🪐Intro to Astronomy Unit 28 – Galaxy Evolution and Distribution

Key Concepts and Definitions

• Galaxies are massive, gravitationally bound systems consisting of stars, planets, gas, dust, and dark matter
• Elliptical galaxies have smooth, elliptical shapes and contain mostly older, redder stars with little gas and dust
  • Classified based on their ellipticity from E0 (nearly spherical) to E7 (highly elongated)
• Spiral galaxies have distinct spiral arms extending from a central bulge and contain a mix of young and old stars, gas, and dust
  • Classified as Sa (tightly wound arms), Sb, or Sc (loosely wound arms) based on the tightness of their spiral arms
• Irregular galaxies lack a distinct shape and often have a chaotic appearance due to gravitational interactions or mergers
• Barred spiral galaxies have a central bar-shaped structure composed of stars extending from the bulge
• Dark matter is a hypothetical form of matter that does not interact with electromagnetic radiation but has gravitational effects on visible matter
• Hubble sequence is a morphological classification scheme for galaxies based on their appearance (elliptical, spiral, or irregular)
• Galactic nuclei are the central regions of galaxies that often contain supermassive black holes and high-density star clusters

Galaxy Formation Theories

• Top-down theory suggests galaxies formed from the collapse of large gas clouds in the early universe
  • As the gas cooled and condensed, it fragmented into smaller clumps that formed stars and galaxies
• Bottom-up theory proposes galaxies formed through the hierarchical merging of smaller structures (dwarf galaxies and star clusters)
  • Smaller structures combined over time to create the larger galaxies we observe today
• Cold dark matter (CDM) model is the most widely accepted theory of galaxy formation
  • Suggests dark matter halos formed first, providing the gravitational framework for baryonic matter to collapse and form galaxies
• Feedback mechanisms (supernovae, active galactic nuclei) play a crucial role in regulating star formation and shaping galaxy evolution
• Initial mass function (IMF) describes the distribution of initial masses for a population of stars forming in a galaxy
• Reionization epoch marks the time when the first stars and galaxies ionized the neutral hydrogen in the early universe
• Cosmic web is the large-scale structure of the universe, consisting of filaments and voids, which influences galaxy formation and evolution

Types and Structures of Galaxies

• Spiral galaxies have distinct components: a central bulge, disk, spiral arms, and a halo
  • Bulge contains mostly older, redder stars and may harbor a supermassive black hole
  • Disk is composed of younger, bluer stars, gas, and dust, organized into spiral arms
• Elliptical galaxies have smooth, elliptical shapes and lack distinct features like spiral arms
  • Contain mostly older, redder stars with little gas and dust
  • Range in size from dwarf ellipticals to giant ellipticals (most massive galaxies in the universe)
• Irregular galaxies have asymmetric shapes and lack a distinct structure
  • Often result from gravitational interactions or mergers with other galaxies
  • Contain a mix of young and old stars, as well as significant amounts of gas and dust
• Dwarf galaxies are small galaxies with lower masses and luminosities compared to larger galaxies
  • Can be classified as dwarf ellipticals, dwarf spheroidals, or dwarf irregulars
• Interacting and merging galaxies are galaxies undergoing gravitational interactions or collisions
  • Can lead to the formation of peculiar galaxies with distorted shapes and enhanced star formation
• Galactic halos are the extended, roughly spherical regions surrounding galaxies
  • Contain older stars, globular clusters, and significant amounts of dark matter

Galaxy Distribution in the Universe

• Galaxies are not uniformly distributed throughout the universe but form large-scale structures
• Galaxy clusters are gravitationally bound groups of galaxies, typically containing hundreds to thousands of galaxies
  • Held together by the gravitational attraction of their combined mass, including dark matter
  • Often have a central dominant galaxy (cD galaxy) and a hot, X-ray emitting intracluster medium
• Superclusters are the largest known structures in the universe, consisting of multiple galaxy clusters and groups
  • Connected by filaments and separated by large voids
  • Examples include the Virgo Supercluster and the Laniakea Supercluster (home to the Milky Way)
• Filaments are long, thin structures of galaxies and dark matter that connect galaxy clusters and form the cosmic web
• Voids are vast, largely empty regions of space between filaments and galaxy clusters
  • Can span tens to hundreds of millions of light-years in diameter
  • May contain a few isolated galaxies or dwarf galaxies
• Hubble's law describes the relationship between a galaxy's distance and its recessional velocity due to the expansion of the universe
  • Expressed as $v = H_0 \times d$, where $v$ is the recessional velocity, $H_0$ is the Hubble constant, and $d$ is the distance

