10.2 Galactic dynamics and rotation
2 min read•july 25, 2024
The Milky Way's structure and rotation are key to understanding our galaxy. Different parts rotate at varying speeds, shaping spiral arms and mixing stellar populations. This , measured by , affects how our galaxy evolves over time.
Our galaxy's rotation curve reveals a surprising twist. Instead of slowing down at the edges as expected, it stays flat. This odd behavior hints at the presence of , an invisible substance that's changing how we view the universe.
Galactic Structure and Rotation
Differential rotation in Milky Way
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- Different parts of galaxy rotate at varying angular velocities inner regions rotate faster than outer regions
- Spiral arm formation and maintenance through shearing of gas clouds and star-forming regions
- Mixing of stellar populations and chemical elements throughout galactic disk
- Oort constants A and B measure differential rotation
- V represents circular velocity, R is galactocentric distance
Rotation curve of Milky Way
- Plot of circular velocity vs distance from galactic center observed using various tracers (HI regions, CO emission)
- Expected rotation curve for visible mass follows
- Observed rotation curve remains flat or slightly rising at large radii deviating from Keplerian expectation
- Mass distribution implications
- Visible mass concentrated in central bulge and disk
- Additional unseen mass required to explain
- Extended inferred to account for discrepancy
Dark Matter and Galactic Dynamics
Dark matter evidence from rotation
- Discrepancy between observed flat rotation curve and expected Keplerian decline indicates presence of unseen mass
- increases with radius suggesting non-luminous matter dominates outer regions
- derived from rotation curve requires more mass than observed in visible matter (stars, gas)
- Dark matter halo models proposed to explain observations
- Navarro-Frenk-White (NFW) profile predicts density distribution
- assumes constant velocity dispersion
- Alternative theories like (MOND) proposed but lack observational support compared to dark matter paradigm
Stellar motions for galactic dynamics
- (angular movement across sky) and (motion along line of sight) provide 3D stellar kinematics
- measure spread in stellar velocities increase with stellar age and distance from galactic plane
- relate velocity dispersions to gravitational potential used to estimate mass distribution
- causes lag in rotational velocity of stars compared to circular orbit increases with velocity dispersion
- Vertical motion studies constrain local dark matter density probe galactic disk structure and evolution
- provides precise 3D positions and velocities for billions of stars revolutionizing understanding of Milky Way dynamics
Key Terms to Review (18)
Asymmetric drift: Asymmetric drift refers to the phenomenon where stars in a galaxy's disk exhibit different average velocities depending on their positions relative to the center of the galaxy. This effect arises due to gravitational interactions and the distribution of mass within the galaxy, leading to differences in orbital speeds and patterns. Understanding asymmetric drift is crucial for studying galactic dynamics and rotation, as it provides insight into the overall structure and behavior of galaxies.
Dark Matter: Dark matter is a mysterious and invisible form of matter that does not emit, absorb, or reflect light, making it undetectable by traditional astronomical methods. Despite being unseen, it makes up about 27% of the universe's total mass-energy content and plays a crucial role in the structure and evolution of galaxies, influencing gravitational interactions in the cosmos.
Dark matter halo: A dark matter halo is a region surrounding a galaxy that is composed primarily of dark matter, which does not emit light or energy and is thus invisible to telescopes. This halo plays a crucial role in the gravitational dynamics of galaxies, affecting their rotation curves and overall structure, while also serving as a major component in the formation and clustering of galaxies throughout the universe.
Differential Rotation: Differential rotation refers to the phenomenon where different parts of a rotating object move at different angular velocities. In the context of galactic dynamics, this term is crucial for understanding how various components of galaxies, including stars and gas, interact and evolve over time. The differing rotational speeds can influence the formation of structures within galaxies, such as spiral arms, and provide insights into the overall dynamics and mass distribution within a galaxy.
Flat rotation curve: A flat rotation curve refers to the observation that the rotational speeds of stars and gas in a galaxy remain constant or 'flat' with increasing distance from the galaxy's center, rather than decreasing as expected from Newtonian dynamics. This phenomenon is critical for understanding the mass distribution within galaxies and suggests the presence of unseen mass, or dark matter, influencing the dynamics of galaxies.
