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.
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.
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. 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
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
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, 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.
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.
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 are a pair of colliding galaxies - the bright, blue knots are young stars that have recently ignited as a result of the merger.
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. 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. 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. 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,
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.
By sequencing several images of different galactic collisions, one can
observe the timeline of two spiral galaxies merging into a single
elliptical galaxy.
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.
Galaxy quenching
Star formation in what are now "dead" galaxies sputtered out billions of years ago.
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.
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".
Stars form out of cold gas,
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.
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.
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.
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. 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.
Ejective processes, which expel cold gas from galaxies, may explain how more massive galaxies are quenched.
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.
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.
Gallery
NGC 3610 shows some structure in the form of a bright disc, implying that it formed only a short time ago.[26]
An image of Messier 101, a prototypical spiral galaxy seen face-on
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.
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassiveblack holes at their active centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.
Recent estimates of the number of galaxies in the observable universe range from 200 billion (2×1011) to 2 trillion (2×1012) or more, containing more stars than all the grains of sand on planet Earth. Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 to 300,000 light years)
and separated by distances on the order of millions of parsecs (or
megaparsecs). For comparison, the Milky Way has a diameter of at least
30,000 parsecs (100,000 LY) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million LY).
The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which is dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. The largest structure of galaxies yet recognised is a cluster of superclusters that has been named Laniakea, which contains the Virgo supercluster.
Etymology
The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias (γαλαξίας, "milky one"), or kyklos galaktikos ("milky circle") due to its appearance as a "milky" band of light in the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's
breast while she is asleep so that the baby will drink her divine milk
and will thus become immortal. Hera wakes up while breastfeeding and
then realizes she is nursing an unknown baby: she pushes the baby away,
some of her milk spills, and it produces the faint band of light known
as the Milky Way.
In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380:
"See yonder, lo, the Galaxyë Which men clepeththe Milky Wey, For hit is whyt."
Galaxies were initially discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae,
and were just thought as a part of the Milky Way', but their true
composition and natures remained a mystery. Observations using larger
telescopes of a few nearby bright galaxies, like the Andromeda Galaxy,
began resolving them into huge conglomerations of stars, but based
simply on the apparent faintness and sheer population of stars, the true
distances of these objects placed them well beyond the Milky Way. For
this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies.
The realization that we live in a galaxy which is one among many
galaxies, parallels major discoveries that were made about the Milky Way
and other nebulae.
Milky Way
The Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.
Aristotle
(384–322 BCE), however, believed the Milky Way to be caused by "the
ignition of the fiery exhalation of some stars that were large, numerous
and close together" and that the "ignition takes place in the upper
part of the atmosphere, in the region of the World that is continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 CE) was critical of this view, arguing that if the Milky Way is sublunary
(situated between Earth and the Moon) it should appear different at
different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way is celestial.
According to Mohani Mohamed, the Arabian astronomer Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way's parallax,
and he thus "determined that because the Milky Way had no parallax, it
must be remote from the Earth, not belonging to the atmosphere." The Persian astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars." The Andalusian astronomer Ibn Bâjjah ("Avempace", d.
1138) proposed that the Milky Way is made up of many stars that almost
touch one another and appear to be a continuous image due to the effect
of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are near. In the 14th century, the Syrian-born Ibn Qayyim proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars."
The shape of the Milky Way as estimated from star counts by William Herschel in 1785; the Solar System was assumed to be near the center.
Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.
In 1750 the English astronomer Thomas Wright, in his An original theory or new hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk. In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.
The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center. Using a refined approach, Kapteyn
in 1920 arrived at the picture of a small (diameter about
15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A
different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our host galaxy, the Milky Way, emerged.
A fish-eye mosaic of the Milky Way arching at a high
inclination across the night sky, shot from a dark-sky location
A few galaxies outside the Milky Way are visible on a dark night to the unaided eye, including the Andromeda Galaxy, Large Magellanic Cloud and the Small Magellanic Cloud. In the 10th century, the Persian astronomer Al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud". In 964, Al-Sufi probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars (referring to "Al Bakr of the southern Arabs", since at a declination of about 70° south it was not visible where he lived); it was not well known to Europeans until Magellan's voyage in the 16th century. The Andromeda Galaxy was later independently noted by Simon Marius in 1612.
