Galaxy merger

Last updated
The Mice Galaxies (NGC 4676 A&B) are in the process of merging. NGC4676.jpg
The Mice Galaxies (NGC 4676 A&B) are in the process of merging.
This artist's impression shows the merger between two galaxies leading to the formation of a disc galaxy.

Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time. [1]



During the merger, stars and dark matter in each galaxy become affected by the approaching galaxy. Toward the late stages of the merger, the gravitational potential (i.e. the shape of the galaxy) begins changing so quickly that star orbits are greatly altered, and lose any trace of their prior orbit. This process is called “violent relaxation”. [2] For example, when two disk galaxies collide they begin with their stars in a orderly rotation in the planes of the two separate disks. During the merger, that ordered motion is transformed into random energy (“thermalized”). The resultant galaxy is dominated by stars that orbit the galaxy in a complicated and random interacting network of orbits, which is what is observed in elliptical galaxies.

NGC 3921 is an interacting pair of disc galaxies in the late stages of its merger. Evolution in slow motion.jpg
NGC 3921 is an interacting pair of disc galaxies in the late stages of its merger.

Mergers are also locations of extreme amounts of star formation. [4] The star formation rate (SFR) during a major merger can reach thousands of solar masses worth of new stars each year, depending on the gas content of each galaxy and its redshift. [5] [6] Typical merger SFRs are less than 100 new solar masses per year. [7] [8] This is large compared to our Galaxy, which makes only a few new stars each year (~2 new stars). [9] Though stars almost never get close enough to actually collide in galaxy mergers, giant molecular clouds rapidly fall to the center of the galaxy where they collide with other molecular clouds.[ citation needed ] These collisions then induce condensations of these clouds into new stars. We can see this phenomenon in merging galaxies in the nearby universe. Yet, this process was more pronounced during the mergers that formed most elliptical galaxies we see today, which likely occurred 1–10 billion years ago, when there was much more gas (and thus more molecular clouds) in galaxies. Also, away from the center of the galaxy gas clouds will run into each other producing shocks which stimulate the formation of new stars in gas clouds. The result of all this violence is that galaxies tend to have little gas available to form new stars after they merge. Thus if a galaxy is involved in a major merger, and then a few billion years pass, the galaxy will have very few young stars (see Stellar evolution) left. This is what we see in today's elliptical galaxies, very little molecular gas and very few young stars. It is thought that this is because elliptical galaxies are the end products of major mergers which use up the majority of gas during the merger, and thus further star formation after the merger is quenched. [ citation needed ]

Galaxy mergers can be simulated in computers, to learn more about galaxy formation. Galaxy pairs initially of any morphological type can be followed, taking into account all gravitational forces, and also the hydrodynamics and dissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium by supernovae. Such a library of galaxy merger simulations can be found on the GALMER website. [10] A study led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Maryland created computer simulations in order to better understand images taken by the Hubble Telescope. [1] Lotz's team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a tiny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles. [1]

One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702. It may form one of the largest galaxies in the Universe. [11]


Galaxy mergers can be classified into distinct groups due to the properties of the merging galaxies, such as their number, their comparative size and their gas richness.

By number

Mergers can be categorized by the number of galaxies engaged in the process:

Binary merger
Two interacting galaxies merge.
Multiple merger
Three or more galaxies merge.

By size

Mergers can be categorized by the extent to which the largest involved galaxy is changed in size or form by the merger:

Minor merger
A merger is minor if one of the galaxies is significantly larger than the other(s). The larger galaxy will often "eat" the smaller, absorbing most of its gas and stars with little other significant effect on the larger galaxy. Our home galaxy, the Milky Way, is thought to be currently absorbing several smaller galaxies in this fashion, such as the Canis Major Dwarf Galaxy, and possibly the Magellanic Clouds. The Virgo Stellar Stream is thought to be the remains of a dwarf galaxy that has been mostly merged with the Milky Way.
Major merger
A merger of two spiral galaxies that are approximately the same size is major; if they collide at appropriate angles and speeds, they will likely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanisms that often include a stage in which there are active galactic nuclei. This is thought to be the driving force behind many quasars. The end result is an elliptical galaxy, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals.

