9.2 Galaxy formation theories and models
3 min read•july 22, 2024
Galaxy formation theories explore how these cosmic structures came to be. Two main models exist: , where galaxies form quickly from massive gas clouds, and , where smaller structures combine over time to create larger ones.
halos play a crucial role in galaxy formation, providing the gravitational framework for gas and stars to coalesce. Galaxy mergers, gas accretion, and feedback processes further shape galactic evolution, influencing star formation rates and overall galaxy structure.
Galaxy Formation Theories and Models
Theories of galaxy formation
Top images from around the web for Theories of galaxy formation
28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original
Is this image relevant?
The Formation and Evolution of Galaxies and Structure in the Universe · Astronomy View original
Is this image relevant?
28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original
Is this image relevant?
28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original
Is this image relevant?
The Formation and Evolution of Galaxies and Structure in the Universe · Astronomy View original
Is this image relevant?
1 of 3
Top images from around the web for Theories of galaxy formation
28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original
Is this image relevant?
The Formation and Evolution of Galaxies and Structure in the Universe · Astronomy View original
Is this image relevant?
28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original
Is this image relevant?
28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original
Is this image relevant?
The Formation and Evolution of Galaxies and Structure in the Universe · Astronomy View original
Is this image relevant?
1 of 3
- Monolithic collapse model proposes galaxies form from the rapid collapse of a single, massive gas cloud early in the universe's history
- Gas cools and forms stars in a relatively short time (few hundred million years)
- Predicts elliptical galaxies form first, followed by spiral galaxies ()
- Hierarchical merging model suggests galaxies form through the gradual merging of smaller structures over time
- Smaller galaxies and dark matter halos merge to form larger galaxies (bottom-up process)
- Process continues throughout the history of the universe (13.8 billion years)
- Predicts that irregular and spiral galaxies form first, with elliptical galaxies forming later through mergers ()
Dark matter halos in galaxies
- Dark matter halos provide the gravitational framework for galaxy formation
- (gas and dust) falls into the gravitational potential wells created by dark matter halos
- Gas cools and condenses within the halos, leading to star formation and galaxy growth (inside-out formation)
- Halo mass influences galaxy properties
- More massive halos tend to host larger, more massive galaxies ( vs. )
- Halo mass affects the gas accretion rate and the efficiency of star formation ()
- Halo mergers drive galaxy evolution
- When dark matter halos merge, their associated galaxies also undergo merging (gravitational interactions)
- Halo mergers can trigger starbursts, , and morphological changes in galaxies ()
Galaxy mergers and evolution
- Galaxy mergers occur when two or more galaxies collide and combine to form a single, larger galaxy
- Merger types
- : merging galaxies have similar masses (mass ratio 1:1 to 1:4)
- Can result in significant morphological changes and the formation of elliptical galaxies ()
- Often triggers intense starbursts and AGN activity (quasars)
- : one galaxy is significantly more massive than the other(s) (mass ratio < 1:4)
- Smaller galaxies are absorbed into the larger galaxy ()
- Can cause more subtle morphological changes and enhance star formation (tidal streams)
- : merging galaxies have similar masses (mass ratio 1:1 to 1:4)
- Impact on star formation
- Mergers can compress gas and trigger intense bursts of star formation ()
- duration depends on the merger parameters and gas content of the galaxies ( to years)
- Impact on morphology
- Major mergers can disrupt the ordered motion of stars, leading to the formation of elliptical galaxies ()
- Minor mergers can cause tidal disturbances, warps, and asymmetries in the galaxy's structure ()
Gas accretion and feedback processes
- Gas accretion
- Galaxies acquire gas from the intergalactic medium through cold and hot accretion modes
- Cold mode: gas flows along filaments directly into the galaxy ( K)
- Hot mode: gas is shock-heated to high temperatures and then cools and falls into the galaxy ( K)
- Accretion rate depends on halo mass and
- Higher accretion rates in the early universe and for lower-mass halos ()
- Accreted gas fuels star formation and galaxy growth (gas recycling)
- Galaxies acquire gas from the intergalactic medium through cold and hot accretion modes
- Feedback processes
- Stellar feedback
- Energy and momentum injected into the interstellar medium by supernovae, stellar winds, and radiation pressure ( erg per supernova)
- Can heat and expel gas from the galaxy, regulating star formation ()
- AGN feedback
- Energy released by the accretion of matter onto the central supermassive black hole ()
- Can heat and expel gas on larger scales, suppressing star formation and gas accretion (radio jets)
- Feedback processes can shape the gas content, star formation history, and mass distribution of galaxies ()
- The efficiency of feedback depends on galaxy mass and environment (cluster vs. field galaxies)
- Stellar feedback
Key Terms to Review (26)
AGN Activity: AGN activity, or Active Galactic Nucleus activity, refers to the phenomenon where the central supermassive black hole of a galaxy actively accretes matter, emitting vast amounts of energy across various wavelengths, including radio, optical, X-ray, and gamma-ray emissions. This energetic activity is linked to the growth and evolution of galaxies, as it influences star formation rates, galactic structure, and the surrounding environment, ultimately providing insights into galaxy formation theories and models.
