🌌Cosmology Unit 9 – The Formation and Evolution of Galaxies

Key Concepts and Definitions

• Galaxies are massive, gravitationally bound systems consisting of stars, planets, gas, dust, and dark matter
• Cosmology studies the origin, evolution, and ultimate fate of the universe, including the formation and evolution of galaxies
• Dark matter is a hypothetical form of matter that does not interact with electromagnetic radiation but has gravitational effects on visible matter
• Redshift measures the increase in wavelength of electromagnetic radiation caused by the expansion of the universe and can be used to determine the distance and age of galaxies
• Hubble's law describes the relationship between a galaxy's distance and its redshift, with more distant galaxies exhibiting greater redshift
• Morphology refers to the classification of galaxies based on their physical structure and appearance (spiral, elliptical, irregular)
• Galactic nuclei are the central regions of galaxies that often contain supermassive black holes and exhibit high luminosity
• Metallicity is the proportion of a galaxy's matter made up of elements heavier than hydrogen and helium, which increases over time as stars undergo nucleosynthesis

The Birth of Galaxies

• Galaxies began to form in the early universe, approximately 13.8 billion years ago, shortly after the Big Bang
• The early universe was nearly homogeneous, with slight fluctuations in density that served as the seeds for galaxy formation
• Dark matter played a crucial role in the formation of galaxies by providing the gravitational framework for matter to accumulate
• As the universe expanded and cooled, gas began to fall into the gravitational wells created by dark matter, forming protogalaxies
• Within protogalaxies, gas clouds collapsed under their own gravity, leading to the formation of the first stars and star clusters
• The first galaxies were likely small, irregular, and lacking well-defined structures compared to present-day galaxies
• Feedback processes, such as supernovae and stellar winds, influenced the early evolution of galaxies by regulating star formation and redistributing matter
  • Supernovae explosions can trigger star formation by compressing nearby gas clouds
  • Stellar winds from massive stars can disperse gas and slow down star formation

Galaxy Types and Structures

• Galaxies are classified into three main types based on their morphology: spiral, elliptical, and irregular
• Spiral galaxies (Milky Way) have a flat, rotating disk with spiral arms, a central bulge, and often a bar-like structure
  • Spiral arms are regions of active star formation and contain younger, bluer stars
  • The central bulge is composed mainly of older, redder stars and may harbor a supermassive black hole
• Elliptical galaxies have a smooth, ellipsoidal shape and lack distinct features like spiral arms
  • They are typically composed of older, redder stars and have little ongoing star formation
  • Elliptical galaxies are further classified based on their ellipticity, ranging from E0 (nearly spherical) to E7 (highly elongated)
• Irregular galaxies lack a distinct regular structure and often have a chaotic appearance
  • They are usually smaller and less massive than spiral and elliptical galaxies
  • Irregular galaxies may result from gravitational interactions or mergers with other galaxies
• Galaxies can also be classified based on their size, mass, and luminosity (dwarf galaxies, giant galaxies)
• The structure and morphology of a galaxy can evolve over time due to internal processes (star formation, feedback) and external interactions (mergers, tidal forces)

Dark Matter and Galaxy Formation

• Dark matter is believed to make up a significant portion of the total mass in the universe and plays a crucial role in the formation and evolution of galaxies
• Dark matter halos provide the gravitational framework for the accumulation of baryonic matter (gas and dust) during galaxy formation
• The distribution of dark matter influences the structure and rotation curves of galaxies
  • Flat rotation curves suggest the presence of a dark matter halo extending beyond the visible matter in a galaxy
• Dark matter halos are thought to have a hierarchical structure, with smaller halos merging to form larger ones over time
• The interaction between dark matter and baryonic matter affects the evolution of galaxies
  • Baryonic matter can cool and condense within dark matter halos, leading to star formation
  • Feedback from star formation and supernovae can affect the distribution of dark matter in a galaxy
• The nature of dark matter remains one of the major unsolved problems in cosmology
  • Candidates for dark matter particles include weakly interacting massive particles (WIMPs) and axions
• Observations of gravitational lensing, galaxy clusters, and the cosmic microwave background provide evidence for the existence of dark matter