One study found that large galaxies merged with each other on average once over the past 9 billion years. Small galaxies coalesced with large galaxies more frequently. [1] Note that the Milky Way and the Andromeda Galaxy are predicted to collide in about 4.5 billion years. The expected result of these galaxies merging would be major as they have similar sizes, and will change from two "grand design" spiral galaxies to (probably) a giant elliptical galaxy.

By gas richness

Mergers can be categorized by the degree to which the gas (if any) carried within and around the merging galaxies interacts:

Wet merger
A wet merger is between gas-rich galaxies ("blue" galaxies). Wet mergers typically produce a large amount of star formation, transform disc galaxies into elliptical galaxies and trigger quasar activity. [12]
Dry merger
A merger between gas-poor galaxies ("red" galaxies) is called dry. Dry mergers typically do not greatly change the galaxies' star formation rates, but can play an important role in increasing stellar mass. [12]
Damp merger
A damp merger occurs between the same two galaxy-types mentioned above ("blue" and "red" galaxies), if there is enough gas to fuel significant star formation but not enough to form globular clusters [13]
Mixed merger
A mixed merger occurs when gas-rich and gas-poor galaxies ("blue" and "red" galaxies) merge.

Merger history trees

In the standard cosmological model, any single galaxy is expected to have formed from a few or many successive mergers of dark matter haloes, in which gas cools and forms stars at the centres of the haloes, becoming the optically visible objects historically identified as galaxies during the twentieth century. Modelling the mathematical graph of the mergers of these dark matter haloes, and in turn, the corresponding star formation, was initially treated either by analysing purely gravitational N-body simulations [14] [15] or by using numerical realisations of statistical ("semi-analytical") formulae. [16]

In a 1992 observational cosmology conference in Milan, [14] Roukema, Quinn and Peterson showed the first merger history trees of dark matter haloes extracted from cosmological N-body simulations. These merger history trees were combined with formulae for star formation rates and evolutionary population synthesis, yielding synthetic luminosity functions of galaxies (statistics of how many galaxies are intrinsically bright or faint) at different cosmological epochs. [14] [15] Given the complex dynamics of dark matter halo mergers, a fundamental problem in modelling merger history tree is to define when a halo at one time step is a descendant of a halo at the previous time step. Roukema's group chose to define this relation by requiring the halo at the later time step to contain strictly more than 50 percent of the particles in the halo at the earlier time step; this guaranteed that between two time steps, any halo could have at most a single descendant. [17] This galaxy formation modelling method yields rapidly calculated models of galaxy populations with synthetic spectra and corresponding statistical properties comparable with observations. [17]

Independently, Lacey and Cole showed at the same 1992 conference [18] how they used the Press–Schechter formalism combined with dynamical friction to statistically generate Monte Carlo realisations of dark matter halo merger history trees and the corresponding formation of the stellar cores (galaxies) of the haloes. [16] Kauffmann, White and Guiderdoni extended this approach in 1993 to include semi-analytical formulae for gas cooling, star formation, gas reheating from supernovae, and for the hypothesised conversion of disc galaxies into elliptical galaxies. [19] Both the Kauffmann group and Okamoto and Nagashima later took up the N-body simulation derived merger history tree approach. [20] [21]


Some of the galaxies that are in the process of merging or are believed to have formed by merging are:

Merging galaxies
Arp 302 (left); NGC 7752/7753; IIZw96 (right).
NGC 2623 or Arp 243 - HST Potw1742a.tif
NGC 2623 – late stage merging of two galaxies. [22]
Galactic glow worm.jpg
Galaxy twistings – possible merger. [23]
Markarian 779.jpg
Markarian 779 – possible merger. [24]
Artist's impression of ancient galaxy megamerger.jpg
Ancient galaxy mega-merger (artist concept). [25]
Cosmic "flying V" of merging galaxies.tif
“Flying V” – two galaxies. [26]

See also

Related Research Articles

Galaxy formation and evolution from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure.

Galaxy Gravitationally bound astronomical structure

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 stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.

Globular cluster Spherical collection of stars

A globular cluster is a spherical collection of stars. Globular clusters are very tightly bound by gravity, giving them their spherical shapes and high concentrations of stars toward their centers. Their name is derived from Latin globulus—a small sphere. Globular clusters are occasionally known simply as globulars.

Hubbles law Observation in physical cosmology

Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance. In other words, the farther they are the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the spectrum.

