Astronomy

How is a Galaxy formed?

How is a Galaxy formed?



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How is a galaxy formed? I know that the center of our galaxy is considered to be Sagittarius A* and it's surrounded by a lot of stars (also a lot of neutron stars).

But what makes this happen? I mean, why are all those stars in that right formation, like other galaxies, rather than just being random clouds of stars in the universe?


Well this is not a question that can be answered in a few sentences!

In short


We still do not know, how exactly the galaxies formed and have the shapes they possess. The large-scale structure (LSS) in the Universe as we see them today are a consequence of tiny primordial density fluctuations that arose right after the Big Bang. The reason for these tiny density fluctuations is believed to be quantum in nature.

So, these tiny fluctuations in the early past lead to agglomeration of gas and dust clouds, leading to certain areas becoming denser. These denser areas slowed down the expansion of the Universe, allowing the gas to accumalate into small protogalactic clouds. Gravity in these clouds casued the gas and dust to collapse, and in turn form stars. These stars burned quickly and became globular clusters (while gravity was still collapsing the dust and gas).

Also, according to the $Lambda$CDM model, the structures form in a "bottom-up" fashion, i.e. small structures forming first (stars and galaxies) followed by large structures (galaxy clusters). This is exactly what we are observing today thanks to surveys that are probing high redshift ranges.


You are closer to the answer than you think with this question. That revolves around the connection between the SMBH central object and the mass of the Galaxy. For instance; all stars rotate around this thing, the Galactic Plane is about its 'equator'(if it ever associates to jetting - thats at right angles to the Galactic plane), a connection in mass and also the density of stars increases, across the galaxy, up to this object. That connection is not really explained by merger, which is unlikely to result in stars orbiting v close to the centre. Matter is 'burping' or spewing from this object in outflow that forms stars locally and that is their origin. Just like in Dwarf Galaxies and Globular Clusters, which seem to have a correlation in general characteristics and morphology, show signs of star forming in the centre with, sometimes, large numbers of young stars in the middle.


How is a Galaxy formed? - Astronomy

I'm a year 8 student and have to find out about the lifecycle of a galaxy. Is there actually a beginning and an end to the life of a galaxy or does it just keep going and joining up with other galaxies?

You ask a good question. Though details of galaxy formation and evolution are still widely debated, the general theory of how they form is now generally accepted.

Small regions in the young Universe collapse under their own gravity, in such a way that "little" structures like galaxies form before "big" structures like clusters of galaxies. This can be explained physically if you assume that the dark matter in the Universe is non-relativistic. People call dark particles like this "cold" thus, many astronomers call this theory of galaxy evolution "the cold dark matter paradigm".

In the cold dark matter paradigm, "halos" of dark matter first become gravitationally bound and "break away" from the general expansion of the Universe that is, instead of expanding with the Universe, the Universe expands around the halos. Each halo gathers more material and eventually end up as a galaxy at some point. So in a sense, these primordial halos represent the "birth" of a galaxy. Once a halo is formed, normal (or "baryonic") matter falls towards its centre, because of the gravitational pull that the halo exerts. During this infall the baryons must conserve angular momentum, and so they begin to spin faster around the centre of the halo as they fall deeper into it. This process yields a disk of star-forming, normal matter inside a halo or dark matter, as we seem to observe in spiral galaxies today.

This "passive" evolution described above is complicated by galaxy collisions, however: numerical simulations have recently revealed that collisions influence the appearance (or "morphology") of an object. As a general rule, the morphology of a galaxy seems to be governed by the types of collisions it has undergone throughout its evolution: major mergers (collisions between two galaxies of comparable mass) produce elliptical galaxies, and minor mergers (collision between a galaxies of different masses) don't disturb the galaxy's evolution towards a thin disk. In other words, elliptical galaxies have recently undergone major mergers, very thin disk galaxies have not collided with anything much in a long time, and other galaxy types are somewhere in between. This evolution via merging is not supported by all observations of galaxies of different types, but the observations do seem to be broadly consistent with the idea. A galaxy's "life" therefore consists of a sequence of star forming events and collisions that it might undergo, depending on its environment and on the properties of the baryons which formed the disk in the first place. Defining the "end" of a galaxy's life is a bit more complicated.

Suppose that a galaxy doesn't collide with any others after a given amount of time: its evolution is then governed by star formation in the disk. Though the latter is cyclic (gas makes stars, stars die and return most of their gas to the interstellar medium), a fraction of the available gas becomes locked up in low-mass stellar remnants in each star-forming cycle, so that eventually no gas will be available to form new stars. As the stellar remnants dim, so will the galaxy, until it no longer emits much light at all. However, the dark matter halo of the galaxy is still ever-present. So if you define galaxy birth by the formation of a halo, then an isolated galaxy never "dies" in the sense that the halo remains even after star formation has ceased.

In reality, of course, we know that most galaxies reside in groups and clusters and hence undergo many collisions. When two galaxies collide, their dark matter halos are believed to coalesce into a single, larger one. One can then look at a minor merger as the "death" of the small galaxy (and the growth of the larger one), and a major merger as the "death" of two galaxies followed by their "reincarnation" as one larger one.

Given sufficient time, will all galaxies collide with each other, and hence merge into one big halo? This depends, actually, on the rate at which the Universe expands. We think today that universal expansion is accelerating with time, in such a way that distinct groups and clusters of galaxies will never merge with one another. So in the future, the members of a given group or cluster may coalesce into a single, monstrous "super-galaxy", but the super-galaxies themselves will not collide with each other. The halos of these objects would then last forever (assuming that the dark matter does not interact with anything at all), giving them ultimate immortality over everything else in the Universe.

