Galaxy formation and evolution

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.

Commonly observed properties of galaxies

Hubble tuning fork diagram of galaxy morphology

Because of the inability to conduct experiments in outer space, the only way to “test” theories and models of galaxy evolution is to compare them with observations. Explanations for how galaxies formed and evolved must be able to predict the observed properties and types of galaxies.

Edwin Hubble created the first galaxy classification scheme known as the Hubble tuning-fork diagram. It partitioned galaxies into ellipticals, normal spirals, barred spirals (such as the Milky Way), and irregulars. These galaxy types exhibit the following properties which can be explained by current galaxy evolution theories:

  • Many of the properties of galaxies (including the galaxy color–magnitude diagram) indicate that there are fundamentally two types of galaxies. These groups divide into blue star-forming galaxies that are more like spiral types, and red non-star forming galaxies that are more like elliptical galaxies.
  • Spiral galaxies are quite thin, dense, and rotate relatively fast, while the stars in elliptical galaxies have randomly-oriented orbits.
  • The majority of giant galaxies contain a supermassive black hole in their centers, ranging in mass from millions to billions of times the mass of our Sun. The black hole mass is tied to the host galaxy bulge or spheroid mass.
  • Metallicity has a positive correlation with the absolute magnitude (luminosity) of a galaxy.

There is a common misconception that Hubble believed incorrectly that the tuning fork diagram described an evolutionary sequence for galaxies, from elliptical galaxies through lenticulars to spiral galaxies. This is not the case; instead, the tuning fork diagram shows an evolution from simple to complex with no temporal connotations intended.[1] Astronomers now believe that disk galaxies likely formed first, then evolved into elliptical galaxies through galaxy mergers.

Current models also predict that the majority of mass in galaxies is made up of dark matter, a substance which is not directly observable, and might not interact through any means except gravity. This observation arises because galaxies could not have formed as they have, or rotate as they are seen to, unless they contain far more mass than can be directly observed.

Formation of disk galaxies

The earliest stage in the evolution of galaxies is the formation. When a galaxy forms, it has a disk shape and is called a spiral galaxy due to spiral-like "arm" structures located on the disk. There are different theories on how these disk-like distributions of stars develop from a cloud of matter: however, at present, none of them exactly predicts the results of observation.

Top-down theories

Olin Eggen, Donald Lynden-Bell, and Allan Sandage[2] in 1962, proposed a theory that disk galaxies form through a monolithic collapse of a large gas cloud. The distribution of matter in the early universe was in clumps that consisted mostly of dark matter. These clumps interacted gravitationally, putting tidal torques on each other that acted to give them some angular momentum. As the baryonic matter cooled, it dissipated some energy and contracted toward the center. With angular momentum conserved, the matter near the center speeds up its rotation. Then, like a spinning ball of pizza dough, the matter forms into a tight disk. Once the disk cools, the gas is not gravitationally stable, so it cannot remain a singular homogeneous cloud. It breaks, and these smaller clouds of gas form stars. Since the dark matter does not dissipate as it only interacts gravitationally, it remains distributed outside the disk in what is known as the dark halo. Observations show that there are stars located outside the disk, which does not quite fit the "pizza dough" model. It was first proposed by Leonard Searle and Robert Zinn [3] that galaxies form by the coalescence of smaller progenitors. Known as a top-down formation scenario, this theory is quite simple yet no longer widely accepted.

Bottom-up theories

More recent theories include the clustering of dark matter halos in the bottom-up process. Instead of large gas clouds collapsing to form a galaxy in which the gas breaks up into smaller clouds, it is proposed that matter started out in these “smaller” clumps (mass on the order of globular clusters), and then many of these clumps merged to form galaxies,[4] which then were drawn by gravitation to form galaxy clusters. This still results in disk-like distributions of baryonic matter with dark matter forming the halo for all the same reasons as in the top-down theory. Models using this sort of process predict more small galaxies than large ones, which matches observations.

Astronomers do not currently know what process stops the contraction. In fact, theories of disk galaxy formation are not successful at producing the rotation speed and size of disk galaxies. It has been suggested that the radiation from bright newly formed stars, or from an active galactic nucleus can slow the contraction of a forming disk. It has also been suggested that the dark matter halo can pull the galaxy, thus stopping disk contraction.[5]

The Lambda-CDM model is a cosmological model that explains the formation of the universe after the Big Bang. It is a relatively simple model that predicts many properties observed in the universe, including the relative frequency of different galaxy types; however, it underestimates the number of thin disk galaxies in the universe.[6] The reason is that these galaxy formation models predict a large number of mergers. If disk galaxies merge with another galaxy of comparable mass (at least 15 percent of its mass) the merger will likely destroy, or at a minimum greatly disrupt the disk, and the resulting galaxy is not expected to be a disk galaxy (see next section). While this remains an unsolved problem for astronomers, it does not necessarily mean that the Lambda-CDM model is completely wrong, but rather that it requires further refinement to accurately reproduce the population of galaxies in the universe.