Andromeda Galaxy Barred spiral galaxy within the Local Group

The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy approximately 2.5 million light-years from Earth and the nearest major galaxy to the Milky Way. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the Ethiopian princess who was the wife of Perseus in Greek mythology.

Elliptical galaxy Galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile

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 classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.

Supermassive black hole Largest type of black hole; usually found at the center of galaxies

A supermassive black hole is the largest type of black hole, with mass on the order of millions to billions of times the mass of the Sun (M). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at the galaxy's center. The Milky Way has a supermassive black hole in its Galactic Center, which corresponds to the location of Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei and quasars.

In physical cosmology, a protogalaxy, which could also be called a "primeval galaxy", is a cloud of gas which is forming into a galaxy. It is believed that the rate of star formation during this period of galactic evolution will determine whether a galaxy is a spiral or elliptical galaxy; a slower star formation tends to produce a spiral galaxy. The smaller clumps of gas in a protogalaxy form into stars.

Galactic bulge Tightly packed group of stars within a larger formation

In astronomy, a galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.

Dark matter halo A theoretical component of a galaxy that envelops the galactic disc and extends well beyond the edge of the visible galaxy

According to modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo may contain multiple virialized clumps of dark matter bound together by gravity, known as subhalos. Modern cosmological models, such as ΛCDM, propose that dark matter halos and subhalos may contain galaxies. The dark matter halo of a galaxy envelops the galactic disc and extends well beyond the edge of the visible galaxy. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through observations of their effects on the motions of stars and gas in galaxies and gravitational lensing. Dark matter halos play a key role in current models of galaxy formation and evolution. Theories that attempt to explain the nature of dark matter halos with varying degrees of success include cold dark matter (CDM), warm dark matter, and massive compact halo objects (MACHOs).

Structure formation Formation of galaxies, galaxy clusters and larger structures from small early density fluctuations

In physical cosmology, structure formation is the formation of galaxies, galaxy clusters and larger structures from small early density fluctuations. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.8 billion years ago. However, looking at the night sky today, structures on all scales can be seen, from stars and planets to galaxies. On even larger scales, galaxy clusters and sheet-like structures of galaxies are separated by enormous voids containing few galaxies. Structure formation attempts to model how these structures formed by gravitational instability of small early ripples in spacetime density.

The Millennium Run, or Millennium Simulation is a computer N-body simulation used to investigate how the distribution of matter in the Universe has evolved over time, in particular, how the observed population of galaxies was formed. It is used by scientists working in physical cosmology to compare observations with theoretical predictions.

Satellite galaxy Galaxy that orbits a larger galaxy due to gravitational attraction

A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within our own solar system are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.

Illustris project Computer-simulated universes

The Illustris project is an ongoing series of astrophysical simulations run by an international collaboration of scientists. The aim is to study the processes of galaxy formation and evolution in the universe with a comprehensive physical model. Early results are described in a number of publications following widespread press coverage. The project publicly released all data produced by the simulations in April, 2015. A follow-up to the project, IllustrisTNG, was presented in 2017.

Galaxy group

A galaxy group or group of galaxies (GrG) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 1010 times the luminosity of the Sun); collections of galaxies larger than groups that are first-order clustering are called galaxy clusters. The groups and clusters of galaxies can themselves be clustered, into superclusters of galaxies.

MACS J0416.1-2403 Galaxy cluster in the constellation Eridanus

MACS J0416.1-2403 is a cluster of galaxies at a redshift of z=0.397 with a mass 160 trillion times the mass of the Sun inside 200 kpc (650 kly). Its mass out to a radius of 950 kpc (3,100 kly) was measured as 1.15 × 1015 solar masses. The system was discovered during the Massive Cluster Survey, MACS. This cluster causes gravitational lensing of distant galaxies producing multiple images. In 2015, the galaxy cluster was announced as gravitationally lensing the most distant galaxy (z = 12). Based on the distribution of the multiple image copies, scientists have been able to deduce and map the distribution of dark matter.

Haro 11 Galaxy in the constellation Sculptor

Haro 11 (H11) is a small galaxy at a distance of 300,000,000 light-years (redshift z=0.020598). It is situated in the southern constellation of Sculptor. Visually, it appears to be an irregular galaxy, as the ESO image to the right shows. H11 is named after Guillermo Haro, a Mexican astronomer who first included it in a study published in 1956 about blue galaxies. H11 is a starburst galaxy that has 'super star clusters' within it and is one of nine galaxies in the local universe known to emit Lyman continuum photons (LyC).