This page was last updated on June 27, 2015

  • Kids/Students
  • Stars
  • Galaxies
  • The Universe
  • Dark Matter
  • Expansion
  • Angular Momentum
  • Formation
  • Collision
  • Spiral Galaxies
  • Halo
  • Disk
  • Mergers
  • Spin
  • Elliptical Galaxies
  • Evolution
  • Baryonic Matter
  • Galaxy Clusters

About the Author

Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.


Gravity paves the way

To begin with, thanks go out to gravity.

In the Universe’s youth, its gas was almost completely evenly distributed. But not entirely!

Ever so small inhomogeneities – the origin of which can be traced back to the quantum mechanical uncertainty principle – implied that some regions were just a wee bit denser than other regions, and thus had just a little more gravity to pull the surrounding matter.

These small inhomogeneities can actually be observed in the earliest light we can see – the so-called cosmic microwave background radiation.

When this light was emitted, less than 400,000 years after the Big Bang, the dense regions were only about one hundred thousandth denser than the dilute regions (see here, here and here).

By attracting more matter, the clumps gradually grew in an earliest, ‘primordial’ collapse. But in the beginning, the expansion of the Universe was tremendously high, and a race between expansion and collapse began.

Dark matter to the rescue! If there had only been ‘normal’ matter, the clumps would not have been able to grow sufficiently before expansion had pulled matter too far apart.

And in that case, neither galaxies, nor stars, planets, or us would ever have formed.


Portrait of young galaxy throws theory of galaxy formation on its head

Gas motion in the distant galaxy ALESS 073.1: gas in blue is moving towards us while gas in red is moving away from us, indicating a rotating disk. Credit: Federico Lelli (2021)

Scientists have challenged our current understanding of how galaxies form by unveiling pictures of a young galaxy in the early life of the Universe which appears surprisingly mature.

The galaxy, dubbed ALESS 073.1, appears to have all of the features expected of a much more mature galaxy and has led the team of scientists to question how it grew so fast.

The new research has been published today in Science.

Galaxies come in a variety of shapes, sizes and colours, and are made up of different components such as rotating disks, spiral arms, and "bulges".

A major goal of present-day astronomy is understanding why different galaxies look the way they are today and when their different components formed.

The team, led by scientists at Cardiff University, used the Atacama Large Millimeter/submillimeter Array (ALMA) telescope as a 'time machine' to peer into the remote past, revealing how ALESS 073.1 looked just 1.2 billion years after the Big Bang.

Because the light emitted from the galaxy took billions of years to reach our telescopes on Earth, the team were able to explore how the galaxy looked during its infancy and determine how it was initially formed.

A picture of ALESS 073.1 just 1.2 billion years after the Big Bang. Credit: Cardiff University

The result was one of the sharpest, direct images of a primordial galaxy ever produced which allowed the team to undertake a detailed study of its internal structure.

"We discovered that a massive bulge, a regular rotating disk, and possibly spiral arms were already in place in this galaxy when the Universe was just 10% of its current age," said lead author of the study Dr. Federico Lelli, who undertook the work at Cardiff University's School of Physics and Astronomy.

"In other words, this galaxy looks like a grown adult, but it should be just a little child."

Co-author of the study Dr. Timothy Davis, from the School of Physics and Astronomy, said: "This spectacular discovery challenges our current understanding of how galaxies form because we believed these features only arose in "mature" galaxies, not in young ones."

One key feature of a galaxy is the presence of a so-called bulge—a tightly packed group of stars usually situated within the centre of the galaxy.

It was believed that massive bulges formed slowly by the merger of smaller galaxies or by specific processes that occurred within the galaxy itself however, the kinematic properties of ALESS 073.1 have revealed that the formation of massive bulges can occur extremely fast—around half of the stars in the galaxy were shown to be in a bulge.

Similarly, some mature galaxies, like our own Milky Way, have been known to have spiral arms extending from their central parts, giving them a distinctive spiral shape.

Similar features were also unexpectedly spotted in ALESS 073.1, much to the team's amazement, as early galaxies are generally thought to be chaotic and turbulent rather than having regular, well-organized structures like spiral arms.

"A galaxy like ALESS 073.1 just defies our understanding of galaxy formation," concluded Dr. Lelli.


Collision Victims and the Multiple Merger Model

In past decades, astronomers have learned that the evolution of the Galaxy has not been quite as peaceful as this monolithic collapse model suggests. In 1994, astronomers discovered a small new galaxy in the direction of the constellation of Sagittarius. The Sagittarius dwarf galaxy is currently about 70,000 light-years away from Earth and 50,000 light-years from the center of the Galaxy. It is the closest galaxy known (Figure 2). It is very elongated, and its shape indicates that it is being torn apart by our Galaxy’s gravitational tides—just as Comet Shoemaker-Levy 9 was torn apart when it passed too close to Jupiter in 1992.

The Sagittarius galaxy is much smaller than the Milky Way, with only about 150,000 stars, all of which seem destined to end up in the bulge and halo of our own Galaxy. But don’t sound the funeral bells for the little galaxy quite yet the ingestion of the Sagittarius dwarf will take another 100 million years or so, and the stars themselves will survive.

Figure 2. Sagittarius Dwarf: In 1994, British astronomers discovered a galaxy in the constellation of Sagittarius, located only about 50,000 light-years from the center of the Milky Way and falling into our Galaxy. This image covers a region approximately 70° × 50° and combines a black-and-white view of the disk of our Galaxy with a red contour map showing the brightness of the dwarf galaxy. The dwarf galaxy lies on the other side of the galactic center from us. The white stars in the red region mark the locations of several globular clusters contained within the Sagittarius dwarf galaxy. The cross marks the galactic center. The horizontal line corresponds to the galactic plane. The blue outline on either side of the galactic plane corresponds to the infrared image in The Architecture of the Galaxy. The boxes mark regions where detailed studies of individual stars led to the discovery of this galaxy. (credit: modification of work by R. Ibata (UBC), R. Wyse (JHU), R. Sword (IoA))

Since that discovery, evidence has been found for many more close encounters between our Galaxy and other neighbor galaxies. When a small galaxy ventures too close, the force of gravity exerted by our Galaxy tugs harder on the near side than on the far side. The net effect is that the stars that originally belonged to the small galaxy are spread out into a long stream that orbits through the halo of the Milky Way (Figure 3).

