👽Galaxies and the Universe Unit 6 – Galactic Dynamics and Interactions

Key Concepts and Terminology

• Galactic dynamics involves the study of the motions and interactions of stars, gas, and dark matter within galaxies
• Gravitational potential energy is the energy stored in an object due to its position in a gravitational field
• Virial theorem relates the average kinetic energy of a system to its average potential energy
• Tidal forces are differential gravitational forces that can stretch and deform galaxies during interactions
• Dynamical friction is the drag force experienced by a massive object moving through a field of smaller objects (stars, dark matter particles)
• Relaxation time is the time required for a system to reach equilibrium through collisions or interactions
  • Two-body relaxation occurs through close encounters between individual stars
  • Violent relaxation is a rapid process that occurs during galaxy mergers or collapses
• Disk galaxies have a flattened, rotating structure with spiral arms (Milky Way) while elliptical galaxies have a more spheroidal shape and little net rotation (M87)

Galactic Structure and Components

• Disk galaxies consist of a thin disk of stars and gas, a central bulge, and a surrounding halo of stars and dark matter
  • Spiral arms are regions of enhanced star formation and density within the disk
  • The bulge is a dense, spheroidal component at the center of the galaxy
• Elliptical galaxies have a smooth, ellipsoidal distribution of stars with little gas or ongoing star formation
• Dark matter halos extend well beyond the visible components of galaxies and dominate their total mass
• Galactic nuclei are the central regions of galaxies and can host supermassive black holes (Sagittarius A* in the Milky Way)
• Globular clusters are dense, spherical collections of old stars that orbit in the halos of galaxies
• Stellar populations in galaxies can be classified as Population I (young, metal-rich) or Population II (old, metal-poor)
• Interstellar medium (ISM) is the gas and dust between stars in a galaxy and plays a crucial role in star formation and galactic evolution

Dynamics of Isolated Galaxies

• Rotation curves describe the orbital velocities of stars and gas as a function of distance from the galactic center
  • Flat rotation curves at large radii indicate the presence of dark matter halos
• Velocity dispersion is a measure of the random motions of stars in a galaxy and is higher in elliptical galaxies compared to disk galaxies
• Jeans equations describe the balance between gravitational forces and the pressure support from random motions in a galaxy
• Stability of galactic disks is influenced by the Toomre Q parameter, which compares the stabilizing effects of rotation and velocity dispersion to the destabilizing effect of self-gravity
  • Spiral arms can form through density wave theory or swing amplification when Q is close to 1
• Galactic fountains and winds are outflows of gas driven by supernovae and star formation in galaxies
• Galactic bars are elongated structures in the central regions of some disk galaxies that can drive gas inflows and star formation (Milky Way has a bar)
• Warps are distortions in the outer regions of galactic disks caused by tidal interactions or intergalactic medium

Galaxy Interactions and Mergers

• Tidal interactions occur when galaxies pass close to each other, leading to the formation of tidal tails, bridges, and shells
  • The Antennae Galaxies (NGC 4038/4039) are a famous example of an ongoing galaxy merger exhibiting tidal features
• Minor mergers involve the accretion of smaller satellite galaxies by a larger galaxy (Sagittarius Dwarf Galaxy merging with the Milky Way)
• Major mergers occur when two galaxies of similar mass collide and merge, often resulting in the formation of an elliptical galaxy
• Starbursts are episodes of intense star formation triggered by galaxy interactions and mergers
• Dual active galactic nuclei (AGN) can be observed in some merging systems as the supermassive black holes from each galaxy coalesce
• Merger remnants are the products of galaxy mergers and can exhibit disturbed morphologies, tidal features, and enhanced star formation rates
• Merger trees trace the hierarchical growth of galaxies through mergers over cosmic time in the context of cosmological structure formation