Alan Duffy (astronomer) Professional astronomer and science communicator (born 1983).

Alan R Duffy is a professional astronomer and science communicator. He was born in England, raised in Ireland, and is currently based in Australia. He is a Research Fellow and Associate Professor at the Centre for Astrophysics and Supercomputing at Swinburne University of Technology, and he is the Lead Scientist at the Royal Institution of Australia.

Red nuggets was the nickname given to rare, unusually small galaxies packed with large amounts of red stars that were originally observed by Hubble Space Telescope in 2005 in the young universe. They are ancient remnants of the first massive galaxies. The environments of red nuggets are usually consistent with the general elliptical galaxy population. Most red nuggets have merged with other galaxies, but some managed to stay unscathed.

Direct collapse black hole High mass black hole seeds

Direct collapse black holes are high-mass black hole seeds, putatively formed within the redshift range , when the Universe was about 100-250 million years old. Unlike seeds formed from the first population of stars (also known as Population III stars), direct collapse black hole seeds are formed by a direct, general relativistic instability. They are very massive, with a typical mass at formation of ~105 M. This category of black hole seeds was originally proposed theoretically to alleviate the challenge in building supermassive black holes already at redshift , as numerous observations to date have confirmed.


  1. 1 2 3 4 "Astronomers Pin Down Galaxy Collision Rate". HubbleSite. 27 October 2011. Retrieved 16 April 2012.
  2. van Albada, T.S. (1982). "[no title cited]". Monthly Notices of the Royal Astronomical Society . 201: 939.[ full citation needed ]
  3. "Evolution in slow motion". Space Telscope Science Institute. Retrieved 15 September 2015.
  4. Schweizer, F. (2005). de Grijs, R.; González-Delgado, R.M. (eds.). [no presentation title cited]. Starbursts: From 30 Doradus to Lyman Break Galaxies; Cambridge, UK; 6–10 September 2004. Astrophysics & Space Science Library. 329. Dordrecht, DE: Springer. p. 143.[ full citation needed ]
  5. Ostriker, Eve C.; Shetty, Rahul (2012). "Maximally star-forming galactic disks I. Starburst regulation via feedback-driven turbulence". The Astrophysical Journal. 731 (1): 41. arXiv: 1102.1446 . Bibcode:2011ApJ...731...41O. doi:10.1088/0004-637X/731/1/41. S2CID   2584335. 41.
  6. Brinchmann, J.; et al. (2004). "The physical properties of star-forming galaxies in the low-redshift Universe". Monthly Notices of the Royal Astronomical Society. 351 (4): 1151–1179. arXiv: astro-ph/0311060 . Bibcode:2004MNRAS.351.1151B. doi:10.1111/j.1365-2966.2004.07881.x. S2CID   12323108.
  7. Moster, Benjamin P.; et al. (2011). "The effects of a hot gaseous halo in galaxy major mergers". Monthly Notices of the Royal Astronomical Society. 415 (4): 3750–3770. arXiv: 1104.0246 . Bibcode:2011MNRAS.415.3750M. doi:10.1111/j.1365-2966.2011.18984.x. S2CID   119276663.
  8. Hirschmann, Michaela; et al. (2012). "Galaxy formation in semi-analytic models and cosmological hydrodynamic zoom simulations". Monthly Notices of the Royal Astronomical Society. 419 (4): 3200–3222. arXiv: 1104.1626 . Bibcode:2012MNRAS.419.3200H. doi:10.1111/j.1365-2966.2011.19961.x. S2CID   118710949.
  9. Chomiuk, Laura; Povich, Matthew S. (2011). "Toward a Unification of Star Formation Rate Determinations in the Milky Way and Other Galaxies". The Astronomical Journal. 142 (6): 197. arXiv: 1110.4105 . Bibcode:2011AJ....142..197C. doi:10.1088/0004-6256/142/6/197. S2CID   119298282. 197.
