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25.3 The Mass of the Galaxy

3 min read•june 12, 2024

The 's mass is a cosmic mystery, with playing a starring role. Astronomers use orbital velocities to uncover the Galaxy's hidden mass, finding that visible matter alone can't explain what they see.

forms a massive halo around our Galaxy, dwarfing the visible components. Scientists are on the hunt for its true nature, with exotic particles like and as prime suspects in this celestial whodunit.

Measuring the Milky Way's Mass

Orbital velocities for dark matter detection

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  • Astronomers measure orbital velocities of objects (stars, gas clouds) in the Milky Way to infer the Galaxy's mass distribution
    • connects to the enclosed mass within the orbit v=GMrv = \sqrt{\frac{GM}{r}}
      • vv represents orbital velocity
      • GG is the
      • MM is the mass enclosed within the orbit
      • rr is the orbital radius
  • Observed orbital velocities in the Milky Way's disk exceed predictions based on visible matter alone
    • Implies the existence of additional unseen mass, attributed to dark matter
  • Orbital velocities remain high even at large Galactic radii
    • Suggests dark matter extends well past the visible disk of the Galaxy
    • This pattern is evident in the Galaxy's

Distribution of dark matter in Galaxy

  • Dark matter believed to form a roughly spherical halo enveloping the Milky Way
    • Halo reaches far beyond the Galaxy's visible disk
  • significantly more massive than the Milky Way's visible components
    • Estimated ~90% of the Galaxy's total mass is dark matter
  • Total mass of the Milky Way including dark matter approximated at 1−2×1012M⊙1-2 \times 10^{12} M_{\odot}
    • Considerably higher than the ~1011M⊙10^{11} M_{\odot} mass of visible components
  • Dark matter density peaks near Galactic center and decreases with distance
    • Density falls off more gradually than visible matter density
  • Extended accounts for the high outer orbital velocities observed in the Milky Way
  • The 's mass distribution can be inferred from these observations

Candidates for dark matter composition

  • Baryonic matter (ordinary protons, neutrons, electrons) proposed as dark matter candidates
    • Possibilities include , , cold gas clouds
  • Baryonic dark matter largely ruled out due to observational constraints
    • and show baryonic matter is a small fraction of total Universal matter
  • Non-baryonic matter now favored as primary dark matter constituent
    • Interacts weakly with electromagnetic radiation making direct detection challenging
  • Weakly Interacting Massive Particles (WIMPs) are leading non-baryonic candidates
    • Hypothetical particles with mass but feeble ordinary matter interactions
    • Supersymmetric particles like are proposed WIMPs
  • Axions also suggested as potential dark matter components
    • Predicted extremely low mass and exceedingly weak coupling to ordinary matter
  • Dark matter particles remain undetected despite extensive searches
    • Nature of dark matter persists as an open question in astronomy and physics

Additional methods for dark matter detection

  • analysis helps identify dark matter presence in galaxies
  • observations provide evidence for dark matter in galaxy clusters

Key Terms to Review (24)

