2.1 Primordial gas and galaxy formation
10 min read•august 20, 2024
The that filled the early universe was mostly and , with trace amounts of . This composition set the stage for the formation of the first stars and galaxies. The absence of heavier elements shaped the and star formation processes in the early cosmos.
Primordial gas clouds collapsed under gravity, aided by . This led to the birth of massive , which were unlike later stellar populations. These first stars produced and heavy elements, kickstarting the of the universe and enriching the .
Composition of primordial gas
- Primordial gas, the material present in the early universe after the Big Bang, had a unique composition that set the stage for the formation of the first stars and galaxies
- This gas was composed almost entirely of the lightest elements, hydrogen and helium, with only trace amounts of lithium and no significant presence of heavier elements
Hydrogen and helium abundance
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- Hydrogen, the lightest and most abundant element, made up approximately 75% of the primordial gas by mass
- Helium, the second lightest element, accounted for nearly 25% of the primordial gas composition
- The relative abundances of hydrogen and helium were determined by the conditions in the early universe, such as temperature and density, during the first few minutes after the Big Bang
Trace amounts of lithium
- Lithium, the third lightest element, was present in the primordial gas, but only in trace amounts
- The abundance of lithium was several orders of magnitude lower than that of hydrogen and helium
- The presence of lithium in the primordial gas is an important test of models, which predict the formation of light elements in the early universe
Absence of heavier elements
- The primordial gas was devoid of elements heavier than lithium, such as carbon, oxygen, and iron
- These heavier elements, known as "metals" in astronomy, were not present in the early universe because they are formed through in the cores of stars
- The absence of metals in the primordial gas had significant implications for the cooling mechanisms and star formation processes in the early universe
Collapse of primordial gas clouds
- The formation of the first stars and galaxies began with the of primordial gas clouds
- These gas clouds, composed mainly of hydrogen and helium, were subject to various physical processes that governed their collapse and the subsequent formation of structures
Jeans instability
- is a critical concept in understanding the collapse of primordial gas clouds
- It refers to the condition where the internal gas pressure of a cloud is insufficient to counteract its own gravitational attraction
- When a gas cloud exceeds the Jeans mass (a critical mass threshold), it becomes gravitationally unstable and begins to collapse under its own gravity
Role of dark matter halos
- Dark matter, a form of matter that does not interact with electromagnetic radiation, played a crucial role in the collapse of primordial gas clouds
- Dark matter halos, gravitationally bound structures composed primarily of dark matter, provided the gravitational potential wells in which primordial gas clouds accumulated and collapsed
- The presence of dark matter halos helped to facilitate the formation of the first stars and galaxies by enhancing the gravitational instability of the gas clouds
Cooling mechanisms in early universe
- Cooling mechanisms were essential for the efficient collapse of primordial gas clouds and the formation of the first stars
- In the absence of heavy elements, the primary cooling mechanism in the early universe was the emission of radiation through atomic transitions of hydrogen and helium
- Other cooling processes, such as molecular hydrogen (H2) cooling, also played a role in the collapse of primordial gas clouds, particularly in the formation of the first stars
Formation of first stars
- The collapse of primordial gas clouds led to the formation of the first stars, known as Population III stars
- These stars were the first luminous objects to form in the universe and had a profound impact on the evolution of the early universe
Population III stars
- Population III stars were the first generation of stars, formed from the pristine primordial gas composed mainly of hydrogen and helium
- These stars are thought to have been very massive, with typical masses ranging from tens to hundreds of times the mass of the Sun
- Population III stars had very short lifetimes, on the order of a few million years, due to their high mass and lack of heavy elements
Characteristics vs later stellar populations
- Population III stars differed significantly from later stellar populations (Population I and II stars) in terms of their composition, mass, and luminosity
- Unlike later generations of stars, Population III stars were composed almost entirely of hydrogen and helium, with no significant amounts of heavier elements
- Population III stars were generally more massive and luminous than their later counterparts, as the absence of heavy elements allowed for the formation of larger stars
