👽Galaxies and the Universe Unit 1 – Galactic Types and Classification

What's the Big Picture?

• Galaxies are vast cosmic islands containing stars, planets, gas, dust, and dark matter gravitationally bound together
• The observable universe contains an estimated two trillion galaxies, each with billions of stars
• Galaxies come in a wide variety of shapes, sizes, and compositions, ranging from dwarf galaxies with a few billion stars to giant elliptical galaxies with trillions of stars
• The study of galactic types and classification helps astronomers understand the formation, evolution, and structure of the universe
• Classification schemes organize galaxies based on their physical properties, such as shape, size, color, and star formation rates
• Observational techniques, including telescopes, spectroscopy, and multi-wavelength astronomy, enable the study of galaxies across the electromagnetic spectrum
• Galaxy evolution and interactions, such as mergers and collisions, play a crucial role in shaping the morphology and properties of galaxies over cosmic time

Key Concepts and Definitions

• Morphology: The study of the physical structure and shape of galaxies
• Hubble sequence: A classification scheme that organizes galaxies based on their shape and structure, ranging from elliptical to spiral galaxies
• Elliptical galaxies: Smooth, ellipsoidal galaxies with little to no gas or dust and minimal star formation (M87)
• Spiral galaxies: Galaxies with a central bulge and spiral arms, rich in gas and dust, and actively forming stars (Milky Way)
• Barred spiral galaxies: Spiral galaxies with a prominent bar-shaped structure extending from the central bulge (NGC 1300)
• Irregular galaxies: Galaxies with no distinct regular shape or structure, often the result of gravitational interactions or mergers (Large Magellanic Cloud)
• Dwarf galaxies: Small galaxies with fewer stars and lower luminosities compared to their larger counterparts (Leo I)
  • Dwarf elliptical galaxies: Small, low-luminosity elliptical galaxies (M32)
  • Dwarf spheroidal galaxies: Faint, diffuse galaxies with no discernible structure and minimal star formation (Draco)
  • Dwarf irregular galaxies: Small, irregular galaxies with ongoing star formation (NGC 1569)
• Active galactic nuclei (AGN): Extremely luminous central regions of some galaxies, powered by accretion onto supermassive black holes (Centaurus A)
• Starburst galaxies: Galaxies undergoing an exceptionally high rate of star formation, often triggered by interactions or mergers (M82)

Types of Galaxies

• Elliptical galaxies (E0-E7) have smooth, ellipsoidal shapes and are classified based on their ellipticity
  • Contain mostly old, red stars and minimal gas and dust
  • Range in size from dwarf ellipticals to giant ellipticals with trillions of stars (M87)
• Spiral galaxies (Sa-Sd) have a central bulge and spiral arms extending from it
  • Classified based on the tightness of their spiral arms and the prominence of their central bulge
  • Contain a mix of old and young stars, with ongoing star formation in the spiral arms
  • Barred spiral galaxies (SBa-SBd) have a bar-shaped structure extending from the central bulge (Milky Way)
• Lenticular galaxies (S0) are a transitional type between elliptical and spiral galaxies
  • Have a central bulge and a disk but lack distinct spiral arms
  • Contain mostly old stars and minimal gas and dust (NGC 3115)
• Irregular galaxies (Irr) have no distinct regular shape or structure
  • Often the result of gravitational interactions or mergers
  • Can be classified as Irr I (some structure) or Irr II (no discernible structure)
  • Examples include the Large and Small Magellanic Clouds
• Dwarf galaxies are small galaxies with fewer stars and lower luminosities compared to their larger counterparts
  • Can be classified as dwarf elliptical (dE), dwarf spheroidal (dSph), or dwarf irregular (dIrr) galaxies
  • Play a crucial role in galaxy evolution through mergers and interactions (Leo I, Draco)

Classification Systems

• Hubble sequence (tuning fork diagram) is the most widely used classification scheme for galaxies
  • Organizes galaxies based on their morphology, from elliptical to spiral galaxies
  • Elliptical galaxies (E0-E7) are classified based on their ellipticity, with E0 being nearly spherical and E7 being highly elongated
  • Spiral galaxies are divided into normal (Sa-Sd) and barred (SBa-SBd) types, based on the tightness of their spiral arms and the prominence of their central bulge
  • Lenticular galaxies (S0) are placed between elliptical and spiral galaxies on the Hubble sequence
• De Vaucouleurs system extends the Hubble sequence to include more detailed subclasses
  • Adds intermediate classes (Sab, Sbc) and a separate branch for irregular galaxies (Irr I, Irr II)
  • Introduces a numerical notation for each subclass (e.g., SA(s)ab for a spiral galaxy with a small central bulge and loosely wound spiral arms)
• Morgan's galaxy classification scheme focuses on the central concentration of light in galaxies
  • Classifies galaxies as a (highly concentrated), af, f (moderately concentrated), fg, or g (low concentration)
  • Correlates with the Hubble sequence, with elliptical galaxies having high central concentrations and late-type spirals having low concentrations
• Yerkes classification system considers both the morphology and the presence of emission lines in a galaxy's spectrum
  • Combines the Hubble sequence with a spectral classification (A, F, G, K) based on the dominant stellar population
  • Example: Sa-K denotes a spiral galaxy with tightly wound arms and a spectrum dominated by K-type stars