Figure 3. Streams in the Galactic Halo: When a small galaxy is swallowed by the Milky Way, its member stars are stripped away and form streams of stars in the galactic halo. This image is based on calculations of what some of these tidal streams might look like if the Milky Way swallowed 50 dwarf galaxies over the past 10 billion years. (credit: modification of work by NASA/JPL-Caltech/R. Hurt (SSC/Caltech))

Such a tidal stream can maintain its identity for billions of years. To date, astronomers have now identified streams originating from 12 small galaxies that ventured too close to the much larger Milky Way. Six more streams are associated with globular clusters. It has been suggested that large globular clusters, like Omega Centauri, are actually dense nuclei of cannibalized dwarf galaxies. The globular cluster M54 is now thought to be the nucleus of the Sagittarius dwarf we discussed earlier, which is currently merging with the Milky Way (Figure 4). The stars in the outer regions of such galaxies are stripped off by the gravitational pull of the Milky Way, but the central dense regions may survive.

Figure 4. Globular Cluster M54: This beautiful Hubble Space Telescope image shows the globular cluster that is now believed to be the nucleus of the Sagittarius Dwarf Galaxy. (credit: ESA/Hubble & NASA)

Calculations indicate that the Galaxy’s thick disk may be a product of one or more such collisions with other galaxies. Accretion of a satellite galaxy would stir up the orbits of the stars and gas clouds originally in the thin disk and cause them to move higher above and below the mid-plane of the Galaxy. Meanwhile, the Galaxy’s stars would add to the fluffed-up mix. If such a collision happened about 10 billion years ago, then any gas in the two galaxies that had not yet formed into stars would have had plenty of time to settle back down into the thin disk. The gas could then have begun forming subsequent generations of population I stars. This timing is also consistent with the typical ages of stars in the thick disk.

The Milky Way has more collisions in store. An example is the Canis Major dwarf galaxy, which has a mass of about 1% of the mass of the Milky Way. Already long tidal tails have been stripped from this galaxy, which have wrapped themselves around the Milky Way three times. Several of the globular clusters found in the Milky Way may also have come from the Canis Major dwarf, which is expected to merge gradually with the Milky Way over about the next billion years.

In about 3 billion years, the Milky Way itself will be swallowed up, since it and the Andromeda galaxy are on a collision course. Our computer models show that after a complex interaction, the two will merge to form a larger, more rounded galaxy (Figure 5).

Figure 5. The Milky Way and Andromeda becoming visibly distorted as Andromeda gets closer to us. In panel 3, at upper right, the sky is ablaze with star forming regions and a riot of dust clouds and star clusters. In panel 4, at lower left, the galaxies further lose their spiral shapes, but dust lanes and star formation persists. By panel 5, at lower center, the two galactic nuclei fill the sky. Finally, in panel 6 at lower right, the nuclei have merged into a huge elliptical mass of stars.

We are thus coming to realize that “environmental influences” (and not just a galaxy’s original characteristics) play an important role in determining the properties and development of our Galaxy. In future chapters we will see that collisions and mergers are a major factor in the evolution of many other galaxies as well.

Key Concepts and Summary

The Galaxy began forming a little more than 13 billion years ago. Models suggest that the stars in the halo and globular clusters formed first, while the Galaxy was spherical. The gas, somewhat enriched in heavy elements by the first generation of stars, then collapsed from a spherical distribution to a rotating disk-shaped distribution. Stars are still forming today from the gas and dust that remain in the disk. Star formation occurs most rapidly in the spiral arms, where the density of interstellar matter is highest. The Galaxy captured (and still is capturing) additional stars and globular clusters from small galaxies that ventured too close to the Milky Way. In 3 to 4 billion years, the Galaxy will begin to collide with the Andromeda galaxy, and after about 7 billion years, the two galaxies will merge to form a giant elliptical galaxy.


Bridging the gap between present day and high redshift galaxies

A group of researchers at IoA are focusing on the so-called "damped Lyman alpha systems" -- galaxies in the process of formation which are not seen directly, but only as absorption when they are in a direct line between us and a yet more distant luminous object. Understanding the nature of these galaxies can ultimately providing a more direct link between them and modern-day galaxies, and thus unveil crucial aspects of galaxy formation. In order to achieve this, some of the most advanced simulations of galaxy formation have been employed and in detail compared with observations.

The figure shows the distribution of the neutral hydrogen responsible for the damped Lyman alpha absorption around a forming galaxy, in the centre.

Figure credits: Andrew Pontzen, based on simulation data run at the Arctic Region Supercomputing Centre thanks to Fabio Governato and the N-Body Shop at the University of Washington.


What is a galaxy?

This is a giant galaxy cluster known as Abell 2744, aka Pandora’s Cluster, located in the direction of the constellation Sculptor. The cluster is about 4 million light-years across and has the mass of 4 trillion suns. It appears to be the result of a simultaneous pile-up of at least 4 separate, smaller galaxy clusters that took place over a span of 350 million years. Read more about this image at HubbleSite. Image via NASA/ ESA/ J. Lotz/ M. Mountain/ A. Koekemoer/ the Hubble Frontier Fields Team.

