🪐Intro to Astronomy Unit 26 – Galaxies

What Are Galaxies?

• Galaxies are vast cosmic islands consisting of stars, planets, gas, dust, and dark matter held together by gravity
• Contain billions of stars, with the Milky Way estimated to have between 100-400 billion stars
• Range in size from dwarf galaxies with a few billion stars to giant galaxies with trillions of stars
• Classified based on their shape and structure (spiral, elliptical, irregular)
• Separated by vast distances, with the nearest galaxy, Andromeda, located 2.5 million light-years away
• Host a variety of celestial objects, including stars, planets, nebulae, star clusters, and supermassive black holes at their centers
• Serve as the building blocks of the universe, tracing the large-scale structure of the cosmos

Types of Galaxies

• Spiral galaxies have a flat, rotating disk with spiral arms winding outward from a central bulge (Milky Way, Andromeda)
  • Spiral arms contain younger, bluer stars and active star formation regions
  • Central bulge consists of older, redder stars
• Elliptical galaxies have a smooth, ellipsoidal shape without distinct features like spiral arms
  • Range from nearly spherical to highly elongated
  • Contain mostly older, redder stars with little ongoing star formation
  • Tend to be more massive than spiral galaxies
• Irregular galaxies lack a distinct shape or structure, often appearing chaotic and asymmetrical
  • May result from gravitational interactions or mergers with other galaxies
  • Examples include the Large and Small Magellanic Clouds, satellite galaxies of the Milky Way
• Lenticular galaxies have characteristics of both spiral and elliptical galaxies
  • Contain a disk component like spiral galaxies but lack distinct spiral arms
  • Have a prominent bulge component similar to elliptical galaxies
• Dwarf galaxies are small galaxies with fewer stars and lower masses compared to their larger counterparts
  • Can be classified as dwarf ellipticals, dwarf spheroidals, or dwarf irregulars
  • Play a crucial role in understanding galaxy formation and evolution

Structure and Components

• Disk: The flat, rotating component of spiral galaxies containing stars, gas, and dust
  • Thin disk hosts younger, metal-rich stars and ongoing star formation
  • Thick disk consists of older, metal-poor stars
• Bulge: The central, spheroidal component of galaxies, particularly prominent in spiral and lenticular galaxies
  • Contains older, redder stars with little ongoing star formation
  • Hosts the supermassive black hole at the galaxy's center
• Halo: The extended, roughly spherical component surrounding a galaxy
  • Consists of older, metal-poor stars and globular clusters
  • Contains a significant portion of a galaxy's dark matter
• Spiral arms: The distinctive, winding structures in spiral galaxies
  • Sites of active star formation and younger, bluer stars
  • Density waves compress gas and trigger star formation
• Bars: Elongated structures found in the central regions of some spiral galaxies
  • Thought to play a role in funneling gas and stars toward the galactic center
  • May contribute to the formation of bulges and supermassive black holes
• Supermassive black holes: Massive black holes residing at the centers of most galaxies
  • Can have masses millions to billions of times that of the Sun
  • Play a crucial role in galaxy evolution and the formation of active galactic nuclei

Galaxy Formation and Evolution

• Galaxies form from the gravitational collapse of massive gas clouds in the early universe
  • Initial density fluctuations in the cosmic microwave background seed galaxy formation
  • Dark matter halos provide the gravitational framework for gas to collapse and form stars
• Hierarchical merging plays a significant role in galaxy growth and evolution
  • Smaller galaxies merge to form larger galaxies over cosmic time
  • Mergers can trigger bursts of star formation and alter galaxy morphology
• Secular evolution describes the internal processes that shape galaxies over time
  • Includes the formation of bars, spiral arms, and galactic bulges
  • Driven by gravitational instabilities, gas dynamics, and stellar feedback
• Feedback from stars and active galactic nuclei regulates galaxy evolution
  • Supernova explosions and stellar winds can drive gas outflows and suppress star formation
  • AGN feedback can heat and expel gas, limiting galaxy growth
• Environmental effects influence galaxy evolution
  • Tidal interactions and ram-pressure stripping can remove gas and stars from galaxies
  • Galaxy clusters provide a dense environment for interactions and mergers
• Studying galaxy evolution requires observations across multiple wavelengths and redshifts
  • High-redshift observations probe earlier stages of galaxy formation
  • Multi-wavelength data reveal different components and processes within galaxies

The Milky Way: Our Home Galaxy

• The Milky Way is a large, barred spiral galaxy with a diameter of approximately 100,000 light-years
• Consists of four main components: the disk, bulge, halo, and central supermassive black hole
• The disk is divided into the thin disk and the thick disk
  • The thin disk contains younger, metal-rich stars and ongoing star formation
  • The thick disk consists of older, metal-poor stars
• The bulge is the central, spheroidal component containing older stars
  • Hosts the Galactic Center, the rotational center of the Milky Way
  • Contains the supermassive black hole Sagittarius A* with a mass of ~4 million solar masses
• The halo is the extended, roughly spherical component surrounding the disk and bulge
  • Contains ancient, metal-poor stars and globular clusters
  • Hosts a significant portion of the Milky Way's dark matter
• The Milky Way has several satellite galaxies, including the Large and Small Magellanic Clouds
• The Sun is located in the disk, approximately 27,000 light-years from the Galactic Center
• The Milky Way is part of the Local Group, a galaxy group that includes Andromeda and dozens of smaller galaxies