In 1734, philosopher Emanuel Swedenborg in his Principia
speculated that there may be galaxies outside our own that are formed
into galactic clusters that are miniscule parts of the universe which
extends far beyond what we can see. These views "are remarkably close to
the present-day views of the cosmos."
In 1750, Thomas Wright speculated (correctly) that the Milky Way is a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. In 1755, Immanuel Kant used the term "island Universe" to describe these distant nebulae.
Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy
Toward the end of the 18th century, Charles Messier compiled a catalog
containing the 109 brightest celestial objects having nebulous
appearance. Subsequently, William Herschel assembled a catalog of 5,000
nebulae. In 1845, Lord Rosse
constructed a new telescope and was able to distinguish between
elliptical and spiral nebulae. He also managed to make out individual
point sources in some of these nebulae, lending credence to Kant's
earlier conjecture.
In 1912, Vesto Slipher
made spectrographic studies of the brightest spiral nebulae to
determine their composition. Slipher discovered that the spiral nebulae
have high Doppler shifts,
indicating that they are moving at a rate exceeding the velocity of the
stars he had measured. He found that the majority of these nebulae are
moving away from us.
In 1917, Heber Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier objectM31, was then known). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs.
He became a proponent of the so-called "island universes" hypothesis,
which holds that spiral nebulae are actually independent galaxies.
In 1920 a debate took place between Harlow Shapley and Heber Curtis (the Great Debate),
concerning the nature of the Milky Way, spiral nebulae, and the
dimensions of the Universe. To support his claim that the Great
Andromeda Nebula is an external galaxy, Curtis noted the appearance of
dark lanes resembling the dust clouds in the Milky Way, as well as the
significant Doppler shift.
In 1922, the Estonian astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. Using the new 100 inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936 Hubble produced a classification of galactic morphology that is used to this day.
Modern research
Rotation curve of a typical spiral galaxy: predicted based on the visible matter (A) and observed (B). The distance is from the galactic core.
In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas;
and in 1951 it was observed. This radiation is not affected by dust
absorption, and so its Doppler shift can be used to map the motion of
the gas in our galaxy. These observations led to the hypothesis of a
rotating bar structure in the center of our galaxy. With improved radio telescopes, hydrogen gas could also be traced in other galaxies.
In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed
and that predicted by the visible mass of stars and gas. Today, the
galaxy rotation problem is thought to be explained by the presence of
large quantities of unseen dark matter. A concept known as the universal rotation curve of spirals, moreover, shows that the problem is ubiquitous in these objects.
Scientists used the galaxies visible in the GOODS survey to recalculate the total number of galaxies.
Beginning in the 1990s, the Hubble Space Telescope
yielded improved observations. Among other things, Hubble data helped
establish that the missing dark matter in our galaxy cannot solely
consist of inherently faint and small stars. The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the observable universe. Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected by Hubble. Particularly, galaxy surveys in the Zone of Avoidance (the region of the sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.
In 2016, a study published in The Astrophysical Journal and led by Christopher Conselice of the University of Nottingham using 3D modeling of images collected over 20 years by the Hubble Space Telescope concluded that there are over 2 trillion (2×1012) galaxies in the observable universe.
Types and morphology
Types of galaxies according to the Hubble classification scheme: an E indicates a type of elliptical galaxy; an S is a spiral; and SB is a barred-spiral galaxy.
Galaxies come in three main types: ellipticals, spirals, and
irregulars. A slightly more extensive description of galaxy types based
on their appearance is given by the Hubble sequence.
Since the Hubble sequence is entirely based upon visual morphological
type (shape), it may miss certain important characteristics of galaxies
such as star formation rate in starburst galaxies and activity in the cores of active galaxies.
Ellipticals
The Hubble classification system rates elliptical galaxies on the
basis of their ellipticity, ranging from E0, being nearly spherical, up
to E7, which is highly elongated. These galaxies have an ellipsoidal
profile, giving them an elliptical appearance regardless of the viewing
angle. Their appearance shows little structure and they typically have
relatively little interstellar matter. Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead they are dominated by generally older, more evolved stars
that are orbiting the common center of gravity in random directions.
The stars contain low abundances of heavy elements because star
formation ceases after the initial burst. In this sense they have some
similarity to the much smaller globular clusters.
The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies,
resulting in a collision and merger. They can grow to enormous sizes
(compared to spiral galaxies, for example), and giant elliptical
galaxies are often found near the core of large galaxy clusters.
Starburst galaxies are the result of a galactic collision that can result in the formation of an elliptical galaxy.