A galaxy is a vast island of stars in an ocean of space. Galaxies are typically separated from one another by huge distances measured in millions of light-years. Galaxies are sometimes said to be the building blocks of our universe. Their distribution isn’t random, as one might suppose: galaxies are strung out along unimaginably long filaments across the universe, a cosmic web of star cities.

A galaxy can contain hundreds of billions of stars and be many thousands of light-years across. Our own galaxy, the Milky Way, is around 100,000 light-years in diameter. That’s about 587,900 trillion miles, nearly a million trillion kilometers.

Galaxies are of widely varying sizes, too.

There are an estimated two trillion galaxies in the universe.

Illustration showing snapshots from a simulation by astrophysicist Volker Springel of the Max Planck Institute in Germany. It represents the growth of cosmic structure (galaxies and voids) when the universe was 0.9 billion, 3.2 billion and 13.7 billion years old (now). Image via Volker Springel / MPE/ Kavli Foundation.

Galaxies group together in clusters. Our own galaxy is part of what is called the Local Group, for example: a cluster comprising 55 galaxies that we know of so far.

In turn, galaxy clusters themselves group into superclusters. Our Local Group is part of the Virgo Supercluster.

The “glue” that binds stars into galaxies, galaxies into clusters, clusters into superclusters and superclusters into filaments is – of course – gravity, the universe’s construction worker, which sculpts all the structures we see in the cosmos.

Distances from the Local Group for selected groups and clusters within the Local Supercluster, which is called the Virgo Supercluster.

There are several basic types of galaxy, each containing sub-types. Galaxies were first systematically classified, based on their visual appearance, by the famous astronomer Edwin P. Hubble in the late 1920s and 30s, during years of painstaking observations. Hubble’s Classification of Galaxies, as it is known, is still very much in use today, although, since Hubble’s time, like any good classification system it has been updated and amended in the light of new observations.

Before Hubble’s study of galaxies, it was believed that our galaxy was the only one in the universe. Astronomers thought that the smudges of light they saw in their telescopes were in fact nebulae within our own galaxy and not, as Hubble discovered, galaxies in their own right. It was Hubble who demonstrated, by measuring their velocities, that they lie at great distances from us, millions of light-years beyond the Milky Way, distances so huge that they appear tiny in all but the largest telescopes. Moreover, he demonstrated that, wherever he looked, galaxies are receding from us in all directions, and the further away they are, the faster they are receding. Hubble had discovered that the universe is expanding.

A diagrammatic representation of Edwin Hubble’s “tuning fork diagram.” In the late 1920s and 30s, Hubble conducted the laborious observations needed to begin to classify galaxies. His original classification scheme was published in 1936 in a book called “The Realm of the Nebulae.” His original scheme is – like all scientific work – continually being modified. But his idea of a “tuning fork diagram” has continued to be useful. Image via Las Cumbres Observatory.

The most common type of galaxy is the one most people are familiar with: the spiral galaxy. The Milky Way is of this family. Spiral galaxies have majestic, sweeping arms, thousands of light years long, made up of millions upon millions of stars. Our solar system is situated about 2/3 of the way out from the galactic center towards the periphery of the galaxy, embedded in one of these spiral arms.

Spiral galaxies are also characterised by having a bright center, made up of a dense concentration of stars, so tightly packed that from a distance the galaxy’s center looks like a solid ball. This ball of stars is known as the galactic bulge. At the center of the Milky Way – within the galactic bulge – the density of stars has been calculated at 1 million per 34 cubic light-years, for example.

Meanwhile, in the vicinity of our sun, the stellar density has been estimated as 0.004 stars per cubic light-year. Big difference!

A stunning view of the center of our Milky Way galaxy as seen by the Murchison Widefield Array (MWA) telescope in Australia in 2019. Image via Natasha Hurley-Walker (ICRAR/ Curtin)/ GLEAM Team/ Phys.org.

The Milky Way is, in fact, in one of Hubble’s spiral galaxy sub-types: it’s a barred spiral, which means it has a bar of stars protruding out from either side of the center. The ends of the bar form the anchors of the spiral arms, the place from where they sweep out in their graceful and enormous arcs. This is a fairly recent discovery: how the bar forms in a galaxy is not yet understood.

Also established recently is the fact that the disk of the Milky Way is not, as most diagrams depict, flat: it is warped, like a long-playing vinyl record left too long in the sun. Exactly why is not known, but it is thought to be the result of a gravitational encounter with another galaxy early in the Milky Way’s history.

Artist’s illustration of our warped Milky Way. Image via Ogle/ Warsaw University/ BBC.

Elliptical galaxies are the universe’s largest galaxies. They are huge and football-shaped.

They come to be because – although most galaxies are flying apart from each other – those astronomically close to each other will be mutually gravitationally attracted. Caught in an inexorable gravitational dance, eventually they merge, passing through each other over millions of years, eventually forming a single, amorphous elliptical galaxy. Such mergers may result in the birth of new generations of stars as gravity’s shock-wave compresses huge clouds of interstellar gas and dust.

The Milky Way is caught in such a gravitational embrace with M31, aka the Andromeda galaxy, which is 2 1/2 million light-years distant. Both galaxies are moving toward each other because of gravitational attraction: they will merge in about 6 billion years from now. However, both galaxies are surrounded by huge halos of gas which may extend for millions of light-years, and it was recently discovered that the halos of the Milky Way and M31 have started to touch.

The two galaxies have had their first kiss.

Galaxy mergers are not uncommon: the universe is filled with examples of galaxies in various stages of merging together, their structures disrupted and distorted by gravity, forming bizarre and beautiful shapes.