  10. "Galaxy merger library". 27 March 2010. Retrieved 27 March 2010.
  11. "Galaxies clash in four-way merger". BBC News . 6 August 2007. Retrieved 7 August 2007.
  12. 1 2 Lin, Lihwal; et al. (July 2008). "The Redshift Evolution of Wet, Dry, and Mixed Galaxy Mergers from Close Galaxy Pairs in the DEEP2 Galaxy Redshift Survey". The Astrophysical Journal. 681 (232): 232–243. arXiv: 0802.3004 . Bibcode:2008ApJ...681..232L. doi:10.1086/587928. S2CID   18628675.
  13. Forbes, Duncan A.; et al. (April 2007). "Damp Mergers: Recent Gaseous Mergers without Significant Globular Cluster Formation?". The Astrophysical Journal. 659 (1): 188–194. arXiv: astro-ph/0612415 . Bibcode:2007ApJ...659..188F. doi:10.1086/512033. S2CID   15213247.
  14. 1 2 3 Roukema, Boudewijn F.; Quinn, Peter J.; Peterson, Bruce A. (January 1993). "Spectral Evolution of Merging/Accreting Galaxies". Observational Cosmology. ASP Conference Series. 51. Astronomical Society of the Pacific. p. 298. Bibcode: 1993ASPC...51..298R .
  15. 1 2 Roukema, Boudewijn F.; Yoshii, Yuzuru (November 1993). "The Failure of Simple Merging Models to Save a Flat, Omega0=1 Universe". The Astrophysical Journal. IOP Publishing. 418: L1. Bibcode: 1993ApJ...418L...1R . doi:10.1086/187101.
  16. 1 2 Lacey, Cedric; Cole, Shaun (June 1993). "Merger rates in hierarchical models of galaxy formation". Monthly Notices of the Royal Astronomical Society. Oxford University Press. 262 (3): 627–649. Bibcode: 1993MNRAS.262..627L . doi: 10.1093/mnras/262.3.627 .
  17. 1 2 Roukema, Boudewijn F.; Quinn, Peter J.; Peterson, Bruce A.; Rocca-Volmerange, Brigitte (December 1997). "Merging History Trees of Dark Matter Haloes: a Tool for Exploring Galaxy Formation Models". Monthly Notices of the Royal Astronomical Society. 292 (4): 835–852. arXiv: astro-ph/9707294 . Bibcode: 1997MNRAS.292..835R . doi:10.1093/mnras/292.4.835. S2CID   15265628.
  18. Lacey, Cedric; Cole, Shaun (January 1993). "Merger Rates in Hierarchical Models of Galaxy Formation" (PDF). Observational Cosmology. ASP Conference Series. 51. Astronomical Society of the Pacific. p. 192. Bibcode: 1993ASPC...51..192L .
  19. Kauffmann, Guinevere; White, Simon D.M.; Guiderdoni, Bruno (September 1993). "Clustering of galaxies in a hierarchical universe - II. Evolution to high redshift". Monthly Notices of the Royal Astronomical Society. IOP Publishing. 264: 201. Bibcode: 1993MNRAS.264..201K . doi: 10.1093/mnras/264.1.201 .
  20. Kauffmann, Guinevere; Kolberg, Jörg M.; Diaferio, Antonaldo; White, Simon D.M. (August 1999). "Clustering of galaxies in a hierarchical universe - II. Evolution to high redshift". Monthly Notices of the Royal Astronomical Society. 307 (3): 529–536. arXiv: astro-ph/9809168 . Bibcode:1999MNRAS.307..529K. doi:10.1046/j.1365-8711.1999.02711.x. S2CID   17636817.
  21. Okamoto, Takashi; Nagashima, Masahiro (January 2001). "Morphology-Density Relation for Simulated Clusters of Galaxies in Cold Dark Matter-dominated Universes". The Astrophysical Journal. 547 (1): 109–116. arXiv: astro-ph/0004320 . Bibcode:2001ApJ...547..109O. doi:10.1086/318375. S2CID   6011298.
  22. "A glimpse of the future". Retrieved 16 October 2017.
  23. "Galactic glow worm". ESA/Hubble. Retrieved 27 March 2013.
  24. "Transforming Galaxies". Picture of the Week. ESA/Hubble. Retrieved 6 February 2012.
  25. "Ancient Galaxy Megamergers - ALMA and APEX discover massive conglomerations of forming galaxies in early Universe". Retrieved 26 April 2018.
  26. "Cosmic "flying V" of merging galaxies". ESA/Hubble Picture of the Week. Retrieved 12 February 2013.