Galactic orientation

Galactic clusters[1] are gravitationally bound large-scale structures of multiple galaxies. The evolution of these aggregates is determined by time and manner of formation and the process of how their structures and constituents have been changing with time. Gamow (1952) and Weizscker (1951) showed that the observed rotations of galaxies are important for cosmology. They postulated that the rotation of galaxies might be a clue of physical conditions under which these systems formed. Thus, understanding the distribution of spatial orientations of the spin vectors of galaxies is critical to understanding the origin of the angular momenta of galaxies.

There are mainly three scenarios for the origin of galaxy clusters and superclusters. These models are based on different assumptions of the primordial conditions, so they predict different spin vector alignments of the galaxies. The three hypotheses are the pancake model, the hierarchy model, and the primordial vorticity theory. The three are mutually exclusive as they produce contradictory predictions. However, the predictions made by all three theories are based on the precepts of cosmology. Thus, these models can be tested using a database with appropriate methods of analysis.


A galaxy is a large gravitational aggregation of stars, dust, gas, and an unknown component termed dark matter. The Milky Way Galaxy[2] is only one of billions of galaxies in the known universe. Galaxies are classified into spirals,[3] ellipticals, irregular, and peculiar. Sizes can range from only a few thousand stars (dwarf irregulars) to 1013 stars in giant ellipticals. Elliptical galaxies are spherical or elliptical in appearance. Spiral galaxies range from S0, the lenticular galaxies, to Sb, which have a bar across the nucleus, to Sc galaxies which have strong spiral arms. In total count, ellipticals amount to 13%, S0 to 22%, Sa, b, c galaxies to 61%, irregulars to 3.5% and peculiars to 0.9%.

At the center of the most galaxies is a high concentration of older stars. This portion of a galaxy is called the nuclear bulge. Beyond the nuclear bulge lies a large disc containing young, hot stars, called the disk of the galaxy. There is a morphological separation: Ellipticals are most common in clusters of galaxies, and typically the center of a cluster is occupied by a giant elliptical. Spirals are most common in the field, i.e., not in clusters.

Primordial Vorticity Model

The primordial vorticity theory predicts that the spin vectors of galaxies are distributed primarily perpendicular to the cluster plane.[4] The primordial vorticity is called top-down scenario. Sometimes it is also called turbulence model. In the turbulence scenario, first flattened rotating proto-clusters formed due to cosmic vorticity in the early universe. Subsequent density and pressure fluctuations caused galaxies to form.

The idea that galaxy formation is initiated by primordial turbulence has a long history. Ozernoy (1971, 1978) proposes that galaxies form from high-density regions behind the shocks produced by turbulence. According to the primordial vorticity theory, the presence of large chaotic velocities generates turbulence, which, in turn, produces density and pressure fluctuations.

Density fluctuations on the scale of clusters of galaxies could be gravitationally bound, but galactic mass fluctuations are always unbound. Galaxies form when unbound galactic mass eddies, expanding faster than their bound cluster background. So forming galaxies collide with each other as clusters start to recollapse. These collisions produce shocks and high-density proto-galaxies at the eddy interfaces. As clusters recollapse, the system of galaxies undergoes a violent collective relaxation.

Pancake Model

The pancake model was first proposed in the 1970s by Yakob B. Zel'dovich at the Institute of Applied Mathematics in Moscow.[5]

The pancake model predicts that the spin vectors of galaxies tend to lie within the cluster plane. In the pancake scenario, formation of clusters took place first and it was followed by their fragmentation into galaxies due to adiabatic fluctuations. According to the non-linear gravitational instability theory, a growth of small inhomogeneities leads to the formation of thin, dense, and gaseous condensations that are called `pancakes'. These condensations are compressed and heated to high temperatures by shock waves causing them to quickly fragment into gas clouds. The later clumping of these clouds results in the formation of galaxies and their clusters.

Thermal, hydrodynamic, and gravitational instabilities arise during the course of evolution. It leads to the fragmentation of gaseous proto-clusters and, subsequently, clustering of galaxies takes place. The pancake scheme follows three simultaneous processes: first, gas cools and new clouds of cold gas form; secondly, these clouds cluster to form galaxies; and thirdly, the forming galaxies and, to an extent, single clouds cluster together to form a cluster of galaxies.

