Galaxy formation and evolution

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

Contents
Commonly observed properties of galaxies
Formation of disk galaxies
Top-down theories
Bottom-up theories
Galaxy mergers and the formation of elliptical galaxies
Galaxy quenching
Gallery
See also
Further reading
References
External links

Commonly observed properties of galaxies

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

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

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

Formation of disk galaxies

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

Top-down theories

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

Bottom-up theories

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

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

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

Galaxy mergers and the formation of elliptical galaxies

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 NGC 4676 (Mice Galaxies) is an
like disk galaxies). A example of a present merger.
Artist's image of a firestorm of distinguishing feature of
star birth deep inside the core of elliptical galaxies is that the
a young, growing elliptical galaxy. velocity of the stars does not
necessarily contribute to
flattening of the galaxy, such as
in spiral galaxies.[7] Elliptical
galaxies have central
supermassive black holes, and
the masses of these black holes
correlate with the galaxy's mass.

Elliptical galaxies have two main

stages of evolution. The first is
due to the supermassive black The Antennae Galaxies are a pair
hole growing by accreting of colliding galaxies – the bright,
cooling gas. The second stage is blue knots are young stars that
marked by the black hole have recently ignited as a result
ESO 325-G004, a typical elliptical of the merger.
stabilizing by suppressing gas
galaxy. cooling, thus leaving the
elliptical galaxy in a stable
state.[8] The mass of the black hole is also correlated to a property
called sigma which is the dispersion of the velocities of stars in their orbits. This relationship, known as the
M-sigma relation, was discovered in 2000.[9] Elliptical galaxies mostly lack disks, although some bulges of
disk galaxies resemble elliptical galaxies. Elliptical galaxies are more likely found in crowded regions of the
universe (such as galaxy clusters).

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

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

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

Galaxy quenching
One observation (see above) that must be explained by a
successful theory of galaxy evolution is the existence of two
different populations of galaxies on the galaxy color-magnitude
diagram. Most galaxies tend to fall into two separate locations on
this diagram: a "red sequence" and a "blue cloud". Red sequence
galaxies are generally non-star-forming elliptical galaxies with little
gas and dust, while blue cloud galaxies tend to be dusty star-
forming spiral galaxies.[15][16] Star formation in what are now
"dead" galaxies sputtered out billions
As described in previous sections, galaxies tend to evolve from of years ago.[14]
spiral to elliptical structure via mergers. However, the current rate
of galaxy mergers does not explain how all galaxies move from the
"blue cloud" to the "red sequence". It also does not explain how star formation ceases in galaxies. Theories
of galaxy evolution must therefore be able to explain how star formation turns off in galaxies. This
phenomenon is called galaxy "quenching".[17]

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

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

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

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

Gallery

NGC 3610 shows NGC 891, a very An image of Messier A spiral galaxy, ESO
some structure in the thin disk galaxy 101, a prototypical 510-G13, was
form of a bright disc, spiral galaxy seen warped as a result of
implying that it face-on colliding with
formed only a short another galaxy. After
time ago.[26] the other galaxy is
completely
absorbed, the
distortion will
disappear. The
process typically
takes millions if not
billions of years.

See also
Big Bang – Cosmological model
Bulge (astronomy)
Chronology of the universe – History and future of the universe
Cosmology – Scientific study of the origin, evolution, and eventual fate of the universe
Galactic disc – Component of disc galaxies comprising gas and stars
Formation and evolution of the Solar System – Formation of the Solar System by
gravitational collapse of a molecular cloud and subsequent geological history
Galactic coordinate system – Celestial coordinate system in spherical coordinates, with the
Sun as its center
 Galactic corona – Hot, ionised, gaseous component in the Galactic halo
Galactic halo
Galactic orientation
Galaxy rotation curve
Illustris project – Computer-simulated universes
List of galaxies
Mass segregation (astronomy) – Process by which heavier members of a gravitationally
bound system tend to move toward the center, while lighter members tend to move away
from the center
Metallicity distribution function – Distribution within a group of stars of the ratio of iron to
hydrogen in a star
Pea galaxy – Possibly a type of luminous blue compact galaxy which is undergoing very
high rates of star formation
Recent development (2018): Galaxies with little or no dark matter – Hypothetical form of
matter comprising most of the matter in the universe
Red nugget, small galaxies packed with large amounts of red stars
Star formation – Process by which dense regions of molecular clouds in interstellar space
collapse to form stars
Structure formation – Formation of galaxies, galaxy clusters and larger structures from small
early density fluctuations
UniverseMachine – Computer simulated universes
Zeldovich pancake – Theoretical condensation of gas out of a primordial density fluctuation
following the Big Bang

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

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External links
NOAO gallery of galaxy images (http://www.noao.edu/image_gallery/galaxies.html)
Image of Andromeda galaxy (M31) (http://www.noao.edu/image_gallery/html/im0685.htm
l)
Javascript passive evolution calculator (http://www.astro.yale.edu/dokkum/evocalc/) for early
type (elliptical) galaxies
Video on the evolution of galaxies by Canadian astrophysicist Doctor P (http://spacegeek.or
g/ep4_flash.shtml)

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