Observing and Studying Galaxies

• Galaxies are observed using telescopes across the electromagnetic spectrum
  • Optical telescopes detect visible light from stars and ionized gas
  • Radio telescopes observe neutral hydrogen, molecular gas, and synchrotron radiation
  • Infrared telescopes study dust, cool stars, and star-forming regions
  • X-ray and gamma-ray telescopes observe high-energy phenomena like active galactic nuclei and supernovae
• Spectroscopy is used to study the composition, kinematics, and distances of galaxies
  • Doppler shifts in spectral lines reveal galaxy rotation and radial velocities
  • Emission and absorption lines provide information about the gas, stars, and chemical composition
• Multi-wavelength observations provide a comprehensive view of galaxy structure and evolution
  • Different wavelengths trace different components and processes within galaxies
  • Combining data from multiple wavelengths allows for a more complete understanding
• Hubble's law relates a galaxy's distance to its recessional velocity due to the expansion of the universe
  • Redshift measurements can be used to determine a galaxy's distance
  • Allows for the construction of a distance ladder and the mapping of large-scale structure
• Surveys and catalogs are used to study large samples of galaxies
  • The Sloan Digital Sky Survey (SDSS) has imaged and cataloged millions of galaxies
  • The Hubble Space Telescope has provided high-resolution images of distant galaxies
• Simulations and theoretical models are used to study galaxy formation and evolution
  • N-body simulations model the gravitational interactions of dark matter and stars
  • Hydrodynamical simulations include the effects of gas dynamics and star formation

Galaxies in the Universe

• The observable universe contains billions of galaxies, with estimates ranging from 100 billion to 2 trillion
• Galaxies are not uniformly distributed throughout the universe, but instead form a complex large-scale structure
  • Galaxies are organized into groups, clusters, and superclusters
  • Clusters can contain hundreds to thousands of galaxies bound by gravity
  • Superclusters are the largest known structures, containing multiple galaxy clusters and groups
• Voids are vast, largely empty regions between galaxy clusters and superclusters
  • Can span tens to hundreds of millions of light-years
  • Contain few galaxies compared to the denser filaments and clusters
• The large-scale structure of the universe is thought to have originated from quantum fluctuations in the early universe
  • These fluctuations were amplified by inflation, a period of rapid expansion in the early universe
  • Dark matter halos formed from these fluctuations, providing the gravitational framework for galaxy formation
• The study of galaxies and their distribution helps constrain cosmological models and parameters
  • The matter-energy content of the universe affects the growth and evolution of large-scale structure
  • Observations of galaxy clusters and their evolution provide evidence for dark matter and dark energy
• Understanding the properties and evolution of galaxies across cosmic time is a key goal of modern astrophysics
  • Surveys like the James Webb Space Telescope aim to study the earliest galaxies and their formation
  • Combining observations, simulations, and theoretical models provides a comprehensive view of galaxy evolution

Cool Galaxy Facts and Mysteries

• The Milky Way and Andromeda galaxies are on a collision course and will merge in about 4.5 billion years
  • The merged galaxy, sometimes called "Milkdromeda," will be an elliptical galaxy
  • Earth and the Solar System are unlikely to be directly affected by the merger
• Some galaxies have active galactic nuclei (AGN) powered by supermassive black holes
  • AGN can outshine their host galaxies and emit radiation across the electromagnetic spectrum
  • Examples include quasars, blazars, and Seyfert galaxies
• The most distant known galaxy, GN-z11, is located 13.4 billion light-years away
  • Observed as it appeared just 400 million years after the Big Bang
  • Provides insights into the early stages of galaxy formation and evolution
• Galactic cannibalism describes the process of larger galaxies consuming smaller galaxies through tidal interactions
  • The Milky Way is currently cannibalizing the Sagittarius Dwarf Spheroidal Galaxy
  • Remnants of past mergers can be seen as stellar streams in the Milky Way's halo
• Some galaxies, known as starburst galaxies, experience exceptionally high rates of star formation
  • Starburst activity can be triggered by galaxy mergers or tidal interactions
  • Examples include the Antennae Galaxies and Messier 82
• Dark matter halos surrounding galaxies extend far beyond the visible matter
  • The nature of dark matter remains one of the greatest mysteries in astrophysics
  • Theories include weakly interacting massive particles (WIMPs) and axions
• Galactic archaeology studies the fossil records of galaxy formation and evolution
  • Uses the ages, chemical compositions, and kinematics of stars to reconstruct a galaxy's history
  • Provides insights into the assembly and growth of galaxies over cosmic time