Hierarchy Model

According to the hierarchy model, the directions of the spin vectors should be distributed randomly. In hierarchy model, galaxies were first formed and then they obtained their angular momenta by tidal force while they were gathering gravitationally to form a cluster. Those galaxies grow by subsequent merging of proto-galactic condensations or even by merging of already fully formed galaxies. In this scheme, one could imagine that large irregularities like galaxies grew under the influence of gravities from small imperfections in the early universe.

The angular momentum transferred to a developing proto-galaxy by the gravitational interaction of the quadrupole moment of the system with the tidal field of the matter.


  1. ^ "Gamow G., 1952, Phys. Rev. 86,251 & Weizscker C.F., 1951, APJ 114, 165".
  2. ^ "The Milky Way Galaxy - SEDS Messier Database".
  3. ^ "Spiral Galaxies (and other disks)". Retrieved 31 July 2014.
  4. ^ "Research Area (Brief Description)". Astro Nepal. Archived from the original on 8 August 2014. Retrieved 31 July 2014.
  5. ^ Pagels, Heinz R. (1985). Perfect Symmetry: The Search for the Beginning of Time. Simon and Schuster. p. 134. ISBN 9780671465483.

Further reading

Coma Filament

Coma Filament is a galaxy filament. The filament contains the Coma Supercluster of galaxies and forms a part of the CfA2 Great Wall.


A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.

Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.

Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion (2×1011) to a suggested 2 trillion (2×1012) or more, containing more stars than all the grains of sand on planet Earth. Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 to 300,000 light years) and separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 LY) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million LY).

The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which is dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. The largest structure of galaxies yet recognised is a cluster of superclusters that has been named Laniakea, which contains the Virgo supercluster.

Galaxy groups and clusters

Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large-scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters.

Green bean galaxy

Green bean galaxies (GBGs) are very rare astronomical objects that are thought to be quasar ionization echos. They were discovered by Mischa Schirmer and colleagues R. Diaz, K. Holhjem, N.A. Levenson, and C. Winge. The authors report the discovery of a sample of Seyfert-2 galaxies with ultra-luminous galaxy-wide narrow-line regions (NLRs) at redshifts z=0.2-0.6.While examining survey images taken with the 3.6-meter Canada-France-Hawaii Telescope (CFHT) atop 4200-m Mauna Kea, Hawaii, Schirmer noticed a galaxy with unusual colors—strongly peaking in the r filter, suggesting a spectral line. In fact, the color is quite similar to the Green Pea galaxies (GPs), which are compact star-forming galaxies. However, the object which became known as a GBG is much larger.

These galaxies are so rare that there is on average only one in a cube about 1.3 billion light-years across. They were nicknamed GBGs because of their color and because they are superficially similar to, but larger than, GPs. The interstellar gas in most GPs is ionized by UV-light from intense star formation, whereas the gas in GBGs is ionized by hard x-rays from an active galactic nucleus (AGN). The scarcity of GBGs indicates that this phenomenon is very rare, and/or very short-lived.GBGs are likely related to the object known as Hanny's Voorwerp, another possible quasar ionization echo. GBGs are substantially different, though, as their luminosities, sizes and gas masses are 10-100 times higher than in other quasar ionisation clouds, for instance the 154 studied in Keel et al. 2012 (nicknamed 'voorwerpjes'). These 'voorwerpjes' are estimated to have bright phases that last between ~20,000 and 200,000 years.Possible formation mechanisms are currently under investigation. Likely, the giant gas outflows have been produced during the last stages in the life of super-luminous quasars, which subsequently experienced a rapid shut-down, e.g. due to a process known as AGN feedback. The escaping X-rays from the former very active quasar state still ionize the gas, causing the ionization echo.

Lynx–Ursa Major Filament

Lynx–Ursa Major Filament (LUM Filament) is a galaxy filament.The filament is connected to and separate from the Lynx–Ursa Major Supercluster.