Evolution of Galaxies Over Time

• Galaxies have undergone significant evolution since the early universe, influenced by various processes and interactions
• Mergers play a crucial role in galaxy evolution, leading to the growth of galaxies and the formation of new structures
  • Major mergers involve galaxies of similar mass and can result in the formation of elliptical galaxies or trigger starbursts
  • Minor mergers occur between galaxies of different masses and can lead to the growth of spiral galaxies or the formation of tidal features
• Star formation rates in galaxies have declined over cosmic time, with the peak of star formation occurring around 10 billion years ago
  • Influenced by factors such as gas availability, feedback mechanisms, and galaxy interactions
• Quenching refers to the suppression of star formation in galaxies, often due to the depletion of cold gas or feedback from active galactic nuclei
• Morphological transformation can occur as galaxies evolve, such as the transition from spiral to elliptical through mergers or the formation of bars in spiral galaxies
• Chemical enrichment of galaxies increases over time as stars produce and expel heavier elements through stellar winds and supernovae
• Environmental effects (ram pressure stripping, tidal interactions) can impact galaxy evolution, particularly in dense environments like galaxy clusters
• Downsizing trend suggests that more massive galaxies formed their stars earlier and more rapidly than lower-mass galaxies

Observational Techniques and Tools

• Multi-wavelength observations are essential for studying galaxy evolution, as different wavelengths probe various components and processes
  • Optical and near-infrared observations reveal stellar populations and morphologies
  • Far-infrared and submillimeter observations trace dust and star formation
  • Radio observations detect neutral hydrogen (HI) and molecular gas, as well as synchrotron emission from cosmic rays
• Hubble Space Telescope (HST) has been instrumental in studying galaxy evolution, providing high-resolution images across a wide range of wavelengths
  • Deep fields (Hubble Deep Field, Ultra Deep Field) have revealed distant galaxies and their evolution over cosmic time
• James Webb Space Telescope (JWST) is a next-generation infrared observatory that will provide unprecedented insights into early galaxy formation and evolution
• Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful radio telescope that studies cold gas and dust in galaxies across cosmic time
• Spectroscopic surveys (SDSS, GAMA) provide detailed information on galaxy properties (redshifts, stellar populations, chemical abundances)
• Integral field spectroscopy (IFS) enables spatially resolved studies of galaxy properties, kinematics, and chemical composition
• Gravitational lensing is a technique that uses the bending of light by massive foreground objects to study distant galaxies and constrain dark matter distributions

Notable Discoveries and Research

• Hubble sequence (tuning fork diagram) established the morphological classification of galaxies based on their appearance
• Hubble-Lemaître law revealed the expansion of the universe and provided a means to measure cosmic distances
• Dark matter was first inferred from the rotation curves of spiral galaxies, which suggested the presence of unseen matter
• Cosmic microwave background (CMB) observations support the Big Bang theory and provide constraints on the early universe and galaxy formation
• Hubble Deep Fields revealed a wealth of distant galaxies and provided insights into galaxy evolution over cosmic time
• Galaxy color-magnitude diagram (red sequence, blue cloud) demonstrates the bimodality of galaxy populations and their evolution
• Tully-Fisher relation correlates the luminosity of a spiral galaxy with its rotation velocity, serving as a distance indicator
• Fundamental plane relates the size, surface brightness, and velocity dispersion of elliptical galaxies, providing insights into their formation and evolution
• Galactic archeology studies the chemical abundances and kinematics of stars in the Milky Way to reconstruct its formation history
• Simulations (Illustris, EAGLE) model the formation and evolution of galaxies in a cosmological context, incorporating various physical processes

Connections to Other Astronomical Topics

• Galaxy evolution is closely tied to cosmology, as the formation and growth of galaxies depend on the properties and evolution of the universe
  • Cosmological parameters (matter density, dark energy) influence the growth of structure and galaxy evolution
• Dark matter plays a crucial role in galaxy formation and evolution, providing the gravitational framework for baryonic matter to collapse and form galaxies
• Stellar evolution is a key driver of galaxy evolution, as stars produce and expel heavy elements, influencing the chemical composition and properties of galaxies
• Supermassive black holes at the centers of galaxies can have a significant impact on galaxy evolution through feedback mechanisms (AGN feedback)
• Gravitational wave astronomy may provide new insights into galaxy evolution, particularly the role of mergers and the formation of supermassive black hole binaries
• Astrobiology and the search for extraterrestrial life are connected to galaxy evolution, as the formation and evolution of galaxies determine the conditions for the emergence of life
• Large-scale structure formation (cosmic web, filaments, voids) is closely related to galaxy formation and evolution, as galaxies form and evolve within this framework