11.2 Galaxy formation and evolution

Galaxies, the cosmic cities of stars, form and evolve through complex processes. Dark matter halos act as gravitational scaffolding, while mergers and interactions shape galactic structures. These processes unfold over billions of years, leaving observable traces in the cosmic tapestry.

Astronomers piece together the story of galaxy evolution using powerful telescopes and surveys. From the Hubble Deep Field to large-scale sky maps, we observe how galaxies grow, change color, and transform their shapes across cosmic time.

Galaxy Formation and Evolution

Galaxy formation and evolution theories

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  The Formation of the Galaxy | Astronomy View original

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  28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original

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  The Formation of the Galaxy | Astronomy View original

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  28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original

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Top images from around the web for Galaxy formation and evolution theories

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  The Formation of the Galaxy | Astronomy View original

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  28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original

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  28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original

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  The Formation of the Galaxy | Astronomy View original

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• Hierarchical structure formation model takes bottom-up approach where smaller structures form first then merge to create larger structures (galaxies, galaxy clusters)
• Cold Dark Matter (CDM) paradigm posits dark matter forms gravitational wells and baryonic matter falls into these wells forming galaxies
• Monolithic collapse model proposes top-down approach where large gas clouds collapse rapidly to form galaxies (elliptical galaxies)
• Cosmic web structure consists of filaments, nodes, and voids in the large-scale universe shaping galaxy distribution
• Galaxy formation stages include primordial density fluctuations, gravitational collapse, gas cooling and star formation, feedback processes (supernovae, AGN)
• Downsizing phenomenon observes massive galaxies form stars earlier and faster than less massive galaxies contradicting hierarchical model
• Cosmic star formation history peaks at redshift $z \approx 2$ followed by decline in star formation rate density since then (cosmic noon)

Dark matter halos in galaxies

• Gravitational potential wells of dark matter form initial structure and baryonic matter falls into these wells shaping galaxy formation
• Halo mass function describes distribution of dark matter halo masses in the universe predicting abundance of different galaxy masses
• Navarro-Frenk-White (NFW) profile models density distribution of dark matter in halos $\rho(r) = \frac{\rho_0}{(r/r_s)(1+r/r_s)^2}$
• Angular momentum acquisition through tidal torques from neighboring structures explains galaxy rotation
• Halo substructure consists of smaller dark matter clumps within larger halos influencing galaxy satellite populations
• Halo merger trees track hierarchical growth of dark matter halos over time showing galaxy assembly history
• Halo concentration-mass relation observes more massive halos are less concentrated affecting galaxy structure
• Halo bias describes how clustering properties of halos depend on their mass influencing large-scale galaxy distribution

Galaxy mergers and interactions

• Types of galaxy mergers include major mergers between similar mass galaxies and minor mergers with large mass difference
• Tidal interactions cause gravitational forces to distort galaxy shapes forming tidal tails and bridges (Antennae Galaxies)
• Dynamical friction leads to orbital decay of merging galaxies bringing them closer together
• Star formation triggering occurs through compression of gas clouds during mergers resulting in starburst events
• Morphological transformations convert disk galaxies to elliptical galaxies and form galactic bulges
• Active Galactic Nuclei (AGN) activation happens as gas is funneled to central black holes during mergers
• Galactic cannibalism involves larger galaxies consuming smaller ones (Andromeda consuming dwarf galaxies)
• Merger rate evolution shows higher merger rates at earlier cosmic times shaping galaxy growth

Observational evidence of galaxy evolution

• Hubble Deep Field and Ultra Deep Field provide observations of distant, early galaxies revealing galaxy formation history
• Lyman-break galaxies are star-forming galaxies at high redshifts offering glimpse into early universe
• Cosmic evolution of galaxy properties shows size evolution with galaxies smaller in the past and mass evolution through build-up of stellar mass over time
• Galaxy luminosity function tracks changes in galaxy brightness distribution over cosmic time revealing galaxy growth
• Color-magnitude diagram illustrates evolution of galaxy colors and luminosities showing star formation history
• Spectral energy distributions (SEDs) reveal changes in galaxy spectra with redshift indicating evolving stellar populations
• Quasar luminosity function traces evolution of quasar populations over cosmic time linking to galaxy-black hole co-evolution
• Metallicity evolution shows increase in heavy element content in galaxies over time due to stellar nucleosynthesis
• Morphological evolution observes prevalence of irregular and clumpy galaxies at high redshifts transforming to ordered structures
• Large-scale surveys like Sloan Digital Sky Survey (SDSS) and Galaxy And Mass Assembly (GAMA) provide statistical data on galaxy properties and evolution