Impact on interstellar medium
- The formation and evolution of Population III stars had a profound impact on the interstellar medium (ISM) in the early universe
- These massive stars produced large amounts of ionizing radiation, which contributed to the reionization of the universe (see next section)
- The deaths of Population III stars as supernovae released the first heavy elements into the ISM, enriching the gas and setting the stage for the formation of later generations of stars
Reionization of the universe
- Reionization is a key event in the history of the universe, marking the transition from a neutral to an ionized state
- This process was driven by the ionizing radiation emitted by the first stars and galaxies, which gradually ionized the neutral hydrogen gas in the intergalactic medium (IGM)
Ionizing radiation from first stars
- The massive Population III stars were a primary source of ionizing radiation in the early universe
- These stars produced large amounts of high-energy photons capable of ionizing neutral hydrogen atoms
- The ionizing radiation from the first stars began the process of reionization, creating expanding bubbles of ionized gas around the stars
Evolution of neutral hydrogen fraction
- As reionization progressed, the fraction of neutral hydrogen in the universe decreased over time
- The evolution of the neutral hydrogen fraction depends on the balance between the ionizing radiation emitted by stars and galaxies and the recombination of ionized hydrogen atoms
- Models of reionization aim to predict the evolution of the neutral hydrogen fraction and the timing of key milestones, such as the completion of reionization
Observational evidence of reionization
- Observational evidence for reionization comes from various sources, including the (CMB) radiation and the spectra of distant quasars
- The CMB provides information about the integrated history of reionization through the Thomson scattering of photons by free electrons
- Quasar spectra can be used to probe the state of the IGM at different redshifts, revealing the presence of neutral hydrogen through absorption features such as the Gunn-Peterson trough
Enrichment of interstellar medium
- The enrichment of the interstellar medium (ISM) with heavy elements was a crucial process in the evolution of the early universe
- This enrichment was driven by the stellar nucleosynthesis in the first stars and the subsequent dispersal of these elements through
Stellar nucleosynthesis in first stars
- Population III stars, despite their lack of initial heavy elements, were sites of stellar nucleosynthesis
- These massive stars produced heavier elements through nuclear fusion reactions in their cores, creating elements such as carbon, oxygen, and silicon
- The nucleosynthesis in Population III stars marked the first production of heavy elements in the universe
Supernova explosions and element dispersal
- The deaths of massive Population III stars as supernova explosions played a crucial role in the dispersal of heavy elements into the ISM
- Supernova explosions ejected the newly synthesized elements into the surrounding gas, enriching the ISM with metals
- The dispersal of heavy elements through supernovae was a key mechanism for the chemical evolution of the universe
Transition to Population II stars
- The enrichment of the ISM with heavy elements set the stage for the formation of the next generation of stars, known as
- Population II stars formed from gas clouds that were enriched with the products of stellar nucleosynthesis from the first stars
- The presence of heavy elements in the gas allowed for more efficient cooling and fragmentation, leading to the formation of stars with a wider range of masses compared to Population III stars
Emergence of first galaxies
- The formation of the first galaxies marks a crucial milestone in the evolution of the universe
- Galaxies are gravitationally bound systems composed of stars, gas, dust, and dark matter, and their emergence is closely tied to the process
Hierarchical structure formation
- The formation of galaxies is thought to have occurred through a hierarchical process, where smaller structures merge to form larger ones
- In this model, dark matter halos play a crucial role, as they provide the gravitational potential wells in which gas can accumulate and form stars
- The hierarchical nature of structure formation means that smaller galaxies formed first, and then merged to create larger galaxies over time
Protogalactic disk formation
- As gas falls into the gravitational potential wells of dark matter halos, it can settle into a rotating disk
- These disks, known as protogalactic disks, are the sites of early star formation in galaxies
- The formation of protogalactic disks is governed by the interplay between gravity, gas dynamics, and feedback processes from star formation and supernovae
Early galaxy morphologies
- The first galaxies likely had a range of morphologies, depending on factors such as the mass and angular momentum of the dark matter halo, the gas accretion history, and the feedback processes at play