Antennae Galaxies: Antennae galaxies, also known as NGC 4038/NGC 4039, are a pair of interacting galaxies located in the constellation Corvus, characterized by their distinct antenna-like structure formed by tidal forces during their merger. This unique shape is a result of gravitational interactions that distort their forms, making them an excellent example of how galaxies can evolve through interactions over cosmic time and provide insights into galaxy formation models.
Baryonic Matter: Baryonic matter refers to the ordinary matter that makes up stars, planets, and living organisms, composed primarily of baryons, which are subatomic particles like protons and neutrons. This form of matter is crucial in understanding the structure and evolution of the universe, as it influences everything from cosmic microwave background radiation to the formation of galaxies and the potential fate of the universe.
Centaurus A: Centaurus A, also known as NGC 5128, is a prominent radio galaxy located in the constellation Centaurus. It is characterized by its peculiar morphology, featuring a bright central core surrounded by a dusty disk, which makes it an important subject for studying galaxy formation and evolution, particularly in understanding the processes that lead to the development of galaxies like our Milky Way.
Cold accretion mode: Cold accretion mode refers to the process by which gas, particularly hydrogen, flows into a galaxy at low temperatures and in a relatively smooth and continuous manner. This mechanism plays a crucial role in the formation and growth of galaxies, as it allows for the accumulation of mass without the disruptive effects seen in hotter, turbulent flows. Understanding this process helps explain how galaxies acquire their gas and subsequently form stars over cosmic time.
Dark Matter: Dark matter is an unseen form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects. It plays a crucial role in the structure and evolution of the universe, influencing galaxy formation, cosmic expansion, and the distribution of galaxies within the cosmic web.
Dwarf Galaxies: Dwarf galaxies are small galaxies that contain a relatively low number of stars, usually less than a billion, and are much less luminous than larger galaxies like the Milky Way. These galaxies often serve as important building blocks in the study of galaxy formation theories, as they provide insights into the processes of star formation, dark matter interactions, and the evolution of larger galactic structures.
Elliptical galaxy: An elliptical galaxy is a type of galaxy characterized by its smooth, featureless light profile and an ellipsoidal shape. These galaxies generally contain older stars and minimal amounts of gas and dust, leading to little ongoing star formation. Understanding elliptical galaxies is important in the study of galaxy classification and in the development of theories about galaxy formation and evolution.
Galactic Fountains: Galactic fountains are a phenomenon where gas and dust are ejected from the central regions of a galaxy into the surrounding interstellar medium, often due to stellar winds and supernova explosions. This process plays a critical role in redistributing materials within galaxies, influencing star formation and the overall evolution of galactic structures. The material that is expelled can eventually fall back towards the galactic center, creating a cycle that impacts both the local environment and the broader galactic ecosystem.
Hierarchical Merging: Hierarchical merging is a process in galaxy formation where smaller galaxies combine to form larger ones, leading to the structure we observe in the universe today. This concept emphasizes that galaxies grow through successive mergers, resulting in a hierarchical structure where smaller systems progressively merge into bigger systems over cosmic time. This process plays a key role in understanding the evolution of galaxies and the large-scale structure of the universe.
Hot Accretion Mode: Hot accretion mode refers to a process during galaxy formation where hot gas, heated to millions of degrees, falls into the gravitational potential well of a forming galaxy. This mode is characterized by the rapid inflow of gas that creates a hot halo around the galaxy, which can significantly affect star formation rates and the overall evolution of the galaxy. It plays a crucial role in how galaxies acquire their mass and energy during the early stages of their development.
Hubble Sequence: The Hubble Sequence is a classification system for galaxies that organizes them into distinct morphological types based on their appearance. It primarily categorizes galaxies into three main groups: elliptical, spiral, and irregular, while also considering the structural characteristics and evolution of these galaxy types. This classification helps astronomers understand the formation and evolution of galaxies, linking their physical properties to their origins.
M82 Starburst Galaxy: M82, also known as the Cigar Galaxy, is a starburst galaxy located approximately 12 million light-years away in the constellation Ursa Major. It is characterized by an exceptionally high rate of star formation, which significantly alters its structure and evolution, making it an important example in the study of galaxy formation theories and models.