Shell galaxy
NGC 3923 Elliptical Shell Galaxy-Hubble Space Telescope photograph
A shell galaxy is a type of elliptical galaxy where the stars in the
galaxy's halo are arranged in concentric shells. About one-tenth of
elliptical galaxies have a shell-like structure, which has never been
observed in spiral galaxies. The shell-like structures are thought to
develop when a larger galaxy absorbs a smaller companion galaxy. As the
two galaxy centers approach, the centers start to oscillate around a
center point, the oscillation creates gravitational ripples forming the
shells of stars, similar to ripples spreading on water. For example,
galaxy NGC 3923 has over twenty shells.
Spiral galaxies resemble spiraling pinwheels.
Though the stars and other visible material contained in such a galaxy
lie mostly on a plane, the majority of mass in spiral galaxies exists in
a roughly spherical halo of dark matter that extends beyond the visible component, as demonstrated by the universal rotation curve concept.
Spiral galaxies consist of a rotating disk of stars and
interstellar medium, along with a central bulge of generally older
stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region. A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.
The speed in which a galaxy rotates is thought to correlate with the
flatness of the disc as some spiral galaxies have thick bulges, while
others are thin and dense.
In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals,
a pattern that can be theoretically shown to result from a disturbance
in a uniformly rotating mass of stars. Like the stars, the spiral arms
rotate around the center, but they do so with constant angular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves".
As stars move through an arm, the space velocity of each stellar system
is modified by the gravitational force of the higher density. (The
velocity returns to normal after the stars depart on the other side of
the arm.) This effect is akin to a "wave" of slowdowns moving along a
highway full of moving cars. The arms are visible because the high
density facilitates star formation, and therefore they harbor many
bright and young stars.
A majority of spiral galaxies, including our own Milky Way
galaxy, have a linear, bar-shaped band of stars that extends outward to
either side of the core, then merges into the spiral arm structure. In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c)
that indicates the form of the spiral arms (in the same manner as the
categorization of normal spiral galaxies). Bars are thought to be
temporary structures that can occur as a result of a density wave
radiating outward from the core, or else due to a tidal interaction with another galaxy. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.
Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011) stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.
Super-luminous spiral
Recently, researchers described galaxies called super-luminous
spirals. They are very large with an upward diameter of 437,000
light-years (compared to the Milky Way's 100,000 light-year diameter).
With a mass of 340 billion solar masses, they generate a significant
amount of ultraviolet and mid-infrared light. They are thought to have
an increased star formation rate around 30 times faster than the Milky
Way.
Other morphologies
Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies.
A ring galaxy
has a ring-like structure of stars and interstellar medium surrounding a
bare core. A ring galaxy is thought to occur when a smaller galaxy
passes through the core of a spiral galaxy. Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.
A lenticular galaxy
is an intermediate form that has properties of both elliptical and
spiral galaxies. These are categorized as Hubble type S0, and they
possess ill-defined spiral arms with an elliptical halo of stars (barred lenticular galaxies receive Hubble classification SB0.)
Irregular galaxies are galaxies that can not be readily classified into an elliptical or spiral morphology.
An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.
Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted. Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.
An ultra diffuse galaxy
(UDG) is an extremely-low-density galaxy. The galaxy may be the same
size as the Milky Way but has a visible star count of only 1% of the
Milky Way. The lack of luminosity is because there is a lack of
star-forming gas in the galaxy which results in old stellar populations.
Dwarfs
Despite the prominence of large elliptical and spiral galaxies, most
galaxies in the Universe are dwarf galaxies. These galaxies are
relatively small when compared with other galactic formations, being
about one hundredth the size of the Milky Way, containing only a few
billion stars. Ultra-compact dwarf galaxies have recently been
discovered that are only 100 parsecs across.
Many dwarf galaxies may orbit a single larger galaxy; the Milky
Way has at least a dozen such satellites, with an estimated 300–500 yet
to be discovered. Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.
A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses,
regardless of whether the galaxy has thousands or millions of stars.
This has led to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.
Other types of galaxies
Interacting
The Antennae Galaxies are undergoing a collision that will result in their eventual merger.
Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.
Collisions occur when two galaxies pass directly through each other and
have sufficient relative momentum not to merge. The stars of interacting
galaxies will usually not collide, but the gas and dust within the two
forms will interact, sometimes triggering star formation. A collision
can severely distort the shape of the galaxies, forming bars, rings or
tail-like structures.