Dark Matter in Galactic Dynamics

• Dark matter is a form of matter that does not interact electromagnetically but exerts gravitational influence
  • Cold dark matter (CDM) is the leading candidate and consists of slow-moving, massive particles
• Missing mass problem refers to the discrepancy between the observed rotation curves of galaxies and the predicted curves based on visible matter alone
• Dark matter halos surround galaxies and extend well beyond their visible components, as inferred from rotation curves and gravitational lensing
• Navarro-Frenk-White (NFW) profile is a commonly used model for the density distribution of dark matter halos, characterized by a cuspy central region and an extended outer profile
• Substructure in dark matter halos arises from the hierarchical nature of structure formation, with smaller halos being accreted by larger ones
• Gravitational lensing provides a direct way to map the distribution of dark matter in galaxies and clusters (bullet cluster)
• Alternative theories to dark matter, such as modified Newtonian dynamics (MOND), have been proposed but face challenges in explaining all observational evidence

Observational Techniques and Evidence

• Doppler spectroscopy is used to measure the radial velocities of stars and gas in galaxies, providing information about their kinematics and rotation curves
• Integral field spectroscopy (IFS) enables spatially resolved measurements of galaxy kinematics and stellar populations (SAURON, CALIFA, MaNGA surveys)
• Multi-wavelength observations are essential for studying different components of galaxies (stars, gas, dust) and their interactions
  • Radio observations trace neutral hydrogen (HI) gas and synchrotron emission from cosmic rays
  • Infrared observations probe dust emission and obscured star formation
  • X-ray observations reveal hot gas and active galactic nuclei
• Tidal streams and shells around galaxies provide evidence for past interactions and mergers (Sagittarius stream around the Milky Way)
• Morphological disturbances, such as tidal tails and bridges, indicate ongoing or recent galaxy interactions (Mice Galaxies, NGC 4676)
• Kinematic anomalies, such as counter-rotating cores or misaligned gas and stellar kinematics, can result from galaxy mergers or interactions
• Stellar population gradients and age-metallicity relations hold clues to the formation and evolutionary history of galaxies

Computational Models and Simulations

• N-body simulations are used to model the gravitational interactions of stars, dark matter particles, and gas in galaxies and cosmological structures
  • Particle-mesh (PM) methods divide the simulation volume into a grid and calculate gravitational forces on particles
  • Tree codes (Barnes-Hut, Oct-tree) hierarchically subdivide space and approximate long-range forces for efficient computation
• Hydrodynamical simulations include the effects of gas dynamics, star formation, and feedback processes in addition to gravity (EAGLE, IllustrisTNG, FIRE simulations)
• Subgrid models are used to describe physical processes that occur on scales smaller than the resolution of the simulation (star formation, supernova feedback, black hole accretion)
• Cosmological simulations model the formation and evolution of galaxies within the larger context of cosmic structure formation (Millennium, Bolshoi, Illustris simulations)
• Isolated galaxy simulations focus on the detailed dynamics and evolution of individual galaxies or mergers
• Merger trees can be extracted from cosmological simulations to study the hierarchical growth of galaxies over time
• Synthetic observations can be generated from simulations to compare with real observational data and test theoretical predictions

Implications for Galaxy Evolution

• Hierarchical structure formation leads to the growth of galaxies through mergers and accretion over cosmic time
  • Disk galaxies form from the cooling and collapse of gas within dark matter halos
  • Elliptical galaxies are thought to result from major mergers of disk galaxies
• Feedback processes, such as supernovae and active galactic nuclei, can regulate star formation and gas content in galaxies
  • Galactic winds can eject gas from galaxies and enrich the intergalactic medium
  • Quenching of star formation can occur through various mechanisms (virial shock heating, AGN feedback, environmental effects)
• Environmental effects, such as ram pressure stripping and tidal interactions, can influence the evolution of galaxies in clusters and groups
• Morphology-density relation describes the trend of early-type (elliptical) galaxies being more common in dense environments compared to late-type (spiral) galaxies
• Galactic archaeology uses the present-day properties of galaxies (kinematics, stellar populations) to infer their formation and evolutionary histories
• Scaling relations, such as the Tully-Fisher relation and Faber-Jackson relation, provide insights into the connections between galaxy properties and their formation processes
• The interplay between dark matter, baryonic matter, and feedback processes shapes the properties and evolution of galaxies over cosmic time