Galaxy mergers and the formation of elliptical galaxies

Artist image of a firestorm of star birth deep inside core of young, growing elliptical galaxy.
NGC 4676 (Mice Galaxies) is an example of a present merger.
Antennae galaxies xl
Antennae Galaxies are a pair of colliding galaxies - the bright, blue knots are young stars that have recently ignited as a result of the merger.
Abell S740, cropped to ESO 325-G004
ESO 325-G004, a typical elliptical galaxy.

Elliptical galaxies (such as IC 1101) are among some of the largest known thus far. Their stars are on orbits that are randomly oriented within the galaxy (i.e. they are not rotating like disk galaxies). A distinguishing feature of elliptical galaxies is that the velocity of the stars does not necessarily contribute to flattening of the galaxy, such as in spiral galaxies.[7] Elliptical galaxies have central supermassive black holes, and the masses of these black holes correlate with the galaxy’s mass.

Elliptical galaxies have two main stages of evolution. The first is due to the supermassive black hole growing by accreting cooling gas. The second stage is marked by the black hole stabilizing by suppressing gas cooling, thus leaving the elliptical galaxy in a stable state.[8] The mass of the black hole is also correlated to a property called sigma which is the dispersion of the velocities of stars in their orbits. This relationship, known as the M-sigma relation, was discovered in 2000.[9] Elliptical galaxies mostly lack disks, although some bulges of disk galaxies resemble elliptical galaxies. Elliptical galaxies are more likely found in crowded regions of the universe (such as galaxy clusters).

Astronomers now see elliptical galaxies as some of the most evolved systems in the universe. It is widely accepted that the main driving force for the evolution of elliptical galaxies is mergers of smaller galaxies. Many galaxies in the universe are gravitationally bound to other galaxies, which means that they will never escape their mutual pull. If the galaxies are of similar size, the resultant galaxy will appear similar to neither of the progenitors,[10] but will instead be elliptical. There are many types of galaxy mergers, which do not necessarily result in elliptical galaxies, but result in a structural change. For example, a minor merger event is thought to be occurring between the Milky Way and the Magellanic Clouds.

Mergers between such large galaxies are regarded as violent, and the frictional interaction of the gas between the two galaxies can cause gravitational shock waves, which are capable of forming new stars in the new elliptical galaxy.[11] By sequencing several images of different galactic collisions, one can observe the timeline of two spiral galaxies merging into a single elliptical galaxy.[12]

In the Local Group, the Milky Way and the Andromeda Galaxy are gravitationally bound, and currently approaching each other at high speed. Simulations show that the Milky Way and Andromeda are on a collision course, and are expected to collide in less than five billion years. During this collision, it is expected that the Sun and the rest of the Solar System will be ejected from its current path around the Milky Way. The remnant could be a giant elliptical galaxy.[13]

Galaxy quenching

Star formation in what are now "dead" galaxies sputtered out billions of years ago.[14]

One observation (see above) that must be explained by a successful theory of galaxy evolution is the existence of two different populations of galaxies on the galaxy color-magnitude diagram. Most galaxies tend to fall into two separate locations on this diagram: a "red sequence" and a "blue cloud". Red sequence galaxies are generally non-star-forming elliptical galaxies with little gas and dust, while blue cloud galaxies tend to be dusty star-forming spiral galaxies.[15][16]

As described in previous sections, galaxies tend to evolve from spiral to elliptical structure via mergers. However, the current rate of galaxy mergers does not explain how all galaxies move from the "blue cloud" to the "red sequence". It also does not explain how star formation ceases in galaxies. Theories of galaxy evolution must therefore be able to explain how star formation turns off in galaxies. This phenomenon is called galaxy "quenching".[17]