At the extreme of interactions are galactic mergers. In this case
the relative momentum of the two galaxies is insufficient to allow the
galaxies to pass through each other. Instead, they gradually merge to
form a single, larger galaxy. Mergers can result in significant changes
to morphology, as compared to the original galaxies. If one of the
merging galaxies is much more massive than the other merging galaxy then
the result is known as cannibalism.
The more massive larger galaxy will remain relatively undisturbed by
the merger, while the smaller galaxy is torn apart. The Milky Way galaxy
is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy.
Starburst
M82, a starburst galaxy that has ten times the star formation of a "normal" galaxy
Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds.
Some galaxies have been observed to form stars at an exceptional rate,
which is known as a starburst. If they continue to do so, then they
would consume their reserve of gas in a time span less than the lifespan
of the galaxy. Hence starburst activity usually lasts for only about
ten million years, a relatively brief period in the history of a galaxy.
Starburst galaxies were more common during the early history of the
Universe, and, at present, still contribute an estimated 15% to the total star production rate.
Starburst galaxies are characterized by dusty concentrations of
gas and the appearance of newly formed stars, including massive stars
that ionize the surrounding clouds to create H II regions. These massive stars produce supernova explosions, resulting in expanding remnants
that interact powerfully with the surrounding gas. These outbursts
trigger a chain reaction of star building that spreads throughout the
gaseous region. Only when the available gas is nearly consumed or
dispersed does the starburst activity end.
Starbursts are often associated with merging or interacting
galaxies. The prototype example of such a starburst-forming interaction
is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity.
Active galaxy
A jet of particles is being emitted from the core of the elliptical radio galaxy M87.
A portion of the observable galaxies are classified as active
galaxies if the galaxy contains an active galactic nucleus (AGN). A
significant portion of the total energy output from the galaxy is
emitted by the active galactic nucleus, instead of the stars, dust and interstellar medium of the galaxy.
The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region of the galaxy. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.
In about 10% of these galaxies, a diametrically opposed pair of
energetic jets ejects particles from the galaxy core at velocities close
to the speed of light. The mechanism for producing these jets is not well understood.
Seyfert galaxies or quasars, are classified depending on the luminosity, are active galaxies that emit high-energy radiation in the form of x-rays.
Blazars
Blazars are believed to be an active galaxy with a relativistic jet that is pointed in the direction of Earth. A radio galaxy
emits radio frequencies from relativistic jets. A unified model of
these types of active galaxies explains their differences based on the
viewing angle of the observer.
LINERS
Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weakly ionized elements. The excitation sources for the weakly ionized lines include post-AGB stars, AGN, and shocks. Approximately one-third of nearby galaxies are classified as containing LINER nuclei.
Seyfert galaxy
Seyfert galaxies are one of the two largest groups of active
galaxies, along with quasars. They have quasar-like nuclei (very
luminous, distant and bright sources of electromagnetic radiation) with
very high surface brightnesses but unlike quasars, their host galaxies
are clearly detectable. Seyfert galaxies account for about 10% of all
galaxies. Seen in visible light, most Seyfert galaxies look like normal
spiral galaxies, but when studied under other wavelengths, the
luminosity of their cores is equivalent to the luminosity of whole
galaxies the size of the Milky Way.
Quasar
Quasars (/ˈkweɪzɑr/) or quasi-stellar radio sources are the most
energetic and distant members of a class of objects called active
galactic nuclei (AGN). Quasars are extremely luminous and were first
identified as being high redshift sources of electromagnetic energy,
including radio waves and visible light, that appeared to be similar to
stars, rather than extended sources similar to galaxies. Their
luminosity can be 100 times greater than that of the Milky Way.
Luminous infrared galaxy
Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above 1011
L☉. LIRGs are more abundant than starburst galaxies, Seyfert galaxies
and quasi-stellar objects at comparable total luminosity. Infrared
galaxies emit more energy in the infrared than at all other wavelengths
combined. A LIRG's luminosity is 100 billion times that of our Sun.
Properties
Magnetic fields
Galaxies have magnetic fields of their own.
They are strong enough to be dynamically important: they drive mass
inflow into the centers of galaxies, they modify the formation of spiral
arms and they can affect the rotation of gas in the outer regions of
galaxies. Magnetic fields provide the transport of angular momentum
required for the collapse of gas clouds and hence the formation of new
stars.