Galaxy Evolution Over Time

• Galaxies have undergone significant evolution since their formation in the early universe
• The rate of star formation in galaxies has declined over cosmic time, with the peak of star formation occurring around 10 billion years ago
• Galaxies in the early universe were typically smaller, more irregular, and had higher rates of star formation compared to present-day galaxies
• As galaxies age, they become more enriched in heavy elements (metals) due to stellar nucleosynthesis and the recycling of matter through supernovae
• Feedback processes, such as supernovae and active galactic nuclei (AGN), can regulate star formation and affect the evolution of galaxies
  • AGN feedback can heat and expel gas from a galaxy, suppressing star formation
• Mergers and interactions between galaxies can significantly impact their evolution
  • Major mergers can lead to the formation of elliptical galaxies and trigger intense star formation (starburst galaxies)
  • Minor mergers and tidal interactions can cause morphological changes and induce star formation
• The evolution of galaxies is influenced by their environment, with galaxies in dense clusters experiencing different evolutionary paths than isolated galaxies
• The study of galaxy evolution helps constrain models of cosmology and improves our understanding of the universe's history

Galaxy Interactions and Mergers

• Galaxies can interact gravitationally with one another, leading to various phenomena and evolutionary consequences
• Tidal interactions occur when galaxies pass close to each other, causing distortions in their shapes due to gravitational forces
  • Tidal tails and bridges can form as material is stripped from the interacting galaxies
• Major mergers happen when two galaxies of similar mass collide and merge, resulting in a single, larger galaxy
  • Major mergers can lead to the formation of elliptical galaxies and trigger intense star formation (starburst galaxies)
  • The Antennae Galaxies are an example of an ongoing major merger
• Minor mergers involve the accretion of a smaller galaxy by a larger one, leading to more gradual changes in morphology and star formation
• Galactic cannibalism refers to the process by which a large galaxy consumes smaller galaxies, growing in mass and size over time
• Mergers can also trigger the activity of supermassive black holes at the centers of galaxies, leading to the formation of active galactic nuclei (AGN)
• The frequency of galaxy mergers and interactions was higher in the early universe when galaxies were closer together
• Studying galaxy interactions and mergers provides insights into the formation and evolution of different galaxy types and the role of hierarchical structure formation in the universe

Observational Techniques and Evidence

• Various observational techniques are used to study the formation and evolution of galaxies across the electromagnetic spectrum
• Optical and near-infrared imaging allows for the study of galaxy morphology, star formation, and the distribution of stellar populations
  • Hubble Space Telescope has provided high-resolution images of galaxies across cosmic time
• Spectroscopy enables the measurement of galaxy redshifts, chemical composition, and kinematics
  • Integral field spectroscopy (IFS) provides spatially resolved spectra, allowing for detailed studies of galaxy dynamics and star formation
• Radio astronomy is used to study neutral hydrogen (HI) in galaxies, which traces the distribution and kinematics of cold gas
  • The 21-cm line of HI is a key diagnostic for studying galaxy evolution and the interstellar medium
• X-ray observations reveal hot gas in galaxies and galaxy clusters, as well as active galactic nuclei (AGN)
  • Chandra X-ray Observatory has provided detailed images of AGN and the hot gas in galaxy clusters
• Infrared astronomy is used to study dust-obscured star formation and the properties of early galaxies
  • Spitzer Space Telescope and James Webb Space Telescope (JWST) are important infrared observatories for studying galaxy evolution
• Gravitational lensing, both strong and weak, provides a means to study the distribution of dark matter in galaxies and galaxy clusters
• Numerical simulations, such as the Illustris and EAGLE projects, play a crucial role in understanding galaxy formation and evolution by modeling the complex physical processes involved

Current Research and Open Questions

• The role of dark matter in galaxy formation and evolution remains an active area of research
  • The nature of dark matter particles and their interactions with baryonic matter are still unknown
  • Alternative theories, such as modified gravity, are being explored to explain the observed properties of galaxies
• The details of the galaxy-black hole connection and the impact of AGN feedback on galaxy evolution are not fully understood
  • The mechanisms by which AGN feedback affects star formation and the interstellar medium are being investigated
• The formation and evolution of the first galaxies in the early universe are key topics of research
  • The James Webb Space Telescope (JWST) is expected to provide new insights into the properties and evolution of early galaxies
• The role of environment in galaxy evolution, particularly in galaxy clusters and groups, is an ongoing area of study
  • The effects of ram-pressure stripping, tidal interactions, and mergers in different environments are being explored
• The origin and evolution of galactic magnetic fields and their impact on star formation and galaxy evolution are not well understood
• The connection between galaxy evolution and the large-scale structure of the universe, including cosmic web filaments and voids, is an active area of research
• Improving our understanding of the various feedback processes in galaxies, such as supernovae, stellar winds, and AGN, is crucial for constructing accurate models of galaxy evolution
• The development of new observational facilities, such as the Extremely Large Telescopes (ELTs) and the Square Kilometre Array (SKA), is expected to revolutionize our understanding of galaxy formation and evolution in the coming decades