Axions: Axions are hypothetical, extremely lightweight particles that were first proposed to resolve an apparent conflict in the theory of quantum chromodynamics (QCD), the fundamental theory of strong interactions. These particles are of great interest in the context of cosmology and astrophysics, as they may provide insights into the nature of dark matter and the overall composition of the universe.
Big Bang Nucleosynthesis: Big Bang nucleosynthesis refers to the production of the lightest atomic nuclei, such as hydrogen, helium, and lithium, in the early stages of the universe's evolution immediately following the Big Bang. This process is a crucial aspect of our understanding of the early universe and the formation of the elements that make up the cosmos.
Black Holes: A black hole is an extremely dense region of spacetime with a gravitational pull so strong that nothing, not even light, can escape from it. Black holes are formed when a massive star collapses in on itself at the end of its life cycle, creating a singularity surrounded by an event horizon.
Brown dwarfs: Brown dwarfs are celestial objects that are too large to be planets but not massive enough to sustain hydrogen fusion in their cores like true stars. They occupy the mass range between the heaviest gas giant planets and the lightest stars.
Brown Dwarfs: Brown dwarfs are substellar objects that are too large to be considered planets, yet not massive enough to sustain the nuclear fusion reactions that power stars. They occupy the mass range between the heaviest gas giant planets and the lightest stars, bridging the gap between these two celestial bodies.
Cold dark matter: Cold dark matter (CDM) consists of slow-moving particles that do not emit, absorb, or reflect light, making them invisible and detectable only through gravitational effects. It plays a crucial role in the formation and clustering of galaxies in the universe.
Cosmic Microwave Background: The cosmic microwave background (CMB) is the oldest light in the universe, a faint glow that permeates all of space and is a remnant of the early stages of the universe's formation. It provides crucial information about the origins and evolution of the universe, as well as its large-scale structure and composition.
Dark matter: Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible to current instruments. It exerts gravitational forces and is thought to constitute approximately 27% of the universe's mass-energy content.
Dark Matter: Dark matter is a hypothetical form of matter that cannot be seen directly but accounts for the majority of the matter in the universe. It is believed to interact gravitationally with itself and with ordinary matter, but does not emit, reflect, or absorb light, making it invisible to traditional astronomical observations.
Dark matter halo: Dark matter halo is a theoretical, spherical region surrounding galaxies, including the Milky Way, composed mostly of dark matter. It extends beyond the visible components of the galaxy and contains most of its mass.
Dark Matter Halo: A dark matter halo is a hypothetical distribution of dark matter that surrounds and extends well beyond the visible component of a galaxy. Dark matter is a mysterious substance that does not emit, reflect, or absorb light, yet it makes up a significant portion of the universe's total mass and plays a crucial role in the formation and evolution of galaxies.
Galactic Halo: The galactic halo is a spherical or ellipsoidal region surrounding the disk and bulge of a galaxy, consisting of diffuse gas, stars, and other matter not concentrated in the galactic plane. It is a crucial component in understanding the architecture, mass, stellar populations, formation, and evolution of galaxies.
Gravitational Constant: The gravitational constant, denoted as 'G', is a fundamental physical constant that describes the strength of the gravitational force between two objects. It is a crucial parameter in Newton's Universal Law of Gravitation, as well as in the study of the mass and center of the Milky Way galaxy.
Gravitational Lensing: Gravitational lensing is the bending of light by the gravitational field of a massive object, such as a galaxy or a black hole. This phenomenon occurs because the presence of matter distorts the fabric of spacetime, causing light to follow a curved path as it travels through this warped spacetime.
Kepler's Third Law: Kepler's Third Law is a fundamental principle in astronomy that describes the relationship between the orbital period and the semi-major axis of a planet or other object orbiting the Sun. It provides a mathematical formula that allows for the prediction of the orbital period of a celestial body based on its distance from the Sun.
Mass-to-light ratio: Mass-to-light ratio is a measurement used in astronomy to compare the total mass of an object, such as a galaxy, to its luminosity. It provides insights into the presence of dark matter and helps astronomers understand the composition and evolution of galaxies.
Mass-to-Light Ratio: The mass-to-light ratio is a measure of the amount of mass, or matter, relative to the amount of light emitted by a celestial object or system. It is a crucial parameter in understanding the properties and composition of galaxies, dark matter, and the overall structure of the universe.
Milky Way: The Milky Way is the galaxy in which our solar system is located, comprising hundreds of billions of stars and vast amounts of gas and dust. It is a spiral galaxy, with a central bulge and a rotating disk of stars, gas, and dust. The Milky Way is an essential component in understanding the structure, formation, and evolution of the universe, as it provides a window into the larger cosmic landscape.
Milky Way Galaxy: The Milky Way Galaxy is the spiral galaxy that includes our Solar System, characterized by its barred structure and multiple spiral arms. It is one of billions of galaxies in the universe and contains over 200 billion stars.
Neutralinos: Neutralinos are hypothetical, electrically neutral particles that are predicted to exist in certain extensions of the Standard Model of particle physics, such as supersymmetry (SUSY) theories. They are considered to be a potential candidate for the mysterious dark matter that makes up a significant portion of the universe's mass.
Orbital Velocity: Orbital velocity is the speed at which an object, such as a planet or satellite, travels in its orbit around another object, typically a larger body like a star or planet. This velocity is a critical factor in determining the stability and characteristics of an object's orbit.
Rotation Curve: The rotation curve is a plot that shows the orbital velocity of objects as a function of their distance from the center of a galaxy. It provides important insights into the mass distribution and dark matter content of a galaxy.
Supermassive black holes: Supermassive black holes are extremely large black holes, typically found at the centers of galaxies, including our Milky Way. They have masses ranging from millions to billions of times that of our Sun and significantly influence their galactic environments.
WIMPs: WIMPs, or Weakly Interacting Massive Particles, are hypothetical subatomic particles that are believed to make up a significant portion of the universe's dark matter. These particles are called 'weakly interacting' because they only interact with ordinary matter through the weak nuclear force and gravity, making them extremely difficult to detect directly.
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Glossary
Glossary
25.4 The Center of the Galaxy