The typical average equipartition strength for spiral galaxies is about 10 μG (microGauss) or 1 nT (nanoTesla). For comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss or 30 μT (microTesla). Radio-faint galaxies like M 31 and M 33, our Milky Way's
neighbors, have weaker fields (about 5 μG), while gas-rich galaxies
with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG
on average. In prominent spiral arms the field strength can be up to 25
μG, in regions where cold gas and dust are also concentrated. The
strongest total equipartition fields (50–100 μG) were found in starburst galaxies, for example in M 82 and the Antennae, and in nuclear starburst regions, for example in the centers of NGC 1097 and of other barred galaxies.
Formation and evolution
Galactic formation and evolution is an active area of research in astrophysics.
Formation
Artist's impression of a protocluster forming in the early Universe
Current cosmological models of the early Universe are based on the Big Bang theory. About 300,000 years after this event, atoms of hydrogen and helium began to form, in an event called recombination.
Nearly all the hydrogen was neutral (non-ionized) and readily absorbed
light, and no stars had yet formed. As a result, this period has been
called the "dark ages". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos. These primordial structures would eventually become the galaxies we see today.
Artist's impression of a young galaxy accreting material
Early galaxies
Evidence for the early appearance of galaxies was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift
of 6.96, corresponding to just 750 million years after the Big Bang and
making it the most distant and primordial galaxy yet seen.
While some scientists have claimed other objects (such as Abell 1835 IR1916)
have higher redshifts (and therefore are seen in an earlier stage of
the Universe's evolution), IOK-1's age and composition have been more
reliably established. In December 2012, astronomers reported that UDFj-39546284
is the most distant object known and has a redshift value of 11.9. The
object, estimated to have existed around "380 million years" after the Big Bang (which was about 13.8 billion years ago), is about 13.42 billion light travel distance years away. The existence of such early protogalaxies suggests that they must have grown in the so-called "dark ages". As of May 5, 2015, the galaxy EGS-zs8-1 is the most distant and earliest galaxy measured, forming 670 million years after the Big Bang. The light from EGS-zs8-1 has taken 13 billion years to reach Earth, and is now 30 billion light-years away, because of the expansion of the universe during 13 billion years.
Early galaxy formation
Different components of near-infrared background light detected by the Hubble Space Telescope in deep-sky surveys
The detailed process by which early galaxies formed is an open
question in astrophysics. Theories can be divided into two categories:
top-down and bottom-up. In top-down correlations (such as the
Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a
large-scale simultaneous collapse lasting about one hundred million
years. In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.
Once protogalaxies began to form and contract, the first halo stars (called Population III stars)
appeared within them. These were composed almost entirely of hydrogen
and helium, and may have been massive. If so, these huge stars would
have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium.
This first generation of stars re-ionized the surrounding neutral
hydrogen, creating expanding bubbles of space through which light could
readily travel.
In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.
Such stars are likely to have existed in the very early universe (i.e.,
at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.
Evolution
Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars
form. The creation of a supermassive black hole appears to play a key
role in actively regulating the growth of galaxies by limiting the total
amount of additional matter added. During this early epoch, galaxies undergo a major burst of star formation.
During the following two billion years, the accumulated matter settles into a galactic disc. A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life.
This matter is mostly hydrogen and helium. The cycle of stellar birth
and death slowly increases the abundance of heavy elements, eventually
allowing the formation of planets.
XDF view field compared to the angular size of the Moon. Several thousand galaxies, each consisting of billions of stars, are in this small view.
XDF (2012) view: Each light speck is a galaxy, some of which are as old as 13.2 billion years – the observable universe is estimated to contain 200 billion to 2 trillion galaxies.
XDF image shows (from left) fully mature galaxies, nearly mature galaxies (from 5 to 9 billion years ago), and protogalaxies, blazing with young stars (beyond 9 billion years).
The evolution of galaxies can be significantly affected by
interactions and collisions. Mergers of galaxies were common during the
early epoch, and the majority of galaxies were peculiar in morphology.
Given the distances between the stars, the great majority of stellar
systems in colliding galaxies will be unaffected. However, gravitational
stripping of the interstellar gas and dust that makes up the spiral
arms produces a long train of stars known as tidal tails. Examples of
these formations can be seen in NGC 4676 or the Antennae Galaxies.