Galaxies may take billions of years to fully merge into a single galaxy. As astronomers look outward in space, they can see only “snapshots” of this long merger process. Located 300 million light-years away in the constellation Coma Berenices, these 2 colliding galaxies have been nicknamed The Mice because of the long tails of stars and gas emanating from each galaxy. Otherwise known as NGC 4676, the pair will eventually merge into a single giant galaxy. Image via Wikimedia Commons.

At the lower end of the galactic size scale, there are the so-called dwarf galaxies, consisting of a few hundred to up to several billion stars. Their origin is not clear. Usually they have no clearly defined structure. Astronomers believe they were born in the same way as larger galaxies like the Milky Way, but for whatever reason they stopped growing. Ensnared by the gravity of a larger galaxy, they orbit its periphery. The Milky Way has around 20 dwarf galaxies orbiting it that we know of, although some models predict there should be many more.

The two most famous dwarf galaxies for us earthlings are, of course, the Small and Large Magellanic Clouds, visible to the unaided eye in Earth’s Southern Hemisphere sky.

Eventually, these and other dwarf galaxies will be ripped apart by the titanic maw of the Milky Way’s gravity, leaving behind a barely noticeable stream of stars across the sky, slowly dissipating over eons.

Lynton Brown captured this beautiful image of the Milky Way over Taylor’s Lake near Horsham, Australia, on April 22, 2019. The 2 objects on the right are the Magellanic Clouds. Thank you, Lynton!

It is believed that all galaxies rotate: the Milky Way takes 226 million years to spin around once, for example. Since its birth, therefore, the Earth has travelled 20 times around the galaxy.

At the center of most galaxies lurks a supermassive black hole, of millions or even billions of solar masses. The record holder, TON 618, has a mass 66 billion times that of our sun.

The origin and evolution of supermassive black holes are not well understood. A few years ago, astronomers uncovered a surprising fact: in spiral galaxies, the mass of the supermassive black hole has a direct linear relationship with the mass of the galactic bulge. The more mass the black hole has, the more stars there are in the bulge. No one knows exactly what the significance of this relationship is, but its existence seems to indicate that the growth of a galaxy’s stellar population and that of its supermassive black hole are inextricably linked.

This discovery comes at a time when astronomers are beginning to realize that a supermassive black hole may control the fate of its host galaxy: the copious amounts of electromagnetic radiation emitted from the maelstrom of material orbiting the central black hole, known as the accretion disk, may push away and dissipate the clouds of interstellar hydrogen from which new stars form. This acts as a throttle on the galaxy’s ability to give birth to new stars. Ultimately, the emergence of life itself may be tied to the activity of supermassive black holes. This is an area of much ongoing research.

While astronomers still know very little about exactly how galaxies formed in the first place – we see them in their nascent forms existing only a few hundred million years after the Big Bang – the study of galaxies is an endless voyage of discovery.

Less than a hundred years after it was realized that other galaxies beside our own exist, we have learned so much about these grand, majestic star cities. And there is still much to learn.

Bottom line: What is a galaxy? Learn about these starry islands in space.


How is a Galaxy formed? - Astronomy

All galaxies began forming at about the same time approximately 13 billion years ago. The origin of galaxies and how they changed over billions of years is an active field of research in astronomy today. Models for galaxy formation have been of two basic types: "top-down" and "bottom-up". The "top-down" model on the origin of the galaxies says that they formed from huge gas clouds larger than the resulting galaxy. The clouds began collapsing because their internal gravity was strong enough to overcome the pressure in the cloud. If the gas cloud was slowly rotating, then the collapsing gas cloud formed most of its stars before the cloud could flatten into a disk. The result was an elliptical galaxy. If the gas cloud was rotating faster, then the collapsing gas cloud formed a disk before most of the stars were made. The result was a spiral galaxy. The rate of star formation may be the determining factor in what type of galaxy will form. But, perhaps the situation is reversed: the type of galaxy determines the rate of star formation. Which is the "cause" and which is the "effect"?

A variation of the "top-down" model says that there were extremely large gas clouds that fragmented into smaller clouds. Each of the smaller clouds then formed a galaxy. This explains why galaxies are grouped in clusters and even clusters of galaxy clusters (superclusters). However, the model predicts a very long time for the collapse of the super-large clouds and fragmentation into individual galaxy clouds. There should still be galaxies forming today. Astronomers looking for these nearby young galaxies focus their attention on the galaxies with very small amounts of "metals" (elements heavier than helium), particularly those with low percentages of oxygen. Recall from the stellar nucleosynthesis section that the metals are made from the stars and oxygen is the third most abundant element in the universe. Younger galaxies with younger generations of stars will have less pollution of metals in them.

The first nearby oxygen-poor galaxy discovered is "I Zwicky 18", just 60 million light years away. It has just 2.6% the amount of oxygen as the Milky Way and appears to have begun forming stars only 500 million years ago. However, further observations have revealed it does have much older stars as well and we're seeing it undergo a burst of star formation now. Its oxygen-poor composition may be due to unpolluted gas falling into the galaxy.

Another even nearer oxygen-poor galaxy, Leo P, is just 5 million light years away and has a very low star formation rate, just 1/50,000 the rate of the Milky Way. Like I Zwicky 18, Leo P also contains very old stars as well. Due to its small mass, Leo P wasn't able to hold on to its metals as supernovae blasted the metals away. It still has gas in it to make stars because it hasn't passed close to a large galaxy and had the gas stolen yet. The current record-holder for lack of oxygen is J0811+4730 with just 1.7% the amount of oxygen as the Milky Way. It is 620 million light years away and it is undergoing a burst of star formation now. Rather than being truly young galaxies, these and other oxygen-poor galaxies have kept low levels of metals because of their environment and small mass. Astronomers are now using these oxygen-poor galaxies to better understand how the universe's first galaxies formed stars billions of years ago. Observations and computer simulations show that the "bottom-up" model is how galaxies developed.