M87: M87, or Messier 87, is a giant elliptical galaxy located in the Virgo Cluster, approximately 53 million light-years away from Earth. It is notable for being one of the largest galaxies in the local universe and for containing a supermassive black hole at its center, which was famously imaged by the Event Horizon Telescope in 2019. The characteristics of M87 provide insights into galaxy formation and evolution, highlighting the processes that govern the growth of massive galaxies and their central black holes.
Major Mergers: Major mergers refer to significant gravitational interactions between galaxies that result in a substantial reorganization of their structures and star populations. These events play a critical role in the formation and evolution of galaxies, leading to the creation of larger galactic systems and influencing their properties, such as morphology, size, and star formation rates.
Mass-metallicity relation: The mass-metallicity relation describes the correlation between the stellar mass of a galaxy and its metallicity, which refers to the abundance of elements heavier than hydrogen and helium. This relationship indicates that more massive galaxies tend to have higher metallicities, reflecting their ability to retain gas and convert it into stars more efficiently over time. This concept connects to our understanding of galaxy formation and evolution, revealing how galaxies build up their stellar populations and enrich their interstellar medium through processes such as star formation and supernova explosions.
Milky Way: The Milky Way is the galaxy that contains our solar system, characterized by its spiral structure and composed of billions of stars, gas, and dust. It is a barred spiral galaxy, meaning it has a central bar-shaped structure made up of stars and rotates around a central supermassive black hole, known as Sagittarius A*. This galaxy is not only vast in size but also rich in diverse stellar populations and celestial phenomena, making it a focal point for understanding galaxy formation and evolution.
Milky Way Warp: The Milky Way warp refers to the bending or distortion of the galactic disk of the Milky Way galaxy, particularly in its outer regions. This phenomenon is thought to arise due to gravitational influences from nearby structures, such as satellite galaxies and dark matter, leading to a misalignment of the galaxy's plane with respect to the surrounding universe. Understanding the warp provides insights into the dynamics and formation processes of galaxies, connecting it to broader galaxy formation theories and models.
Minor mergers: Minor mergers refer to the gravitational interaction and subsequent merging of smaller galaxies with larger host galaxies. These events are significant in the context of galaxy formation theories, as they contribute to the growth and evolution of galaxies over cosmic time. Minor mergers can enhance star formation and trigger various dynamical processes within galaxies, playing a crucial role in understanding how galaxies assemble and evolve.
Monolithic Collapse: Monolithic collapse refers to a theory in galaxy formation suggesting that galaxies formed from a single, massive cloud of gas that collapsed under its own gravity. This process theorizes that the initial collapse led to the rapid formation of stars and structures within the galaxy, creating a cohesive and unified system. The concept emphasizes the role of gravitational instabilities in the early universe and contrasts with hierarchical models where galaxies form through mergers and interactions over time.
Redshift: Redshift is the phenomenon where light from an object moving away from an observer is stretched to longer wavelengths, making it appear redder. This effect is crucial in understanding the universe's expansion and provides essential insights into the formation of galaxies, the evidence for the Big Bang, and the large-scale structure of the cosmos.
Sagittarius Dwarf Galaxy: The Sagittarius Dwarf Galaxy is a small satellite galaxy of the Milky Way, located about 70,000 light-years from Earth. It is significant for its role in the study of galaxy formation and evolution, particularly as it is being gravitationally pulled apart by the Milky Way's tidal forces, providing insight into how smaller galaxies interact with larger ones during the cosmic dance of formation and growth.
Schmidt-Kennicutt Law: The Schmidt-Kennicutt Law is an empirical relationship that links the star formation rate (SFR) of a galaxy to its gas surface density. This law suggests that the rate at which stars form in a galaxy is proportional to the density of its molecular gas, revealing key insights into the processes governing galaxy evolution and star formation efficiency.
Spiral galaxy: A spiral galaxy is a type of galaxy characterized by its flat, rotating disk containing stars, gas, and dust, as well as a central concentration of stars known as the bulge. The spiral arms are regions of higher density that extend from the center and give these galaxies their distinctive shape. Understanding spiral galaxies is essential for studying galaxy formation theories and classification systems in astronomy.
Starburst: A starburst is a rapid and intense phase of star formation that occurs in galaxies, where the rate of star creation dramatically increases, often leading to the formation of many new stars over a short period of time. This phenomenon is typically triggered by interactions such as galaxy mergers or gravitational encounters, which can compress gas and dust, providing the necessary conditions for star formation. Understanding starbursts is essential for comprehending galaxy evolution and the processes that govern star formation in the universe.
Ultra-luminous infrared galaxies: Ultra-luminous infrared galaxies (ULIRGs) are a class of galaxies that emit an enormous amount of infrared radiation, typically exceeding 10^12 solar luminosities. These galaxies are often characterized by intense star formation and active galactic nuclei, which can provide insights into galaxy formation and evolution during the early universe.