- Some early galaxies may have resembled the irregular or dwarf galaxies observed in the nearby universe, while others could have had more disk-like or spheroidal morphologies
- The study of provides insights into the physical processes that shaped the formation and evolution of galaxies in the early universe
Feedback processes in early galaxies
- Feedback processes play a crucial role in regulating star formation and shaping the evolution of galaxies
- In the context of early galaxies, two main types of feedback are particularly important: and active galactic nuclei (AGN) feedback
Stellar winds and supernovae
- Massive stars can influence their surroundings through stellar winds and supernova explosions
- Stellar winds, driven by the radiation pressure from luminous stars, can push gas away from star-forming regions, reducing the gas density and slowing down star formation
- Supernova explosions release large amounts of energy and momentum into the ISM, heating the gas and potentially driving galactic-scale outflows
Active galactic nuclei feedback
- AGN feedback refers to the impact of the energy released by supermassive black holes at the centers of galaxies
- As matter accretes onto the central black hole, it releases enormous amounts of energy in the form of radiation and relativistic jets
- AGN feedback can heat and expel gas from galaxies, suppressing star formation and regulating the growth of the central black hole
Impact on star formation rates
- Feedback processes can have a significant impact on the in early galaxies
- Stellar feedback can locally suppress star formation by heating and expelling gas from star-forming regions
- AGN feedback can operate on larger scales, heating and removing gas from the entire galaxy, effectively quenching star formation
- The interplay between these feedback processes and the available gas supply determines the overall star formation history of early galaxies
Observing high-redshift galaxies
- Observing galaxies in the early universe is a challenging but crucial task for understanding galaxy formation and evolution
- High-redshift galaxies are very distant and faint, requiring sophisticated observational techniques and state-of-the-art telescopes
Lyman-break technique
- The is a widely used method for identifying high-redshift galaxies
- This technique relies on the strong absorption of ultraviolet light by neutral hydrogen in the ISM of galaxies
- By looking for a characteristic "break" in the spectrum of a galaxy at the wavelength corresponding to the Lyman limit (912 Å) at the galaxy's redshift, astronomers can identify candidate high-redshift galaxies
Gravitational lensing
- is another powerful tool for studying high-redshift galaxies
- Massive foreground objects, such as galaxy clusters, can act as natural gravitational lenses, magnifying and distorting the light from background galaxies
- This magnification effect allows astronomers to detect and study galaxies that would otherwise be too faint to observe, providing a glimpse into the early stages of galaxy formation
Future observational prospects
- Upcoming and future observational facilities, such as the James Webb Space Telescope (JWST) and the next generation of extremely large telescopes, will revolutionize the study of high-redshift galaxies
- These telescopes will have the sensitivity and resolution to detect and characterize galaxies from the earliest epochs of the universe
- By combining observations from these facilities with theoretical models and simulations, astronomers will gain new insights into the formation and evolution of galaxies across cosmic time
Key Terms to Review (25)
Active galactic nuclei feedback: Active galactic nuclei feedback refers to the energy and momentum output from the central supermassive black hole in a galaxy, particularly when it is actively accreting matter. This process can influence star formation and the surrounding interstellar medium by heating gas, driving outflows, and regulating the growth of the galaxy. The feedback mechanisms are crucial in understanding how galaxies evolve over time, especially in relation to primordial gas and galaxy formation.
Big Bang Nucleosynthesis: Big Bang nucleosynthesis refers to the process that occurred during the first few minutes after the Big Bang, when protons and neutrons combined to form the lightest atomic nuclei, primarily hydrogen, helium, and small amounts of lithium and beryllium. This process laid the foundation for the primordial gas that eventually formed galaxies and stars, shaping the early universe's chemical composition and structure.
Cooling mechanisms: Cooling mechanisms refer to the processes through which gas loses energy and cools down, enabling the formation of stars and galaxies in the universe. This cooling is essential for the transition from hot primordial gas to denser regions that can collapse under gravity, leading to star formation and ultimately the development of galaxies. These mechanisms include radiative cooling, where gas emits radiation, and adiabatic cooling, where gas expands and loses temperature without heat exchange with its environment.
