🚀Astrophysics II Unit 7 – Galactic Structure and Dynamics

Key Concepts and Terminology

• Galaxies are massive, gravitationally bound systems consisting of stars, gas, dust, and dark matter
• Morphological classification of galaxies based on their appearance and structure (elliptical, spiral, irregular)
• Hubble sequence organizes galaxies according to their morphology and provides a framework for studying galaxy evolution
• Bulge refers to the central, spheroidal component of a galaxy, typically containing older stars
• Disk is the flat, rotating component of a galaxy, often featuring spiral arms and younger stars
• Halo surrounds the disk and bulge, containing older stars, globular clusters, and a significant portion of a galaxy's dark matter
• Metallicity represents the abundance of elements heavier than hydrogen and helium in stars and interstellar medium
• Stellar populations are groups of stars with similar ages, chemical compositions, and kinematic properties

Galactic Components and Their Properties

• Interstellar medium (ISM) fills the space between stars, consisting of gas (mostly hydrogen and helium) and dust
  • Molecular clouds are dense regions of the ISM where star formation occurs
  • HII regions are areas of ionized hydrogen surrounding young, hot stars
• Globular clusters are dense, spherical collections of old stars that orbit the galaxy's halo
• Open clusters are loosely bound groups of young stars that formed together in the galaxy's disk
• Supermassive black holes reside at the centers of most galaxies and can significantly influence their host galaxy's evolution
• Stellar nurseries are regions of active star formation, typically found in spiral arms or irregular galaxies
• Galactic outflows are streams of gas and dust expelled from a galaxy by supernovae, stellar winds, or active galactic nuclei
• Galactic winds can transport heavy elements and regulate star formation in galaxies

Stellar Populations and Distribution

• Population I stars are young, metal-rich stars found in the disk and spiral arms of galaxies
• Population II stars are old, metal-poor stars that inhabit the halo and bulge of galaxies
• Population III stars are the hypothetical first generation of stars, believed to be extremely massive and metal-free
• Stellar initial mass function (IMF) describes the distribution of initial masses for a population of stars
• Stellar metallicity gradient refers to the decrease in metal abundance with increasing distance from the galactic center
• Stellar age gradient describes the trend of older stars being more concentrated in the bulge and inner regions of a galaxy
• Disk scale length characterizes the exponential decline in stellar density with increasing distance from the galactic center
• Bulge-to-disk ratio compares the mass or luminosity of a galaxy's bulge to its disk, providing insight into its morphology and evolution

Galactic Rotation and Kinematics

• Differential rotation describes the variation in rotational velocity with radius in a galaxy's disk
  • Flat rotation curves indicate that rotational velocity remains constant with increasing distance from the galactic center
  • Rotation curves provide evidence for the presence of dark matter in galaxies
• Velocity dispersion measures the range of stellar velocities within a galaxy or a specific component (bulge, disk, or halo)
• Asymmetric drift refers to the lagging of older stellar populations behind the disk's rotation due to their higher velocity dispersion
• Galactic shear is the difference in rotational velocity between inner and outer regions of a galaxy's disk
• Tidal interactions occur when galaxies pass close to each other, leading to distortions in their morphology and potentially triggering star formation
• Galactic mergers are violent collisions between galaxies that can significantly alter their structure and properties
• Kinematic heating is the increase in velocity dispersion of stars over time due to gravitational interactions or mergers

Dark Matter in Galaxies

• Dark matter is a hypothetical form of matter that does not interact with electromagnetic radiation but has gravitational effects
  • Cold dark matter (CDM) is a model where dark matter particles have low velocities and can form small-scale structures
  • Warm dark matter (WDM) is a model where dark matter particles have higher velocities than CDM, suppressing the formation of small-scale structures
• Dark matter halos surround galaxies and extend well beyond their visible components, providing additional gravitational support
• Gravitational lensing is the distortion of light from background sources by the gravitational field of intervening matter, including dark matter
  • Strong lensing occurs when a massive object (like a galaxy cluster) creates multiple images of a background source
  • Weak lensing refers to the subtle distortions in the shapes of background galaxies due to the gravitational influence of intervening matter
• Bullet Cluster is a system of two colliding galaxy clusters that provides strong evidence for the existence of dark matter
• Missing satellites problem refers to the discrepancy between the observed number of satellite galaxies and the predictions of CDM simulations
• Cusp-core problem is the discrepancy between the predicted cuspy dark matter density profiles and the observed cored profiles in some galaxies

Galactic Evolution and Formation Theories

• Hierarchical structure formation is the process by which larger structures (like galaxies) form through the merging of smaller structures
• Press-Schechter formalism is a statistical approach to predicting the abundance of dark matter halos as a function of mass and redshift
• Jeans instability is the process by which a self-gravitating system (like a molecular cloud) collapses if its gravitational energy exceeds its internal energy
• Monolithic collapse model proposes that galaxies formed early in the universe through the rapid collapse of massive gas clouds
• Secular evolution refers to the gradual changes in a galaxy's structure and properties over long timescales, driven by internal processes
• Feedback processes (like supernovae or active galactic nuclei) can regulate star formation and influence galaxy evolution
• Reionization is the process by which neutral hydrogen in the early universe was ionized by the first generation of stars and galaxies
• Cosmic downsizing refers to the observation that more massive galaxies formed their stars earlier and over shorter timescales than less massive galaxies

Observational Techniques and Tools

• Multi-wavelength observations are crucial for studying different components of galaxies (stars, gas, dust, and dark matter)
  • Radio observations probe neutral hydrogen, molecular gas, and synchrotron emission from cosmic rays
  • Infrared observations trace dust emission and can reveal obscured star formation
  • Optical and ultraviolet observations are sensitive to stellar populations and ionized gas
  • X-ray observations can detect hot gas, active galactic nuclei, and the intracluster medium in galaxy clusters
• Integral field spectroscopy (IFS) provides spatially resolved spectra across a galaxy, enabling studies of kinematics and chemical composition
• Adaptive optics (AO) systems correct for atmospheric distortions, allowing ground-based telescopes to achieve higher angular resolution
• Gravitational lensing can be used as a tool to study the distribution of dark matter in galaxies and galaxy clusters
• Simulations play a crucial role in understanding galaxy formation and evolution, providing insights into the complex physical processes involved
• Citizen science projects (like Galaxy Zoo) engage the public in classifying galaxies and contribute to our understanding of galaxy evolution

Current Research and Open Questions

• The role of supermassive black holes in galaxy evolution, particularly through feedback processes like active galactic nuclei
• The nature of dark matter and its distribution within galaxies, as well as its role in galaxy formation and evolution
• The origin and properties of ultra-diffuse galaxies (UDGs), which have low surface brightnesses and large sizes
• The formation and evolution of the first galaxies in the early universe, and their contribution to cosmic reionization
• The interplay between galaxy mergers, star formation, and the growth of supermassive black holes
• The environmental effects on galaxy evolution, such as the role of galaxy clusters and the intergalactic medium
• The assembly history of the Milky Way and its satellite galaxies, and what it reveals about galaxy formation in a cosmological context
• The connection between the properties of galaxies and their large-scale structure, as traced by galaxy surveys like SDSS and upcoming surveys like LSST and Euclid