The "bottom-up" model builds galaxies from the merging of smaller clumps about the size of a million solar masses (the sizes of the globular clusters). These clumps would have been able to start collapsing when the universe was still very young. Then galaxies would be drawn into clusters and clusters into superclusters by their mutual gravity. This model predicts that there should be many more small galaxies than large galaxies---that is observed to be true. The dwarf irregular galaxies may be from cloud fragments that did not get incorporated into larger galaxies. Also, the galaxy clusters and superclusters should still be in the process of forming---observations suggest this to be true, as well.

The radio galaxy MRC 1138-262, also called the "Spiderweb Galaxy" is a large galaxy in the making. At 10.6 billion light years away, we see it in the process of forming only 3 billion years after the Big Bang. Note the small, thin "tadpole" and "chain" galaxies that are merging together to create a giant galaxy.

Astronomers are now exploring formation models with supercomputer simulations that incorporate the dark matter which makes up most of the matter in the universe. Huge dark matter clumps the size of superclusters gather together under the action of gravity into a network of filaments to make the "cosmic web" described in the previous section. Where the dark matter filaments intersect, regular matter concentrates into galaxies and galaxy clusters. The densest places with many intersecting filaments would have had more rapid star formation to make the elliptical galaxies while the lower density concentrations along more isolated filaments would have made the spiral galaxies and dwarf galaxies. In such a model, the visible galaxies ablaze in starlight are like the tip of an iceberg---the visible matter is at the very densest part of much larger dark matter chunks. Some dark matter clumps may have cold hydrogen and helium gas making "dark galaxies" that have not become concentrated enough to start star formation (see also the news site of the Dragonfly Telescope Array).

A dark matter map was published at the beginning of 2007 that probed the dark matter distribution over a large expanse of sky and depth (distance---see the "Distribution of Dark Matter" figure below). The map is large enough and has high enough resolution to show the dark matter becoming more concentrated with time. The map stretches halfway back to the beginning of the universe. It also shows the visible matter clumping at the densest areas of the dark matter filaments (see the "Distribution of Visible and Dark Matter" figure below). The dark matter distribution was measured by the weak gravitational lensing of the light from visible galaxies by the dark matter (see the relativity chapter).

Galaxy Collisions and Mergers

Collisions take place over very long timescales compared to the length of our lifetime---several tens of millions of years. In order to study the collisions, astronomers use powerful computers to simulate the gravitational interactions between galaxies. The computer can run through a simulation in several hours to a few days depending on the computer hardware and the number of interacting points. The results are checked with observations of galaxies in different stages of interaction. Note that this is the same process used to study the evolution of stars. The physics of stellar interiors are input into the computer model and the entire star's life cycle is simulated in a short time. Then the results are checked with observations of stars in different stages of their life.

In the past, computer simulations used several million points to represent a galaxy to save on computer processing time. A simulation of several million points could take many weeks to process. However, galaxies are made of billions to trillions of stars, so each point in the simulation actually represented large clusters of stars. The simulations were said to be of "low resolution" because many individual stars were smeared together to make one mass point in the simulation. The resolution of a computer simulation does affect the result, but it is not known how much of the result is influenced by the resolution of the simulation and how much the ignorance of the physics plays a role. Computer hardware speeds and the programming techniques have greatly improved, so astronomers are now getting to the point where they can run simulations with several billion mass points in a few weeks time. The computer simulations are also now incorporating more physical effects than just gravity and simplified hydrodynamics (gas motion) such as star formation, supernovae, formation of very large black holes at the centers of galaxies, electromagnetic fields, and other processes associated with ordinary matter. While dark matter makes up most of the matter in the universe and acts by the force of gravity alone, it turns out that smaller-scale effects from ordinary matter can make a significant impact on the structure and evolution of galaxies and clusters, much like differences in seasonings and leavening can greatly change the taste and texture of a baked clump of flour.

When two galaxies collide the stars will pass right on by each other without colliding. The distances between stars is so large compared to the sizes of the stars that star-star collisions are very rare when the galaxies collide. The orbits of the stars can be radically changed, though. Gravity is a long-range force and is the primary agent of the radical changes in a galaxy's structure when another galaxy comes close to it. Computer simulations show that a small galaxy passing close to a disk galaxy can trigger the formation of spiral arms in the disk galaxy. Alas! The simulations show that spiral arms formed this way do not last long. Part of the reason may be in the low resolution of the simulations.

The stars may be flung out from the colliding galaxies to form long arcs. Several examples of very distorted galaxies are seen with long antenna-like arcs. In some collisions a small galaxy will collide head-on with a large galaxy and punch a hole in the large galaxy. The stars are not destroyed. The star orbits in the large galaxy are shifted to produce a ring around a compact core.

Select the "Antennae Galaxies formation movie" link below to show a movie of a computer simulation from Joshua Barnes showing the formation of the Antennae Galaxies. It is a Quicktime movie, so you will need a Quicktime viewer. The red particles are the dark matter particles and the white and green are stars and gas, respectively. Other collision movies are available on Barnes' Galaxy Transformations web site and on Chris Mihos' Galaxy Collisions and Mergers website.

Here are some photographs of examples of these collisions. The first is the Antennae Galaxies (NGC 4038 & NGC 4039) as viewed from the ground (left) and from the Hubble Space Telescope (right). Note the large number of H II regions produced from the collision. The second is the Cartwheel Galaxy as seen by the Hubble Space Telescope. A large spiral was hit face-on by one of the two galaxies to the right of the ring. The insets on the left show details of the clumpy ring structure and the core of the Cartwheel. Selecting the images will bring up an enlarged version in another window. See Mihos' galaxy modelling website for simulations of the creation of the Cartwheel Galaxy.

