© ESA/Wolfram Freudling

In physical cosmology, a protogalaxy, also known as a “primeval galaxy,” is a cloud of gas forming into a galaxy. The star formation rate during this period of galactic evolution plays a crucial role in determining whether a galaxy will become a spiral or elliptical galaxy. A slower rate of star formation tends to produce spiral galaxies. The smaller clumps of gas within a protogalaxy eventually form into stars.

The term “protogalaxy” generally refers to “progenitors of the present day (normal) galaxies, in the early stages of formation.” However, the phrase “early stages of formation” lacks a clear definition. It could be interpreted as:

• The first major burst of star formation in a progenitor of a present-day elliptical galaxy.
• The peak merging epoch of dark halos of the fragments that assemble to produce an average galaxy today.
• A still gaseous body before any star formation has taken place.
• An over-dense region of dark matter in the very early universe, destined to become gravitationally bound and collapse.

FORMATION OF PROTOGALAXY

The formation of a protogalaxy is a complex process that involves the gravitational collapse of a gas cloud in the early universe. This process is thought to have occurred around 13.6 billion years ago, during the reionization era.

INITIAL CONDITION

The universe began as a hot, dense plasma, with temperatures exceeding 10 billion degrees Celsius. Over time, this plasma cooled and expanded, leading to the formation of subatomic particles such as protons, neutrons, and electrons. These particles eventually came together to form neutral atoms, primarily hydrogen and helium, which were dispersed throughout the universe.

DARK MATTER CLUSTERING

The universe also contained dark matter, which is an invisible form of matter that does not interact with light. Dark matter particles began to clump together under the influence of gravity, forming large-scale structures such as galaxy clusters and superclusters. These structures were the result of tiny fluctuations in the density of the universe, which were amplified by gravity over time.

GAS CLOUD FORMATION

As the universe continued to expand and cool, the gas clouds began to collapse under their gravity. These clouds were primarily composed of hydrogen and helium, with some heavier elements formed through nuclear reactions within the first stars. The gas clouds collapsed into denser regions, which eventually formed the first stars and protogalaxies.

FIRST STAR AND PROTOGALAXIES

The first stars were massive and short-lived, burning through their fuel quickly and exploding as supernovae. These explosions dispersed heavy elements into the surrounding gas, enriching the interstellar medium. The gas clouds continued to collapse, and the first protogalaxies formed. These protogalaxies were small, irregular, and highly turbulent, with gas and dust swirling around the first stars.

GALAXY MERGERS AND EVOLUTION

Over time, the protogalaxies collided and merged, forming larger and more massive galaxies. This process of hierarchical assembly continued, with smaller galaxies merging to form larger ones. The galaxies that formed through this process were often elliptical or spiral in shape, with the spiral galaxies having a central bulge and a disk of stars and gas.

STAR FORMATION AND FEEDBACK

As the galaxies continued to evolve, star formation became more efficient. Stars formed in bursts, releasing energy and heavy elements into the interstellar medium. This energy and matter influenced the surrounding gas, regulating the rate of star formation and shaping the galaxy’s structure. Feedback mechanisms, such as supernovae explosions and radiation from stars, played a crucial role in regulating the star formation rate and the overall evolution of the galaxy.

GALAXY CLUSTERS AND LARGE-SCALE STRUCTURE

The galaxies that formed through this process were not isolated but were part of larger structures such as galaxy clusters and superclusters. These structures were the result of the gravitational attraction between galaxies and the dark matter that held them together. The formation of these large-scale structures was influenced by the distribution of dark matter and the mergers between galaxies.

PROPERTIES

The properties of protogalaxies are characterized by several key features that distinguish them from more developed galaxies.

MECHANICS

Protogalaxies are vast clouds of gas that are forming into galaxies. Once a protogalaxy begins to form, all particles bound by its gravity start to fall towards it. The time taken for this free-fall to conclude can be approximated using the free-fall equations. Most galaxies have completed this free-fall stage to become stable elliptical or disk galaxies, with the disks taking longer to fully form. The formation of galaxy clusters takes much longer and is still in progress now. During this stage, galaxies acquire most of their angular momentum due to gravitational influence from neighboring dense clumps in the early universe, with the gas farther away from the center receiving more spin.

LUMINOSITY

The luminosity of protogalaxies is primarily due to the radiation from the nuclear fusion of hydrogen into helium in early stars. This early burst of star formation is thought to have made a protogalaxy’s luminosity comparable to that of a present-day starburst galaxy or quasar. Additionally, the release of excess gravitational binding energy also contributes to the luminosity of protogalaxies.

The primary wavelength expected from a protogalaxy is a variety of ultraviolet (UV) radiation, particularly Lyman-alpha, which is emitted by hydrogen gas when it is ionized by radiation from stars. This radiation is thought to be a key indicator of the early stages of galaxy formation and evolution.

