standard ruler for measuring cosmic expansion history
  • Clustering statistics constrain parameters of ΛCDM model
• Redshift surveys map three-dimensional distribution of galaxies
  • Reveal complex network of filaments, clusters, and voids
  • (Sloan Digital Sky Survey, 2dF Galaxy Redshift Survey)
• Weak lensing measurements trace dark matter distribution
  • Distortions in shapes of background galaxies reveal foreground mass concentrations
  • Allows mapping of dark matter on large scales, independent of luminous tracers
• Studying large-scale structure at high redshifts probes earlier epochs of cosmic history
  • Provides insights into initial conditions of universe and growth of structure over time