Gaia Mission: The Gaia Mission is a groundbreaking space observatory launched by the European Space Agency in 2013, aimed at creating the most detailed three-dimensional map of our galaxy, the Milky Way. This mission involves measuring the positions, distances, and motions of approximately one billion stars, enabling astronomers to investigate the structure, formation, and evolution of the galaxy, as well as its dynamics and rotation.
Galactic rotation curve: A galactic rotation curve is a plot that shows the rotational velocity of stars or gas in a galaxy as a function of their distance from the galaxy's center. This curve provides important insights into the distribution of mass within galaxies and plays a crucial role in understanding galactic dynamics and rotation, particularly in revealing the presence of dark matter.
Gravitational potential: Gravitational potential is the work done per unit mass to move an object from a reference point to a specific point in a gravitational field, usually measured in joules per kilogram (J/kg). This concept is essential in understanding how gravity affects the motion of celestial bodies, influencing their dynamics and rotation within galaxies.
Isothermal Sphere Model: The isothermal sphere model is a theoretical framework used to describe the distribution of matter in a spherical region of space, where the temperature remains constant throughout. This model assumes that the gravitational forces and thermal pressure balance each other, leading to a stable configuration of stars and gas within galaxies. It provides insights into the dynamics of galaxies, particularly in understanding their rotation curves and mass distributions.
Jeans Equations: The Jeans equations are a set of mathematical expressions that describe the dynamical behavior of a system of stars in a gravitational field, essentially linking the motions of stars to the distribution of mass within galaxies. These equations help in understanding how galaxies rotate and how their mass is distributed, providing insights into galactic dynamics and the stability of stellar systems under gravitational influence.
Keplerian decline: Keplerian decline refers to the observed phenomenon where the density of stars in a galaxy decreases with increasing distance from the center, typically following a specific mathematical pattern inspired by Kepler's laws of planetary motion. This decline is crucial for understanding the distribution of mass within galaxies, influencing their dynamics and rotation. The concept also links closely to how galaxies evolve over time and how their gravitational interactions affect their structures.
Mass-to-light ratio: The mass-to-light ratio is a measure that compares the mass of an astronomical object to its luminosity, providing insights into its composition and structure. This ratio helps astronomers understand how much mass is present in stars or galaxies compared to the light they emit, which can indicate the presence of dark matter or the efficiency of star formation.
Modified newtonian dynamics: Modified Newtonian Dynamics (MOND) is a theoretical framework that aims to explain the observed discrepancies in galactic rotation curves without invoking dark matter. It suggests that at low accelerations, such as those found in the outer regions of galaxies, the laws of gravity and motion differ from Newton's laws, leading to modified gravitational effects.
Navarro-Frenk-White Profile: The Navarro-Frenk-White (NFW) profile is a mathematical model that describes the density distribution of dark matter halos, which are crucial for understanding the structure of galaxies and their formation. This profile predicts that dark matter density decreases with radius, following a specific formula where the density is high at small radii and falls off as one moves outward, demonstrating a characteristic 'cusp' behavior. The NFW profile plays a vital role in explaining the dynamics of galaxies and the evolution of structures in the universe.
Oort Constants: Oort constants are two parameters, denoted as A and B, that describe the distribution of stars in the Milky Way galaxy, particularly in relation to its rotation and dynamics. These constants help in understanding the density of stars and their gravitational effects on the overall motion of the galaxy, linking kinematics and dynamics in galactic studies.
Proper motions: Proper motions refer to the apparent angular movement of a star across the sky relative to more distant stars, measured in seconds of arc per year. This motion occurs due to the actual motion of the star through space, combined with the effects of parallax and perspective. Proper motions are essential for understanding the dynamics of stars within the galaxy, providing insight into their velocities and trajectories as they orbit the galactic center.
Radial Velocities: Radial velocities refer to the speed at which an object moves toward or away from an observer along the line of sight. This measurement is crucial in understanding the motion of stars and galaxies, especially in determining their distances and the dynamics of galactic rotation. By analyzing the Doppler shift in spectral lines, radial velocities provide insight into the gravitational interactions within galaxies and the overall structure of the universe.
Velocity dispersions: Velocity dispersions refer to the range of velocities observed among stars or other objects within a particular system, indicating how much individual velocities vary from the average velocity. This concept is crucial in understanding the dynamics of galaxies, as it helps reveal information about the mass distribution, gravitational forces at play, and the overall stability of the galactic structure.