Observational Techniques

• Optical telescopes, such as the Hubble Space Telescope, capture visible light from galaxies
  • Provide detailed images of galaxy morphology and structure
  • Enable the study of star formation, stellar populations, and galactic environments
• Radio telescopes, like the Very Large Array (VLA), observe galaxies at radio wavelengths
  • Detect emission from neutral hydrogen gas (HI), which traces the distribution and kinematics of gas in galaxies
  • Study the properties of active galactic nuclei (AGN) and the presence of radio jets
• Infrared telescopes, such as the James Webb Space Telescope (JWST), observe galaxies at infrared wavelengths
  • Penetrate through dust obscuration to reveal hidden star formation regions and the distribution of cold gas and dust
  • Enable the study of high-redshift galaxies, whose visible light is shifted into the infrared due to cosmic expansion
• X-ray telescopes, like the Chandra X-ray Observatory, detect high-energy emission from galaxies
  • Study the hot gas in galaxy clusters, the accretion processes in AGN, and the presence of supernova remnants
  • Provide insights into the interactions between galaxies and their environments
• Spectroscopy is the study of the wavelength distribution of light from galaxies
  • Reveals the composition, temperature, and velocity of stars and gas within galaxies
  • Enables the measurement of redshifts, which are used to determine the distances and recessional velocities of galaxies
• Multi-wavelength astronomy combines observations from different parts of the electromagnetic spectrum
  • Provides a comprehensive view of the physical processes occurring within galaxies
  • Allows for the study of the interplay between stars, gas, dust, and active galactic nuclei

Galaxy Evolution and Interactions

• Hierarchical formation: Galaxies grow through the accretion of smaller galaxies and the merging of similarly sized systems over cosmic time
  • Leads to the formation of larger, more massive galaxies with complex morphologies
  • Explains the observed diversity of galaxy types and the presence of galaxy clusters and superclusters
• Mergers: The collision and coalescence of two or more galaxies
  • Can be classified as major (similar-sized galaxies) or minor (a large galaxy accreting a smaller one) mergers
  • Trigger intense star formation, reshape galaxy morphology, and drive the growth of supermassive black holes
  • Example: The Antennae Galaxies (NGC 4038/4039) are a pair of interacting spiral galaxies in the early stages of a merger
• Tidal interactions: Gravitational forces between galaxies that distort their shapes and strip away stars and gas
  • Can lead to the formation of tidal tails, bridges, and shells (Mice Galaxies, NGC 4676)
  • Trigger star formation and can result in the formation of dwarf galaxies from the stripped material
• Feedback processes: The impact of star formation, supernovae, and active galactic nuclei on the surrounding gas and dust
  • Can heat and expel gas from galaxies, regulating star formation and influencing galaxy evolution
  • Example: Galactic winds driven by supernova explosions can enrich the intergalactic medium with heavy elements
• Environmental effects: The influence of a galaxy's surroundings on its evolution and properties
  • Galaxy clusters: High-density regions where galaxies experience frequent interactions and the effects of a hot, X-ray emitting intracluster medium (Coma Cluster)
  • Void galaxies: Isolated galaxies in low-density regions of the universe, which may have different properties and evolutionary histories compared to galaxies in clusters (MCG+01-02-015)

Cool Facts and Mind-Blowing Stuff

• The Milky Way is on a collision course with the Andromeda Galaxy (M31), with the merger expected to occur in about 4.5 billion years
  • The resulting galaxy, sometimes called "Milkdromeda," will be an elliptical galaxy with a combined mass of about 1.5 trillion solar masses
• The most distant known galaxy, GN-z11, is located approximately 13.4 billion light-years away
  • We observe it as it appeared just 400 million years after the Big Bang, providing insights into the early universe
• The Ultra Deep Field image taken by the Hubble Space Telescope reveals thousands of galaxies in a tiny patch of sky
  • Covering an area about 1/10th the size of the full Moon, this image showcases the vast number and diversity of galaxies in the observable universe
• Some galaxies, known as "red and dead" galaxies, have ceased star formation and are composed mainly of old, red stars
  • These galaxies, often massive ellipticals, have used up or lost their gas supply, preventing the formation of new stars
• The most luminous known galaxy, W2246-0526, is a hyperluminous infrared galaxy with a luminosity of about 350 trillion suns
  • Its extreme brightness is thought to be powered by a combination of intense star formation and a supermassive black hole at its center
• Galactic cannibalism is a process by which a larger galaxy consumes a smaller one through tidal forces and mergers
  • The Milky Way has undergone multiple cannibalistic events, with evidence found in the form of stellar streams and remnants of dwarf galaxies in its halo

Real-World Applications

• Understanding galaxy formation and evolution helps cosmologists constrain models of the universe
  • The distribution and properties of galaxies across cosmic time provide crucial insights into the nature of dark matter, dark energy, and the overall structure of the universe
• Studying the chemical composition and evolution of galaxies informs our understanding of the origin and distribution of heavy elements
  • This knowledge is crucial for fields such as astrobiology, as it helps determine the likelihood of life emerging in different cosmic environments
• Galactic astronomy has led to advancements in technology and instrumentation
  • The development of sensitive detectors, adaptive optics systems, and data processing techniques has benefited fields beyond astronomy, such as medical imaging and remote sensing
• The study of active galactic nuclei and supermassive black holes has implications for fundamental physics
  • Observations of these extreme environments test our understanding of general relativity, accretion processes, and high-energy astrophysics
• Outreach and education programs based on galactic astronomy help inspire public interest in science
  • Stunning images of galaxies, such as those from the Hubble Space Telescope, captivate audiences and encourage the pursuit of careers in STEM fields
• Collaborations between professional and amateur astronomers contribute to our knowledge of galaxies
  • Citizen science projects, such as Galaxy Zoo, engage the public in classifying galaxies and identifying unique or interesting objects, leading to new discoveries and increased scientific literacy