Stars form out of cold gas (see also the Kennicutt-Schmidt law), so a galaxy is quenched when it has no more cold gas. However, it is thought that quenching occurs relatively quickly (within 1 billion years), which is much shorter than the time it would take for a galaxy to simply use up its reservoir of cold gas.[18][19] Galaxy evolution models explain this by hypothesizing other physical mechanisms that remove or shut off the supply of cold gas in a galaxy. These mechanisms can be broadly classified into two categories: (1) preventive feedback mechanisms that stop cold gas from entering a galaxy or stop it from producing stars, and (2) ejective feedback mechanisms that remove gas so that it cannot form stars.[20]

One theorized preventive mechanism called “strangulation” keeps cold gas from entering the galaxy. Strangulation is likely the main mechanism for quenching star formation in nearby low-mass galaxies.[21] The exact physical explanation for strangulation is still unknown, but it may have to do with a galaxy’s interactions with other galaxies. As a galaxy falls into a galaxy cluster, gravitational interactions with other galaxies can strangle it by preventing it from accreting more gas.[22] For galaxies with massive dark matter halos, another preventive mechanism called “virial shock heating” may also prevent gas from becoming cool enough to form stars.[19]

Ejective processes, which expel cold gas from galaxies, may explain how more massive galaxies are quenched.[23] One ejective mechanism is caused by supermassive black holes found in the centers of galaxies. Simulations have shown that gas accreting onto supermassive black holes in galactic centers produces high-energy jets; the released energy can expel enough cold gas to quench star formation.[24]

Our own Milky Way and the nearby Andromeda Galaxy currently appear to be undergoing the quenching transition from star-forming blue galaxies to passive red galaxies.[25]


A young elliptical

NGC 3610 shows some structure in the form of a bright disc, implying that it formed only a short time ago.[26]


NGC 891, a very thin disk galaxy

M101 hires STScI-PRC2006-10a

An image of Messier 101, a prototypical spiral galaxy seen face-on

Warped galaxy

A spiral galaxy, ESO 510-G13, was warped as a result of colliding with another galaxy. After the other galaxy is completely absorbed, the distortion will disappear. The process typically takes millions if not billions of years.

See also

Further reading

  • Mo, Houjun; van den Bosch, Frank; White, Simon (June 2010), Galaxy Formation and Evolution (1 ed.), Cambridge University Press, ISBN 978-0521857932


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  17. ^ Faber, S. M.; Willmer, C. N. A.; Wolf, C.; Koo, D. C.; Weiner, B. J.; Newman, J. A.; Im, M.; Coil, A. L.; C. Conroy (2007-01-01). "Galaxy Luminosity Functions to z 1 from DEEP2 and COMBO-17: Implications for Red Galaxy Formation". The Astrophysical Journal. 665 (1): 265–294. arXiv:astro-ph/0506044. Bibcode:2007ApJ...665..265F. doi:10.1086/519294. ISSN 0004-637X.
  18. ^ Blanton, Michael R. (2006-01-01). "Galaxies in SDSS and DEEP2: A Quiet Life on the Blue Sequence?". The Astrophysical Journal. 648 (1): 268–280. arXiv:astro-ph/0512127. Bibcode:2006ApJ...648..268B. doi:10.1086/505628. ISSN 0004-637X.
  19. ^ a b Gabor, J. M.; Davé, R.; Finlator, K.; Oppenheimer, B. D. (2010-09-11). "How is star formation quenched in massive galaxies?". Monthly Notices of the Royal Astronomical Society. 407 (2): 749–771. arXiv:1001.1734. Bibcode:2010MNRAS.407..749G. doi:10.1111/j.1365-2966.2010.16961.x. ISSN 0035-8711.
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External links


Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space". Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. Emissions from these objects are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists apply concepts and methods from many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

In practice, modern astronomical research often involves a substantial amount of work in the realms of theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter, dark energy, and black holes; whether or not time travel is possible, wormholes can form, or the multiverse exists; and the origin and ultimate fate of the universe. Topics also studied by theoretical astrophysicists include Solar System formation and evolution; stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics.


Bottom-up may refer to:

Bottom-up analysis, a fundamental analysis technique in accounting and finance

Bottom-up parsing, a computer science strategy

Bottom-up processing, in Pattern recognition (psychology)

Bottom-up theories of galaxy formation and evolution

Bottom-up tree automaton, in data structures

Bottom-up integration testing, in software testing

Top-down and bottom-up design, strategies of information processing and knowledge ordering

Bottom-up proteomics, a laboratory technique involving proteins

Bottom Up Records, a record label founded by Shyheim

Bottom-up approach of the Holocaust, a viewpoint on the causes of the Holocaust

Dark matter halo

A dark matter halo is a theoretical component of a galaxy that envelops the galactic disc and extends well beyond the edge of the visible galaxy. The halo's mass dominates the total mass. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through their effects on the motions of stars and gas in galaxies. Dark matter halos play a key role in current models of galaxy formation and evolution. The dark matter halo is not fully explained by the presence of massive compact halo objects (MACHOs).