Cosmic Microwave Background: The cosmic microwave background (CMB) is the afterglow radiation from the Big Bang, permeating the universe and providing a snapshot of the early universe when it was just about 380,000 years old. This faint glow, detected in the microwave part of the electromagnetic spectrum, is crucial for understanding the formation and evolution of structures in the universe, linking various aspects of cosmology and astrophysics.
Dark Matter Halos: Dark matter halos are vast, invisible regions surrounding galaxies that contain dark matter, a mysterious form of matter that does not emit, absorb, or reflect light. These halos are critical in galaxy formation, as they provide the gravitational framework necessary for normal matter to accumulate and form stars and galaxies. Essentially, dark matter halos shape the structure of the universe and influence the dynamics of galaxies within them.
Early galaxy morphologies: Early galaxy morphologies refer to the various shapes and structures of galaxies in the early universe, particularly those formed from primordial gas. These morphologies include features such as irregular, elliptical, and spiral shapes, which reflect the processes involved in galaxy formation and evolution during the universe's infancy. Understanding these early forms helps astronomers piece together how galaxies grew and transformed over billions of years.
Gravitational Collapse: Gravitational collapse is the process by which an astronomical object contracts under its own gravity, leading to the formation of denser structures like stars, galaxies, or even black holes. This fundamental process plays a critical role in the evolution of the universe, shaping the distribution of matter and energy throughout space and influencing the formation of cosmic structures.
Gravitational Lensing: Gravitational lensing is a phenomenon that occurs when a massive object, such as a galaxy or a cluster of galaxies, bends the light from a more distant object due to its gravitational field. This effect not only magnifies and distorts the image of the background object but can also provide crucial information about the mass and distribution of dark matter in the lensing object, connecting it to various cosmic structures and dynamics.
Helium: Helium is a colorless, odorless, inert gas and the second lightest element in the universe, primarily produced through nuclear fusion processes in stars. Its significance is evident in the early universe, where it formed alongside hydrogen during the Big Bang, playing a crucial role in the formation of primordial gas and influencing galaxy evolution. Helium's unique properties make it an essential component for understanding stellar nucleosynthesis and the chemical makeup of the cosmos.
Hierarchical Structure Formation: Hierarchical structure formation is a cosmological model that describes how the universe evolves from small, simple structures to larger, more complex ones, often involving the merging of smaller entities to form bigger systems. This process plays a vital role in shaping the formation and distribution of galaxies, leading to the diversity of galaxy types and their environments. As smaller structures collapse under gravity, they create gravitational wells that attract more mass, eventually resulting in the formation of galaxy clusters and large-scale structures in the universe.
Hydrogen: Hydrogen is the simplest and most abundant chemical element in the universe, represented by the symbol 'H' and atomic number 1. It plays a crucial role in the formation of stars and galaxies, as it is the primary building block of primordial gas that filled the early universe, leading to the formation of galaxies and structures we observe today.
Interstellar Medium: The interstellar medium (ISM) is the matter that exists in the space between stars within a galaxy, comprising gas, dust, and cosmic rays. It plays a crucial role in the life cycle of galaxies by providing the raw materials for star formation and influencing the dynamics of stellar evolution and galactic structure.
Ionizing Radiation: Ionizing radiation refers to high-energy particles or electromagnetic waves that can remove tightly bound electrons from atoms, thus creating charged ions. This type of radiation plays a crucial role in various astrophysical processes, including the interactions within primordial gas during the early universe, influencing galaxy formation and the conditions for star development.
Jeans Instability: Jeans Instability refers to a phenomenon in astrophysics where a gas cloud becomes unstable and begins to collapse under its own gravity when the gravitational forces exceed the thermal pressure within the cloud. This concept is crucial for understanding the formation of structures in the universe, such as stars and galaxies, as it explains how primordial gas clouds can fragment and lead to the birth of new celestial objects.
Lithium: Lithium is a lightweight, highly reactive chemical element with the symbol Li and atomic number 3. It plays a significant role in astrophysics, particularly during the early universe's formation and in the process of primordial nucleosynthesis, where light elements were formed shortly after the Big Bang. Lithium's presence in the universe helps astronomers understand the conditions of the early universe and provides clues about the processes that led to galaxy formation.