The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s,
and—depending upon the lateral movements—the two might collide in about
five to six billion years. Although the Milky Way has never collided
with a galaxy as large as Andromeda before, evidence of past collisions
of the Milky Way with smaller dwarf galaxies is increasing.
Such large-scale interactions are rare. As time passes, mergers
of two systems of equal size become less common. Most bright galaxies
have remained fundamentally unchanged for the last few billion years,
and the net rate of star formation probably also peaked approximately
ten billion years ago.
Future trends
Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms. Elliptical galaxies are largely devoid of this gas, and so form few new stars.
The supply of star-forming material is finite; once stars have
converted the available supply of hydrogen into heavier elements, new
star formation will come to an end.
The current era of star formation is expected to continue for up
to one hundred billion years, and then the "stellar age" will wind down
after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest, longest-lived stars in our universe, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes. Eventually, as a result of gravitational relaxation,
all stars will either fall into central supermassive black holes or be
flung into intergalactic space as a result of collisions.
Larger-scale structures
Deep sky surveys show that galaxies are often found in groups and clusters.
Solitary galaxies that have not significantly interacted with another
galaxy of comparable mass during the past billion years are relatively
scarce. Only about 5% of the galaxies surveyed have been found to be
truly isolated; however, these isolated formations may have interacted
and even merged with other galaxies in the past, and may still be
orbited by smaller, satellite galaxies. Isolated galaxies can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies.
On the largest scale, the Universe is continually expanding,
resulting in an average increase in the separation between individual
galaxies.
Associations of galaxies can overcome this expansion on a local scale
through their mutual gravitational attraction. These associations formed
early in the Universe, as clumps of dark matter pulled their respective
galaxies together. Nearby groups later merged to form larger-scale
clusters. This on-going merger process (as well as an influx of
infalling gas) heats the inter-galactic gas within a cluster to very
high temperatures, reaching 30–100 megakelvins.
About 70–80% of the mass in a cluster is in the form of dark matter,
with 10–30% consisting of this heated gas and the remaining few percent
of the matter in the form of galaxies.
Most galaxies in the Universe are gravitationally bound to a number of other galaxies. These form a fractal-like
hierarchical distribution of clustered structures, with the smallest
such associations being termed groups. A group of galaxies is the most
common type of galactic cluster, and these formations contain a majority
of the galaxies (as well as most of the baryonic mass) in the Universe.
To remain gravitationally bound to such a group, each member galaxy
must have a sufficiently low velocity to prevent it from escaping. If there is insufficient kinetic energy, however, the group may evolve into a smaller number of galaxies through mergers.
Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity. Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy, which, over time, tidally destroys its satellite galaxies and adds their mass to its own.
Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids. Above this scale, the Universe appears to be the same in all directions (isotropic and homogeneous).
The Milky Way galaxy is a member of an association named the Local Group,
a relatively small group of galaxies that has a diameter of
approximately one megaparsec. The Milky Way and the Andromeda Galaxy are
the two brightest galaxies within the group; many of the other member
galaxies are dwarf companions of these two galaxies. The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large, extended structure of groups and clusters of galaxies centered on the Virgo Cluster. And the Virgo Supercluster itself is a part of the Pisces-Cetus Supercluster Complex, a giant galaxy filament.
Multi-wavelength observation
This ultraviolet image of Andromeda shows blue regions containing young, massive stars.
The peak radiation of most stars lies in the visible spectrum, so the observation of the stars that form galaxies has been a major component of optical astronomy. It is also a favorable portion of the spectrum for observing ionized H II regions, and for examining the distribution of dusty arms.
The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail. Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier in the history of the Universe. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy.
The southern plane of the Milky Way from submillimeter wavelengths
The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. (The ionosphere blocks signals below this range.) Large radio interferometers have been used to map the active jets emitted from active nuclei. Radio telescopes can also be used to observe neutral hydrogen (via 21 cm radiation), including, potentially, the non-ionized matter in the early Universe that later collapsed to form galaxies.
Ultraviolet and X-ray telescopes
can observe highly energetic galactic phenomena. Ultraviolet flares are
sometimes observed when a star in a distant galaxy is torn apart from
the tidal forces of a nearby black hole.
The distribution of hot gas in galactic clusters can be mapped by
X-rays. The existence of supermassive black holes at the cores of
galaxies was confirmed through X-ray astronomy.
NGC 891, a very thin disk galaxy
An image of Messier 101, a prototypical spiral galaxy seen face-on
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.
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