ESO 510-G13

ESO 510-G13 is a spiral galaxy approximately 150 million light-years away in the constellation Hydra. The equatorial dust cloud is heavily warped; this may indicate that ESO 510-G13 has interacted with another galaxy. If this is the case, it would provide an excellent illustration of the distortion caused by interacting galaxies, discussed in the article Galaxy formation and evolution under the Spiral galaxy heading.

This galaxy was examined by the Hubble Space Telescope in 2001.

Extragalactic astronomy

Extragalactic astronomy is the branch of astronomy concerned with objects outside the Milky Way galaxy. In other words, it is the study of all astronomical objects which are not covered by galactic astronomy.

As instrumentation has improved, distant objects can now be examined in more detail. It is therefore useful to sub-divide this branch into Near-Extragalactic Astronomy and Far-Extragalactic Astronomy. The former deals with objects such as the galaxies of the Local Group, which are close enough to allow very detailed analyses of their contents (e.g. supernova remnants, stellar associations).

Some topics include:

Galaxy groups

Galaxy clusters, Superclusters

Galaxy filaments

Active galactic nuclei, Quasars

Radio galaxies


Intergalactic stars

Intergalactic dust

the observable universe

Fibre multi-object spectrograph

Fibre multi-object spectrograph (FMOS) is facility instrument for the Subaru telescope on Mauna Kea in Hawaii. The instrument consists of a complex fibre-optic positioning system mounted at the prime focus of the telescope. Fibres are then fed to a pair of large spectrographs, each weighing nearly 3000 kg. The instrument will be used to look at the light from up to 400 stars or galaxies simultaneously over a field of view of 30 arcminutes (about the size of the full moon on the sky. The instrument will be used for a number of key programmes, including galaxy formation and evolution and dark energy via a measurement of the rate at which the universe is expanding.

It is currently being built by a consortium of institutes led by Kyoto University and Oxford University with parts also being manufactured by the Rutherford Appleton Laboratory, Durham University and the Anglo-Australian Observatory. The instrument is scheduled for engineering first-light in late 2008.

The spectrographs use a technique called OH-suppression to increase the sensitivity of the observations: The incoming light from the fibres is dispersed to a relatively high resolution and this spectrum forms an image on a pair of spherical mirrors which have been etched at the positions corresponding to the bright OH-lines. This spectrum is then re-imaged through a second diffraction grating to allow the full spectrum (without the OH lines) to be imaged onto a single infrared detector.

Galactic astronomy

Galactic astronomy is the study of the Milky Way galaxy and all its contents. This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies.

Galactic astronomy should not be confused with galaxy formation and evolution, which is the general study of galaxies, their formation, structure, components, dynamics, interactions, and the range of forms they take.

The Milky Way galaxy, where the Solar System belongs, is in many ways the best studied galaxy, although important parts of it are obscured from view in visible wavelengths by regions of cosmic dust. The development of radio astronomy, infrared astronomy and submillimetre astronomy in the 20th Century allowed the gas and dust of the Milky Way to be mapped for the first time.

Galaxy And Mass Assembly survey

The Galaxy And Mass Assembly (GAMA) survey is a project to exploit the latest generation of ground-based wide-field survey facilities to study cosmology and galaxy formation and evolution. GAMA will bring together data from a number of world class instruments:

The Anglo-Australian Telescope (AAT),

The VLT Survey Telescope (VST)

The Visible and Infrared Survey Telescope for Astronomy (VISTA)

The Australian Square Kilometre Array Pathfinder (ASKAP)

The Herschel Space Observatory

The Galaxy Evolution Explorer (GALEX)Data from these instruments will be used to construct a state-of-the-art multi-wavelength database of ~375,000 galaxies in the local Universe over a 360 deg2 region of sky,

based on a spectroscopic redshift survey on the AAT's AAOmega spectrograph.

The main objective of GAMA is to study structure on scales of 1 kpc to 1 Mpc. This includes galaxy clusters, groups, mergers and coarse measurements of galaxy structure (i.e., bulges and discs). It is on these scales where baryons play a critical role in the galaxy formation and subsequent evolutionary processes and where our understanding of structure in the Universe breaks down.