The gas clouds in galaxies are much larger than the stars, so they will very likely hit the clouds in another galaxy when the galaxies collide. When the clouds hit each other, they compress and collapse to form a lot of stars in a short time. Galaxies undergoing such a burst of star formation are called starburst galaxies and they can be the among the most luminous of galaxies.


Messier 82 (a starburst in the M81 group)

Though typical galaxy collisions take place over what to us seems a long timescale, they are short compared to the lifetimes of galaxies. Some collisions are gentler and longer-lasting. In such collisions the galaxies can merge. Computer simulations show us that the big elliptical galaxies can form from the collisions of galaxies, including spiral galaxies. Elliptical galaxies formed in this way have faint shells of stars or dense clumps of stars that are probably debris left from the merging process. Mergers of galaxies to form ellipticals is probably why ellipticals are common in the central parts of rich clusters. The spirals in the outer regions of the clusters have not undergone any major interactions yet and so retain their original shape. Large spirals can merge with small galaxies and retain a spiral structure.

Some satellite galaxies of the Milky Way are in the process of merging with our galaxy. The dwarf elliptical galaxy SagDEG in the direction of the Milky Way's center is stretched and distorted from the tidal effects of the Milky Way's strong gravity. The Canis Major Dwarf galaxy about 25,000 light years from us is in a more advanced stage of "digestion" by the Milky Way---just the nucleus of a former galaxy is all that is left. A narrow band of neutral hydrogen from other satellite galaxies, the Magellanic Clouds, appears to be trailing behind those galaxies as they orbit the Milky Way. The band of hydrogen gas, called the "Magellanic Stream", extends almost 90 degrees across the sky away from the Magellanic Clouds and may be the result of an encounter they experienced with the Milky Way about 200 million years ago. At least eight other streams in the Milky Way from other dwarf galaxies have been found. The Andromeda Galaxy and the Milky Way will collide with each other 4.5 billion years from now and over the following two billion years after that initial encounter, they will merge to form an elliptical galaxy. The smaller spiral galaxy of the Local Group, the Triangulum Galaxy (M33) will also probably merge with us after that. (See the 2012 Hubblesite.org story which pegged the collision at 3.9 billion years from now for images and videos of the future collision.)

The giant ellipticals (called "cD galaxies") found close to the centers of galaxies were formed from the collision and merging of galaxies. When the giant elliptical gets large enough, it can gobble up nearby galaxies whole. This is called galactic cannibalism. The cD galaxies will have several bright concentrations in them instead of just one at the center. The other bright points are the cores of other galaxies that have been gobbled up.

Messier 87 (a cD galaxy that has grown large by swallowing smaller galaxies)

If collisions and mergers do happen, then more interactions should be seen when looking at regions of space at very great distances. When you look out to great distances, you see the universe as it was long ago because the light from those places takes such a long time to reach us over the billions of light years of intervening space. Edwin Hubble's discovery of the expansion of the universe means that the galaxies were once much closer together, so collisions should have been more common. Pictures from the Hubble Space Telescope of very distant galaxies show more distorted shapes, bent spiral arms, and irregular fragments than in nearby galaxies (seen in a more recent stage of their evolution).


The Hubble Ultra Deep Field (HUDF)---a narrow look back through time past many intervening galaxies to the universe as it looked billions of years ago near the start of the expansion. There were more distorted (interacting) galaxies back then! The larger fuzzy patches in the picture are closer galaxies and the smallest bright points are very distant galaxies. It is a 278-hour exposure (over 412 orbits) of a single piece of sky in the Fornax constellation.

In early 2010, astronomers announced that they were able to detect galaxies from the time of just 600 million to 800 million years after the birth of the universe (the Big Bang) using the new camera on the Hubble Space Telescope. A later study in 2012 found a cluster of galaxies beginning to form 600 million years after the Big Bang. Another study in 2012 called the Extreme Deep Field honed in on the center of the HUDF and detected galaxies forming just 450 million years after the Big Bang. A very deep look with the Spitzer Space Telescope in 2019 at two other patches of sky near the HUDF patch was able to get spectra of the H II regions (emission nebulae) in 135 galaxies at a stage less than a billion years after the Big Bang. Although these nebulae glow in the visible band, the light has been greatly redshifted into the infrared by the expansion of the universe to be detectable by Spitzer. It found that the early galaxies produced much more ionizing radiation that do modern galaxies. A detailed analysis of these early galaxies to figure out why they are so different than modern galaxies will have to wait until the much larger James Webb Space Telescope is trained on them.

The formation of galaxies is one major field of current research in astronomy. Astronomers are close to solving the engineering problem of computer hardware speeds and simulation techniques so that they can focus on the physical principles of galaxy formation. One major roadblock in their progress is the lack of understanding of the role that dark matter plays in the formation and interaction of galaxies. Since the dark matter's composition is unknown and how far out it extends in the galaxies and galaxy clusters is only beginning to be mapped (and see also link), it is not known how to best incorporate it into the computer simulations. Faced with such ignorance of the nature of dark matter, astronomers try inputting different models of the dark matter into the simulations and see if the results match the observations. As mentioned in the previous section, models that use "cold dark matter" of "WIMPs" provide the best fit to the observed structures. The recent discovery of "dark energy" is another major unknown in galaxy evolution models, though its effect may be more important to the future of the universe than to the origin and early history of the galaxies in which gravity and gas dynamics played the significant role. On the observational side, the earliest stages of galaxy formation can be studied spectroscopically only in the infrared due to the expansion of the universe, so large infrared space telescopes like Webb (launch date in early 2021) or the Wide Field Infrared Survey Telescope (launch date in 2025) are required to test the computer models.

