physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fu ...

, a protogalaxy, which could also be called a "primeval galaxy", is a cloud of gas which is forming into a

galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...

. It is believed that the rate of

star formation Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...

during this period of galactic evolution will determine whether a galaxy is a

spiral In mathematics, a spiral is a curve which emanates from a point, moving further away as it revolves around the point. It is a subtype of whorled patterns, a broad group that also includes concentric objects. Two-dimensional A two-dimension ...

elliptical galaxy An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main galaxy morphological classification, classes of galaxy described by Edwin Hubble in his Hub ...

; a slower star formation tends to produce a spiral galaxy. The smaller clumps of gas in a protogalaxy form into

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s. The term "protogalaxy" itself is generally accepted to mean "Progenitors of the present day (normal) galaxies, in the early stages of formation." However, the "early stages of formation" is not a clearly defined phrase. It could be defined as: "The first major burst of

in a progenitor of a present day elliptical galaxy"; "The peak merging

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

of dark halos of the fragments which assemble to produce an average galaxy today"; "A still gaseous body before any star formation has taken place."; or " an over-dense region of

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

in the very early universe, destined to become gravitationally bound and to collapse."

Formation

It is thought that the early universe began with a nearly uniform distribution (each particle an equal distance from the next) of matter and dark matter. The dark matter then began to clump together under gravitational attraction due to the initial density perturbation spectrum caused by

quantum fluctuation In quantum physics, a quantum fluctuation (also known as a vacuum state fluctuation or vacuum fluctuation) is the temporary random change in the amount of energy in a point in space, as prescribed by Werner Heisenberg's uncertainty principle. ...

s. This derives from Heisenberg's uncertainty principle which shows that there can be tiny temporary changes in the amount of energy in empty space. Particle/

antiparticle In particle physics, every type of particle of "ordinary" matter (as opposed to antimatter) is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the ...

pairs can form from this energy through

mass–energy equivalence In physics, mass–energy equivalence is the relationship between mass and energy in a system's rest frame. The two differ only by a multiplicative constant and the units of measurement. The principle is described by the physicist Albert Einstei ...

, and gravitational pull causes other nearby particles to move towards it, disturbing the even distribution and creating a centre of gravity, pulling nearby particles closer. When this happens at the universe's present size it is negligible, but the state of these tiny fluctuations as the universe began expanding from a single point left an impression which scaled up as the universe expanded, resulting in large areas of increased density. The gravity of these denser clumps of dark matter then caused nearby matter to start falling into the denser region. This sort of process was reportedly observed and analysed by Nilsson et al. in 2006. This resulted in the formation of clouds of gas, predominantly

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

, and the first stars began to form within these clouds. These clouds of gas and early stars, many times smaller than our galaxy, were the first protogalaxies. Stellar Fireworks Finale

The established theory is that the groups of small protogalaxies were attracted together by gravity and collided, which resulted in the formation of the much larger "adult" galaxies we have today. This follows the process of hierarchical assembly, which is an ongoing process where larger bodies are continually formed from the merging of smaller ones.

Properties

Composition

Since there had been no previous

to create other elements, protogalaxies would have been made up almost entirely of hydrogen and helium. The hydrogen would bond to form H₂ molecules, with some exceptions. This would change as star formation began and produced more elements through the process of

nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...

Mechanics

Once a protogalaxy begins to form, all particles bound by its gravity begin to

free fall In classical mechanics, free fall is any motion of a physical object, body where gravity is the only force acting upon it. A freely falling object may not necessarily be falling down in the vertical direction. If the common definition of the word ...

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, the disks taking longer to fully form. The formation of galaxy clusters takes much longer and is still in progress now. This stage is also where galaxies acquire most of their

angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...

. A protogalaxy acquires this due to gravitational influence from neighbouring dense clumps in the early universe, and the further the gas is away from the centre, the more spin it gets.

Luminosity

The luminosity of protogalaxies comes from two sources. First and foremost is the

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

from nuclear fusion of Hydrogen into

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

in early stars. This early burst of star formation is thought to have made a protogalaxy's luminosity comparable to a present-day

starburst galaxy A starburst galaxy is one undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy, or the star formation rate observed in most other galaxies. For example, the star format ...

or a

quasar A quasar ( ) is an extremely Luminosity, luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass rangi ...

. The other is the release of excess gravitational binding energy. The primary wavelength expected from a protogalaxy is a variety of UV called Lyman-alpha, which is the wavelength emitted by Hydrogen gas when it is ionised by radiation from a star.

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, in some places long enough that we see them at the stage where they are populated by protogalaxies. There have been many attempts to find protogalaxies with telescopes over the last 30 years because of the value of such a discovery in confirming how galaxies form, but the sheer distance any light would have to travel for it to be old enough to come from a protogalaxy is very large. This, coupled with the fact that the Lyman-alpha wavelength is quite readily absorbed by dust, made some astronomers think protogalaxies may be too faint to detect. In 1996, a protogalaxy candidate was discovered by Yee et al. using the Canadian Network for Observational Cosmology (CNOC). The object was a disk-like galaxy at high

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...

with a very high luminosity. It was later debated that the incredible luminosity was caused by the

gravitational lens A gravitational lens is matter, such as a galaxy cluster, cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's Ge ...

ing of a foreground galactic cluster. In 2006, K. Nilsson et al. reported finding a "blob" emitting Lyman alpha UV radiation. Analysis concluded that this was a giant cloud of hydrogen gas falling onto a clump of dark matter in the early universe, creating a protogalaxy. In 2007, Michael Rauch et al. were using the VLT to search for a signal from intergalactic gas, when they spotted dozens of discrete objects emitting large amounts of the Lyman-alpha type UV radiation. They concluded that these 27 objects were examples of protogalaxies from 11 billion years ago.

References

External links

Universe-Review:Formation and Evolution of Galaxy
{{Galaxy Protogalaxies Physical cosmology