GAMA's primary goal is to test the CDM paradigm of structure formation. In particular, the key scientific objectives are:

A measurement of the dark matter halo mass function of groups and clusters using group velocity dispersion measurements.

A comprehensive determination of the galaxy stellar mass function to Magellanic Cloud masses to constrain baryonic feedback processes.

A direct measurement of the recent galaxy merger rates as a function of mass, mass ratio, local environment and galaxy type.In August 2012 GAMA received worldwide attention for its announcement of a galaxy system very similar to our own Milky-Way Magellanic Cloud system, centred on GAMA202627.

Hyron Spinrad

Hyron Spinrad (February 17, 1934 – December 7, 2015) was an American astronomer. His research has ranged from the study of planet atmospheres to the evolution of galaxies. From 2010 until his death in late 2015 he was an emeritus professor of astronomy at the University of California, Berkeley.

Illustris project

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 followup to the project, IllustrisTNG, was presented in 2017.

Instituto de Astrofísica de Andalucía

The Institute of Astrophysics of Andalusia (Spanish: Instituto de Astrofísica de Andalucía, IAA-CSIC) is a research institute funded by the High Council of Scientific Research of the Spanish government Consejo Superior de Investigaciones Científicas (CSIC), and is located in Granada, Andalusia, Spain. IAA activities are related to research in the field of astrophysics, and instrument development both for ground-based telescopes and for space missions. Scientific research at the Institute covers the solar system, star formation, stellar structure and evolution, galaxy formation and evolution and cosmology. The IAA was created as a CSIC research institute in July 1975. Presently, the IAA operates the Sierra Nevada Observatory, and (jointly with the also the Max-Planck Institute of Heidelberg) the Calar Alto Observatory.

The Instituto de Astrofísica de Andalucía is divided in the following departments, each with an (incomplete) outline of research avenues and groups:

Department of Extragalactic Astronomy

Violent Stellar Formation Group

AMIGA Group (Analysis of the interstellar Medium of Isolated Galaxies)

Department of Stellar Physics

Department of Radio Astronomy and Galactic Structure

Stellar Systems Group

Department of Solar SystemThe technological needs of IAA's research groups are fulfilled by the Instrumental and Technological Developments Unit.

Jane C. Charlton

Jane C. Charlton (born June 5, 1965) is a professor of astronomy and astrophysics at Pennsylvania State University where she is a specialist in galaxy formation and evolution. She also has a daughter named Thomasin.

Julianne Dalcanton

Julianne Dalcanton is an American astronomer.

She is Professor of Astronomy, University of Washington, and researcher for Sloan Digital Sky Survey. Her main work is on the area of galaxy formation and evolution. She led the ACS Nearby Galaxy Survey Treasury (ANGST) and is leading the Panchromatic Hubble Andromeda Treasury (PHAT) programs on the Hubble Space Telescope.She became known worldwide by her discovery of the comet C/1999 F2 Dalcanton. She is also a contributor to the physics blog Cosmic Variance.

In 2018, Professor Dalcanton was awarded the Beatrice M. Tinsley Prize by the American Astronomical Society in recognition of her work in Astronomy and “contributions that are of an exceptionally creative or innovative character and that have played a seminal role in furthering our understanding of the universe.”

Large UV Optical Infrared Surveyor

The Large UV Optical Infrared Surveyor (LUVOIR) is a multi-wavelength space observatory concept being developed by NASA under the leadership of a Science and Technology Definition Team drawn from the scientific and technical community.

LUVOIR is one of four large astrophysics space mission concepts being studied in preparation for the National Academy of Sciences 2020 Astronomy and Astrophysics Decadal Survey.While LUVOIR is a concept for a general-purpose observatory, it has the key science goal of characterizing a wide range of exoplanets, including those that might be habitable. An additional goal is to enable a broad range of astrophysics, from the reionization epoch, through galaxy formation and evolution, to star and planet formation.

Powerful imaging and spectroscopy observations of Solar System bodies would also be possible.

LUVOIR would be a Large Strategic Science Mission and will be considered for a development start sometime after 2020. The LUVOIR Team has produced designs for two variants of LUVOIR: one with a 15 m diameter telescope mirror (LUVOIR-A) and one with an 8 m diameter mirror (LUVOIR-B).

LUVOIR can observe ultraviolet, visible, and near-infrared wavelengths of light and has often been described as a "super-duper Hubble Space Telescope".

