Glossary

26.3 Properties of Galaxies

3 min read•june 12, 2024

are cosmic marvels, their properties revealing hidden secrets. Mass estimation techniques and unveil the presence of , a mysterious substance shaping galactic structure and evolution.

Mass-to-light ratios vary across galaxy types, hinting at 's crucial role. From spirals to ellipticals and dwarf spheroidals, these ratios offer clues about galactic composition and the universe's unseen matter.

Galactic Properties and Dark Matter

Mass estimation of galaxies

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  • Astronomers estimate galaxy mass using rotation curves that plot orbital velocities of stars and gas vs. distance from galactic center
    • Construct curves using of spectral lines from different galaxy regions
  • predict orbital velocities should decrease as distance from galactic center increases
    • Orbital velocity v=GMrv = \sqrt{\frac{GM}{r}} where GG is gravitational constant, MM is mass enclosed within orbit, rr is orbital radius
  • Observed rotation curves often show flat or rising velocities at large distances (outer regions of galaxies)
    • Implies additional mass beyond visible matter in galaxy
  • Discrepancy between observed and predicted rotation curves allows estimation of galaxy's total mass
    • Accounts for both visible matter (stars, gas, dust) and dark matter
  • , extending far beyond the visible disk, contribute significantly to the total mass

Mass-to-light ratios across galaxy types

  • (M/LM/L) is a galaxy's total mass divided by its luminosity
    • Expressed in solar units (M⊙/L⊙M_{\odot}/L_{\odot})
  • Spiral galaxies have M/LM/L ratios of 10-20 M⊙/L⊙M_{\odot}/L_{\odot} (, )
    • Indicates significant amounts of dark matter present
  • Elliptical galaxies have higher M/LM/L ratios, typically 50-100 M⊙/L⊙M_{\odot}/L_{\odot} (, )
    • Suggests even greater proportion of dark matter compared to spirals
  • Dwarf spheroidal galaxies have the highest M/LM/L ratios, up to 1000 M⊙/L⊙M_{\odot}/L_{\odot} (, )
    • Implies these small, faint galaxies are dominated by dark matter
  • Variation in M/LM/L ratios among galaxy types suggests dark matter plays crucial role in galactic structure and evolution
    • Particularly important in low-luminosity and low-surface-brightness galaxies (dwarf irregulars, ultra-diffuse galaxies)
  • within galaxies influence their mass-to-light ratios

Dark matter in galactic evolution

  • Flat rotation curves provide strong evidence for presence of dark matter halos surrounding galaxies
    • Extended halos explain observed orbital velocities at large galactic radii (beyond visible disk)
  • observations reveal unseen mass in galaxies and
    • Gravitational deflection of light from background sources greater than expected from visible matter alone (, )
  • of galaxies in clusters suggest additional mass present
    • Needed to explain high velocities and prevent clusters from dispersing
  • Cosmological simulations of structure formation require dark matter to reproduce observed large-scale structure of universe
    • (CDM) models successfully explain formation and evolution of galaxies and clusters (, )
  • Dark matter halos influence formation and evolution of galaxies by:
    1. Providing gravitational potential wells for baryonic matter to collapse and form stars
    2. Shaping morphology and dynamics of galaxies through gravitational interactions (tidal stripping, mergers)
  • Nature of dark matter remains major unsolved problem in astrophysics
    • Candidates include weakly interacting massive particles (), , modified gravity theories ()

Galaxy Composition and Structure

  • describes the overall shape and structure of galaxies (e.g., spiral, elliptical, irregular)
  • The , consisting of gas and dust, plays a crucial role in star formation and galactic evolution
  • are highly energetic central regions found in some galaxies, powered by supermassive black holes
  • are large-scale structures containing hundreds to thousands of galaxies bound by gravity

Key Terms to Review (39)