There will be many new fundamental discoveries made in the coming years, so this section of the web site will surely undergo major revisions of the content. Although the content of our knowledge will be changed and expanded, the process of figuring out how things work will be the same. Theories and models will be created from the past observations and the fundamental physical laws and principles. Predictions will be made and then tested against new observations. Nature will veto our ideas or say that we are on the right track. Theories will be dropped, modified, or broadened. Having to reject a favorite theory can be frustrating but the excitement of meeting the challenge of the mystery and occasionally making a breakthrough in our understanding motivates astronomers and other scientists to keep exploring.


Galaxies in the Infant Universe Were Surprisingly Mature

Massive galaxies were already much more mature in the early universe than previously expected. This was shown by an international team of astronomers who studied 118 distant galaxies with the Atacama Large Millimeter/submillimeter Array (ALMA).

Most galaxies formed when the universe was still very young. Our own galaxy, for example, likely started forming 13.6 billion years ago, in our 13.8 billion-year-old universe. When the universe was only ten percent of its current age (1-1.5 billion years after the Big Bang), most of the galaxies experienced a “growth spurt”. During this time, they built up most of their stellar mass and other properties, such as dust, heavy element content, and spiral-disk shapes, that we see in today’s galaxies. Therefore, if we want to learn how galaxies like our Milky Way formed, it is important to study this epoch.

In a survey called ALPINE (the ALMA Large Program to Investigate C+ at Early Times), an international team of astronomers studied 118 galaxies experiencing such a “growth spurt” in the early universe. “To our surprise, many of them were much more mature than we had expected,” said Andreas Faisst of the Infrared Processing and Analysis Center (IPAC) at the California Institute of Technology (Caltech).

Galaxies are considered more “mature” than “primordial” when they contain a significant amount of dust and heavy elements. “We didn’t expect to see so much dust and heavy elements in these distant galaxies,” said Faisst. Dust and heavy elements (defined by astronomers as all elements heavier than hydrogen and helium) are considered to be a by-product of dying stars. But galaxies in the early universe have not had much time to build stars yet, so astronomers don’t expect to see much dust or heavy elements there either.

“From previous studies, we understood that such young galaxies are dust-poor,” said Daniel Schaerer of the University of Geneva in Switzerland. “However, we find around 20 percent of the galaxies that assembled during this early epoch are already very dusty and a significant fraction of the ultraviolet light from newborn stars is already hidden by this dust,” he added.

Many of the galaxies were also considered to be relatively grown-up because they showed a diversity in their structures, including the first signs of rotationally supported disks – which may later lead to galaxies with a spiral structure as is observed in galaxies such as our Milky Way. Astronomers generally expect that galaxies in the early universe look like train wrecks because they often collide. “We see many galaxies that are colliding, but we also see a number of them rotating in an orderly fashion with no signs of collisions,” said John Silverman of the Kavli Institute for the Physics and Mathematics of the Universe in Japan.

ALMA has spotted very distant galaxies before, such as MAMBO-9 (a very dusty galaxy) and the Wolfe Disk (a galaxy with a rotating disk). But it was hard to say whether these discoveries were unique, or whether there were more galaxies like them out there. ALPINE is the first survey that enabled astronomers to study a significant number of galaxies in the early universe, and it shows that they might evolve faster than expected. But the scientists don’t yet understand how these galaxies grew up so fast, and why some of them already have rotating disks.

Observations from ALMA were crucial for this research because the radio telescope can see the star formation that is hidden by dust and trace the motion of gas emitted from star-forming regions. Surveys of galaxies in the early universe commonly use optical and infrared telescopes. These allow the measurement of the unobscured star formation and stellar masses. However, these telescopes have difficulties measuring dust obscured regions, where stars form, or the motions of gas in these galaxies. And sometimes they don’t see a galaxy at all. “With ALMA we discovered a few distant galaxies for the first time. We call these Hubble-dark as they could not be detected even with the Hubble telescope,” said Lin Yan of Caltech.

To learn more about distant galaxies, the astronomers want to point ALMA at individual galaxies for a longer time. “We want to see exactly where the dust is and how the gas moves around. We also want to compare the dusty galaxies to others at the same distance and figure out if there might be something special about their environments,” added Paolo Cassata of the University of Padua in Italy, formerly at the Universidad de Valparaíso in Chile.

ALPINE is the first and largest multi-wavelength survey of galaxies in the early universe. For a large sample of galaxies the team collected measurements in the optical (including Subaru, VISTA, Hubble, Keck and VLT), infrared (Spitzer), and radio (ALMA). Multi-wavelength studies are needed to get the full picture of how galaxies are built up. “Such a large and complex survey is only possible thanks to the collaboration between multiple institutes across the globe,” said Matthieu Béthermin of the Laboratoire d’Astrophysique de Marseille in France.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Media contact:
Iris Nijman
NRAO News and Public Information Manager
[email protected]

A list of ALPINE publications to date can be found here (including eight papers appearing in Astronomy & Astrophysics today): http://alpine.ipac.caltech.edu/#publications

All ALPINE papers are dedicated to the memory of Olivier Le Fèvre, Principal Investigator of ALPINE.

Co-Principal Investigators of ALPINE are:
– Andreas Faisst, Caltech/IPAC, USA
– Lin Yan, Caltech, USA
– Peter Capak, Caltech/IPAC, USA
– John Silverman, Kavli Institute for the Physics and Mathematics of the Universe, Japan
– Matthieu Béthermin, Laboratoire d’Astrophysique de Marseille, France
– Paolo Cassata, University of Padua, Italy
– Daniel Schaerer, University of Geneva, Switzerland

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.