Perseus–Pegasus Filament

Perseus–Pegasus Filament is a galaxy filament containing the Perseus-Pisces Supercluster and stretching for roughly a billion light years (or over 300/h Mpc). Currently, it is considered to be one of the largest known structures in the universe. This filament is adjacent to the Pisces–Cetus Supercluster Complex.

Pisces–Cetus Supercluster Complex

The Pisces–Cetus Supercluster Complex is a galaxy filament. It includes the Virgo Supercluster which in turn contains the Local Group, the galaxy cluster that includes the Milky Way.

This filament is adjacent to the Perseus–Pegasus Filament.

However, a 2014 study indicates that the Virgo Supercluster is only a lobe of a greater supercluster, Laniakea.

Radio Galaxy Zoo

Radio Galaxy Zoo (RGZ) is an internet crowdsourced citizen science project that seeks to locate supermassive black holes in distant galaxies. It is hosted by the web portal Zooniverse. The scientific team want to identify black hole/jet pairs and associate them with the host galaxies. Using a large number of classifications provided by citizen scientists they hope to build a more complete picture of black holes at various stages and their origin. It was initiated in 2010 by Ray Norris in collaboration with the Zooniverse team, and was driven by the need to cross-identify the millions of extragalactic radio sources that will be discovered by the forthcoming Evolutionary Map of the Universe survey. RGZ is now led by scientists Julie Banfield and Ivy Wong. RGZ started operations on 17 December 2013.

Ursa Major Filament

Ursa Major Filament is a galaxy filament. The filament is connected to the CfA Homunculus, a portion of the filament forms a portion of the "leg" of the Homunculus.

Z8 GND 5296

z8_GND_5296 is a dwarf galaxy discovered in October 2013 which has the highest redshift that has been confirmed through the Lyman-alpha emission line of hydrogen, placing it among the oldest and most distant known galaxies at approximately 13.1 billion light-years (4.0 Gpc) from Earth. It is "seen as it was at a time just 700 million years after the Big Bang [...] when the universe was only about 5 percent of its current age of 13.8 billion years". The galaxy is at a redshift of 7.51, and it is a neighbour to what was announced then as the second-most distant galaxy with a redshift of 7.2. The galaxy in its observable timeframe was producing stars at a phenomenal rate, equivalent in mass to about 330 Suns per year.The light reaching Earth from z8_GND_5296 shows its position over 13 billion years ago, having traveled a distance of more than 13 billion light-years. Due to the expansion of the universe, this position is now at about 30 billion light-years (9.2 Gpc) (comoving distance) from Earth.

Zeldovich pancake

A Zel'dovich pancake is a theoretical condensation of gas out of a primordial density fluctuation following the Big Bang. In 1970, Yakov B. Zel'dovich showed that for an ellipsoid of gas on a supergalactic scale, an approximation can be used that will model the collapse as occurring most rapidly along the shortest axis, resulting in a pancake form. This approximation assumes that the ellipsoid of gas is sufficiently large that the effect of pressure is negligible and only gravitational attraction needs to be considered. That is, the gas will collapse without being significantly perturbed by outward pressure. This assumption is especially valid if the collapse occurs before the recombination era that resulted in the formation of hydrogen atoms.In 1989, Zel'dovich and S. F. Shandarin showed that initial overlapping density fluctuations of random Gaussian fields would result in "dense pancakes, filaments, and compact clumps of matter". This model became known as a top-down model of

galactic formation, with the supergalactic condensation

fragmenting into protogalaxies. The formation of plane concentrations would compress the gas through shock waves generated during the collapse, increasing the temperature.At a higher level, the collapse of larger structures according to the Zel'dovich Approximation is known as second-generation pancakes or superpancakes. At still higher levels there is a transition to a hierarchical clustering model in which a hierarchy of collapsing structures exist. The Truncated Zel'dovich Approximation allowed the method to be applied to these hierarchical, or bottom-up models of cosmological structure. This approach truncates the power law fluctuation spectrum at large values of k before applying the Zel'dovich Approximation. Generalizations of the Zel'dovich Approximation have also been shown to apply in the case of a non-zero cosmological constant.The first example of a Zeldovich pancake may have been identified in 1991 using the Very Large Array in New Mexico.

Active nuclei
Energetic galaxies
Low activity
See also

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