Lyman-break technique: The Lyman-break technique is an observational method used in astronomy to identify and study high-redshift galaxies, particularly those that formed during the early universe. This technique exploits the distinct drop in brightness (the 'break') of galaxies at specific wavelengths, particularly the Lyman-alpha line, due to the absorption of light by neutral hydrogen in the intergalactic medium. By focusing on this break, astronomers can pinpoint the presence of young galaxies that are forming in a primordial environment and gain insights into their properties and formation processes.
Population II stars: Population II stars are a class of stars that are older, metal-poor, and typically found in the halo and globular clusters of galaxies. They play a crucial role in understanding the early universe, as they formed from primordial gas before significant amounts of heavy elements were produced by earlier generations of stars. Their existence provides insights into the processes of galaxy formation and the history of star formation across cosmic time.
Population III stars: Population III stars are the first generation of stars formed in the universe, consisting entirely of hydrogen and helium with no heavier elements. These stars played a crucial role in the early stages of cosmic evolution, influencing the formation of galaxies and contributing to the reionization of the universe. Their formation occurred from primordial gas, setting the stage for subsequent star generations that would include heavier elements produced through nuclear fusion.
Primordial gas: Primordial gas refers to the hydrogen and helium that filled the universe shortly after the Big Bang, serving as the fundamental building blocks for the formation of stars and galaxies. This gas, composed mainly of these two light elements, played a critical role in the cooling and clumping processes that led to the creation of structures in the universe, ultimately influencing galaxy formation and evolution over billions of years.
Protogalactic Disk Formation: Protogalactic disk formation refers to the process by which a rotating disk of primordial gas, primarily hydrogen and helium, evolves into a galaxy. This formation occurs in the early universe, where gravitational interactions and density fluctuations cause the gas to collapse and spin, leading to the creation of a structured disk. This process is essential for understanding how galaxies like our Milky Way were born from the simple ingredients present in the cosmos shortly after the Big Bang.
Reionization: Reionization refers to the process that occurred in the early universe when neutral hydrogen atoms were ionized, leading to the re-establishment of ionized plasma in the intergalactic medium. This event is crucial for understanding the formation of galaxies and the large-scale structure of the universe, as it marked a significant transition from a mostly neutral state to an ionized one, influencing star formation and the evolution of cosmic structures.
Star Formation Rates: Star formation rates refer to the speed at which new stars are formed in a given region of space, typically expressed in solar masses per year. This concept is crucial in understanding how galaxies evolve over time, particularly regarding the amount of gas available for star formation and the efficiency with which that gas transforms into stars. A higher star formation rate indicates a vigorous process of star birth, often linked to the presence of dense molecular clouds, while a lower rate suggests more quiescent conditions, impacting the overall dynamics and evolution of galaxies.
Stellar feedback: Stellar feedback refers to the various processes by which stars influence their surrounding environment, particularly in terms of energy and material exchange. This occurs through mechanisms like stellar winds, supernova explosions, and radiation pressure, which can trigger or suppress star formation in nearby regions. Understanding stellar feedback is crucial for comprehending galaxy formation, the lifecycle of molecular clouds, and the broader feedback processes that shape the universe.
Stellar nucleosynthesis: Stellar nucleosynthesis is the process by which elements are formed through nuclear reactions in the cores of stars. This process plays a crucial role in the chemical evolution of the universe, as it creates new elements from primordial gas, which eventually contributes to galaxy formation and evolution. Stellar nucleosynthesis not only produces light elements like helium and carbon but also heavier elements like iron and beyond, enriching the interstellar medium and influencing subsequent generations of stars and planetary systems.
Supernova Explosions: A supernova explosion is a catastrophic explosion of a star at the end of its life cycle, resulting in a sudden and extremely bright release of energy. These events are significant in the context of cosmic evolution as they contribute to the formation of heavier elements and the dispersal of materials into space, which are essential for creating new stars and galaxies from primordial gas.