Protogalaxies are believed to have undergone a significant burst of star formation, which would have released a large amount of energy into the interstellar medium. This energy release would have ionized the surrounding gas, leading to the emission of UV radiation. The luminosity of protogalaxies is thus closely tied to the rate of star formation during this period, which in turn determines the eventual shape and structure of the galaxy.

DETECTION

Protogalaxies can theoretically still be seen today, as the light from the farthest reaches of the universe takes a very long time to reach Earth. However, the sheer distance any light would have to travel for it to be old enough to come from a protogalaxy is very large, and the Lyman-alpha wavelength is readily absorbed by dust, making some astronomers think protogalaxies may be too faint to detect. Despite these challenges, several candidates have been discovered, such as a protogalaxy candidate discovered by Yee et al. in 1996 and dozens of discrete objects emitting large amounts of Lyman-alpha UV radiation found by Michael Rauch et al. in 2007.

CLUSTERING

The clustering of protogalaxies is a crucial aspect of their formation and evolution. In the early universe, protogalaxies are thought to have formed through the gravitational collapse of dark matter halos. These halos were initially small and numerous, but over time, they merged to form larger structures, such as galaxy clusters and superclusters. This hierarchical clustering process is believed to have continued, with smaller protogalaxies merging to form larger galaxies, including elliptical and spiral galaxies.

The clustering of protogalaxies is influenced by several factors, including the initial conditions of the universe, the properties of dark matter, and the interactions between protogalaxies themselves. For example, the rate of star formation within a protogalaxy can affect its ability to interact with other protogalaxies, potentially leading to mergers and the formation of larger galaxies.

MORPHOLOGY

Protogalaxies are expected to have a morphology that is distinct from more developed galaxies. They are likely to be highly irregular and may not yet have developed the characteristic spiral or elliptical shapes seen in modern galaxies. The morphology of protogalaxies is still a topic of ongoing research and is influenced by the complex interactions between dark matter and baryonic gas during their formation.

COMPOSITION

The composition of a protogalaxy is primarily composed of hydrogen and helium, with some exceptions. Hydrogen molecules (H2) are formed through the bonding of hydrogen atoms. This composition is a result of the universe’s early stages, where no previous star formation had occurred to create other elements through nuclear fusion. The hydrogen and helium in a protogalaxy are thought to be trapped in dark matter potential, which eventually collapses to form the galaxy.

The initial conditions of protogalaxies are characterized by a nearly uniform density and temperature. These conditions are thought to have been influenced by the gravitational influence of neighboring dense clumps in the early universe, which imparted angular momentum to the gas as it fell toward the center of the protogalaxy.

The luminosity of protogalaxies comes from two primary sources: the radiation from nuclear fusion of hydrogen into helium in early stars and the release of excess gravitational binding energy. The primary wavelength emitted by protogalaxies is Lyman-alpha radiation, which is the wavelength emitted by hydrogen gas when it is ionized by radiation from stars.

In summary, the composition of a protogalaxy is dominated by hydrogen and helium, with the gas trapped in dark matter potential. The initial conditions of protogalaxies are characterized by uniform density and temperature, and their luminosity is primarily due to nuclear fusion and gravitational binding energy.

EVOLUTION

The evolution of a protogalaxy is a complex process that involves the formation and interaction of various components. Here is a detailed description of this process in more than 200 words:

GALAXY EVOLUTION

Once the first stars have formed, the protogalaxy begins to evolve. The accumulated matter settles into a galactic disc, and the galaxy continues to absorb infalling material from high-velocity clouds and dwarf galaxies. This process leads to the formation of a galactic bulge and the creation of a supermassive black hole at the center of the galaxy. The galaxy also undergoes a major burst of star formation, which gradually increases the abundance of heavy elements, eventually allowing the formation of planets.

INTERACTIONS AND COLLISIONS

The evolution of a protogalaxy can be significantly affected by interactions and collisions with other galaxies. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology. These interactions can lead to the formation of tidal tails and the stripping of interstellar gas and dust from the spiral arms.

CURRENT UNDERSTANDING

The detailed process by which the earliest galaxies formed is still an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up. Top-down theories propose that protogalaxies form in a large-scale simultaneous collapse, while bottom-up theories suggest that small structures such as globular clusters form first and then accrete to form a larger galaxy. The existence of galaxies so soon after the Big Bang suggests that protogalaxies must have grown in the so-called “dark ages” before the first stars formed.

FUTURE RESEARCH

Future advances in telescopes and instrumentation will hopefully yield firm detections of protogalaxies, from which we can finally study their formation in detail. The discovery of protogalaxies will provide valuable insights into the early universe and the formation of galaxies, shedding light on the fundamental processes that have shaped the cosmos as we know it today.