Rachel Somerville

Rachel S. Somerville is an American astronomer and holds the George A. and Margaret M. Downsbrough Chair in Astrophysics at Rutgers University. She is known for theoretical research into galaxy formation and evolution. She was awarded the 2013 Dannie Heineman Prize for Astrophysics

“For providing fundamental insights into galaxy formation and evolution using semi-analytic modeling, simulations and observations.”

Radiation pressure

Radiation pressure is the pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength which is absorbed, reflected, or otherwise emitted (e.g. black-body radiation) by matter on any scale (from macroscopic objects to dust particles to gas molecules).The forces generated by radiation pressure are generally too small to be noticed under everyday circumstances; however, they are important in some physical processes. This particularly includes objects in outer space where it is usually the main force acting on objects besides gravity, and where the net effect of a tiny force may have a large cumulative effect over long periods of time. For example, had the effects of the sun's radiation pressure on the spacecraft of the Viking program been ignored, the spacecraft would have missed Mars orbit by about 15,000 km (9,300 mi). Radiation pressure from starlight is crucial in a number of astrophysical processes as well. The significance of radiation pressure increases rapidly at extremely high temperatures, and can sometimes dwarf the usual gas pressure, for instance in stellar interiors and thermonuclear weapons.

Radiation pressure can equally well be accounted for by considering the momentum of a classical electromagnetic field or in terms of the momenta of photons, particles of light. The interaction of electromagnetic waves or photons with matter may involve an exchange of momentum. Due to the law of conservation of momentum, any change in the total momentum of the waves or photons must involve an equal and opposite change in the momentum of the matter it interacted with (Newton's third law of motion), as is illustrated in the accompanying figure for the case of light being perfectly reflected by a surface. This transfer of momentum is the general explanation for what we term radiation pressure.

Richard Hunstead

Professor Richard (Dick) Waller Hunstead is a member and former head of the Sydney Institute for Astronomy (SIfA) and the Director of the Molonglo Observatory Synthesis Telescope (MOST), within the University of Sydney. Dick is internationally recognised for his work in the field of quasars and radio galaxies. In 1995, he was awarded the Robert Ellery Lectureship of the Astronomical Society of Australia in recognition of his outstanding contributions in astronomy. One of 33 Australian Science Citation Laureates (of which only nine are astronomers), he is the author of multiple high impact papers which are frequently cited by other scientists around the world. The minor planet 171429 Hunstead is named in his honour.His most notable achievements to date include the discovery of the variability of radio sources at low frequencies, resulting in a large number of related research projects, conferences and workshops internationally, and his research into the redshift evolution of the Lyman alpha absorption forest. Dick's work on the Lyman alpha forest resolved much of the confusion surrounding the nature of the cloud population responsible for these absorption lines, by confirming that their comoving number density does indeed evolve with redshift. He has also contributed to our understanding of the metallicity, dust content and star formation rate in high redshift galaxies from his comprehensive study of damped Lyman alpha systems. This has subsequently motivated interest in galaxy formation and evolution. More recently, he was involved in a study that showed radio loud quasars have distinct spectroscopic signatures that depend on their orientation and size. He is currently leading an international team to locate and study the first massive galaxies formed in the Universe.

Sofia Feltzing

Johanna Sofia Nikolina Feltzing (born June 26, 1965 in Högsbo, Gothenburg, Sweden) is a Swedish astronomer and Professor of Astronomy at Lund University since 2011. She completed her PhD at Uppsala University in 1996, publishing a dissertation about the chemical evolution of the Milky Way. Feltzing was the first woman to complete a PhD in astronomy at Uppsala, and the tenth in Sweden. She was a postdoctoral researcher at Royal Greenwich Observatory and the Institute of Astronomy, Cambridge at Cambridge University from 1996 to 1998. In 1998 she moved to Lund Observatory.

Sofia Feltzing's research primarily concerns understanding galaxy formation and evolution by studying the stars and gas of the Milky Way. She has also studied dwarf spheroidal galaxies and globular star clusters.

In 2013, Sofia Feltzing was awarded the Strömer-Ferrnerska prize of 20,000SEK by the Royal Swedish Academy of Sciences for "her spectroscopic and photometric studies which have been crucial contributions to a deeper understanding of the development of the Milky Way and its surrounding galaxies." In 2015, Feltzing was elected to the Royal Swedish Academy of Sciences.

Yūko Kakazu

Yūko Kakazu (嘉数悠子 Kakazu Yūko) is a Japanese astronomer. Her specialty is galaxy formation and evolution. She is the Public Outreach Specialist for the Subaru Telescope at the NAOJ Hawaii Observatory.

Active nuclei
Energetic galaxies
Low activity
See also

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