Abell 1689: Abell 1689 is a massive galaxy cluster located approximately 2.2 billion light-years from Earth. It is one of the most studied and well-known galaxy clusters in the universe, known for its remarkable gravitational lensing effects and the insights it provides into the properties of galaxies and dark matter.
Active Galactic Nuclei: Active galactic nuclei (AGN) refer to the extremely luminous and energetic centers of some galaxies, powered by supermassive black holes that are actively accreting matter. These galactic nuclei emit radiation across the entire electromagnetic spectrum, from radio waves to high-energy gamma rays, making them some of the most energetic phenomena in the universe.
Andromeda: Andromeda is the nearest major galaxy to the Milky Way, located approximately 2.5 million light-years away. It is a spiral galaxy similar in structure and size to our own Milky Way galaxy, and it is a prominent feature in the northern night sky, visible to the naked eye under clear conditions.
Andromeda galaxy: The Andromeda Galaxy, also known as M31, is the closest spiral galaxy to the Milky Way and is on a collision course with it. It is approximately 2.537 million light-years from Earth and is the largest galaxy in the Local Group.
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.
Bullet Cluster: The Bullet Cluster is a collection of galaxies that provides some of the strongest evidence for the existence of dark matter. It is a system of two colliding galaxy clusters that have become separated, revealing the distribution of dark matter within the system.
Centaurus A: Centaurus A is a prominent elliptical galaxy located approximately 12 million light-years from Earth. It is one of the closest and most studied active galactic nuclei, known for its distinctive radio lobes and jets emanating from a supermassive black hole at its center. Centaurus A is a key object of study in the fields of galaxy properties, formation, and evolution.
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.
Cold Dark Matter: Cold Dark Matter (CDM) is a hypothetical form of dark matter that is believed to be the dominant component of the universe's matter content. It is called 'cold' because it is thought to consist of slow-moving, non-relativistic particles that do not interact with electromagnetic radiation, making them invisible to telescopes and other conventional astronomical instruments.
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.
Doppler Shifts: Doppler shifts refer to the change in the observed frequency or wavelength of a wave due to the relative motion between the source of the wave and the observer. This phenomenon is widely used in astronomy to study the motion and properties of celestial objects.
Draco: Draco is a northern circumpolar constellation, known as the Dragon, that wraps around the north celestial pole. It is one of the largest constellations in the night sky and is characterized by its serpentine shape, with a head at one end and a tail at the other.
Galactic Halos: Galactic halos are extended, roughly spherical regions of diffuse matter that surround the visible components of galaxies, including the disk and bulge. These halos are composed primarily of dark matter, which cannot be directly observed but exerts a gravitational influence on the visible matter within the galaxy.
Galactic mass: Galactic mass is the total mass of a galaxy, encompassing stars, gas, dust, and dark matter. It is crucial for understanding the dynamics and evolution of galaxies.
Galaxies: Galaxies are vast systems composed of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity. They vary greatly in size and structure and are fundamental building blocks of the universe.
Galaxy clusters: Galaxy clusters are large groups of galaxies bound together by gravity, typically containing hundreds to thousands of member galaxies. They represent the largest gravitationally bound structures in the universe and provide crucial insights into cosmic evolution and distribution.
Galaxy Clusters: Galaxy clusters are massive, gravitationally bound groups of galaxies that can contain hundreds or even thousands of individual galaxies. They are the largest known structures in the universe held together by their mutual gravitational attraction.
Galaxy Morphology: Galaxy morphology refers to the classification and study of the physical shapes and structures of galaxies. It is a fundamental aspect of understanding the diverse types of galaxies that exist in the universe and the processes that shape their evolution.
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.
IllustrisTNG: IllustrisTNG is a state-of-the-art cosmological simulation that models the formation and evolution of galaxies in a large volume of the universe. It is an advanced version of the original Illustris simulation, incorporating new and improved physical models to provide a more accurate representation of galaxy properties and their cosmic environment.
Interstellar medium: Interstellar medium (ISM) is the matter that exists in the space between star systems within a galaxy. It consists of gas (both ionized and neutral) and dust, playing a crucial role in the life cycle of cosmic material.
Interstellar Medium: The interstellar medium refers to the vast expanse of gas and dust that fills the space between stars within a galaxy. It is the material that exists in the space between solar systems and plays a crucial role in the formation and evolution of stars, as well as the overall structure and dynamics of galaxies.
Kepler's Laws: Kepler's laws are a set of three fundamental principles that describe the motion of planets around the Sun. Formulated by the 17th-century astronomer Johannes Kepler, these laws provide a mathematical framework for understanding the dynamics of the solar system and laid the groundwork for Newton's universal law of gravitation.
Line broadening: Line broadening is the phenomenon where spectral lines from astronomical objects are wider than expected. This occurs due to various factors such as thermal motion, rotation, and magnetic fields.
M87: M87, also known as Virgo A, is a giant elliptical galaxy located in the Virgo Cluster, approximately 55 million light-years from Earth. It is one of the most massive and luminous galaxies in the local universe and is notable for its active galactic nucleus and powerful jet of high-energy particles.
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.
Millennium Simulation: The Millennium Simulation is a large-scale computer simulation of the formation and evolution of the universe, focusing on the properties and structure of galaxies. It provides a comprehensive model of the cosmic structure formation process, allowing researchers to study the complex interactions between dark matter, gas, and stars that shape the observable universe.
MOND: MOND, or Modified Newtonian Dynamics, is a proposed alternative to the standard model of dark matter, which seeks to explain the observed discrepancies in the motion of galaxies and galaxy clusters without invoking the presence of undetected dark matter. MOND suggests that the laws of gravity may need to be modified at very low accelerations, rather than relying on the existence of an unseen dark matter component.
Rotation Curves: Rotation curves are graphs that depict the rotational velocity of matter, typically stars or gas, in a galaxy as a function of their distance from the galactic center. These curves provide valuable insights into the mass distribution and gravitational field within a galaxy.
Stellar Populations: Stellar populations refer to the different groups or categories of stars that make up the various components of a galaxy. These populations are distinguished by their age, chemical composition, and kinematics, which provide insights into the formation and evolution of galaxies.
Ursa Minor: Ursa Minor, also known as the Little Bear or the North Star, is a prominent constellation in the northern celestial hemisphere. It is one of the most recognizable constellations in the night sky and holds great significance in both the study of Earth and the sky, as well as the properties of galaxies.
Velocity Dispersions: Velocity dispersion is a measure of the spread or distribution of velocities of objects within a system, such as a galaxy or a cluster of galaxies. It provides insights into the dynamical properties and mass distribution of these systems.
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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