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. Hydrodynamics simulation, which simulates
both baryons and dark matter, is widely used to study galaxy formation and evolution.

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 theories: Hubble tuning fork diagram of galaxy
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 the 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 their 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 theory

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 (most notably
supergiant ellipticals, such as ESO
306-17) are among some of the
largest known thus far. Their stars
are on orbits that are randomly
oriented within the galaxy (i.e. NGC 4676 (Mice Galaxies) is an
they are not rotating like disk example of a present merger.
Artist's image of a firestorm of galaxies). A distinguishing feature
star birth deep inside the core of 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
The Antennae Galaxies are a pair
to the supermassive black hole of colliding galaxies – the bright,
growing by accreting cooling gas. blue knots are young stars that
The second stage is marked by the have recently ignited as a result
ESO 325-G004, a typical black hole stabilizing by of the merger.
elliptical galaxy. suppressing gas cooling, thus
leaving the elliptical galaxy in a
stable state.[8] The mass of the black hole is also correlated to a
property called sigma which is the dispersion of the velocities of stars in their orbits. This relationship,
known as the M-sigma relation, was discovered in 2000.[9] Elliptical galaxies mostly lack disks, although
some bulges of disk galaxies resemble elliptical galaxies. Elliptical galaxies are more likely found in
crowded regions of the universe (such as galaxy clusters).

Astronomers now see elliptical galaxies as some of the most evolved systems in the universe. It is widely
accepted that the main driving force for the evolution of elliptical galaxies is mergers of smaller galaxies.
Many galaxies in the universe are gravitationally bound to other galaxies, which means that they will never
escape their mutual pull. If those colliding 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 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
As described in previous sections, galaxies tend to evolve from "dead" galaxies sputtered out billions
spiral to elliptical structure via mergers. However, the current rate of of years ago.[14]
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]

Hydrodynamics Simulation
Dark energy and dark matter account for most of the Universe's energy, so it is valid to ignore baryons
when simulating large-scale structure formation (using methods such as N-body simulation). However,
since the visible components of galaxies consist of baryons, it is crucial to include baryons in the simulation
to study the detailed structures of galaxies. At first, the baryon component consists of mostly hydrogen and
helium gas, which later transforms into stars during the formation of structures. From observations, models
used in simulations can be tested and the understanding of different stages of galaxy formation can be
improved.

Euler equations

In cosmological simulations, astrophysical gases are typically modeled as inviscid ideal gases that follow
the Euler equations, which can be expressed mainly in three different ways: Lagrangian, Eulerian, or
arbitrary Lagrange-Eulerian methods. Different methods give specific forms of hydrodynamical
equations.[26] When using the Lagrangian approach to specify the field, it is assumed that the observer
tracks a specific fluid parcel with its unique characteristics during its movement through space and time. In
contrast, the Eulerian approach emphasizes particular locations in space that the fluid passes through as time
progresses.

Baryonic Physics

To shape the population of galaxies, the hydrodynamical equations must be supplemented by a variety of
astrophysical processes mainly governed by baryonic physics.

Gas cooling

Processes, such as collisional excitation, ionization, and inverse Compton scattering, can cause the internal
energy of the gas to be dissipated. In the simulation, cooling processes are realized by coupling cooling
functions to energy equations. Besides the primordial cooling, at high temperature, ,
heavy elements (metals) cooling dominates. [27] When , the fine structure and molecular
cooling also need to be considered to simulate the cold phase of the interstellar medium.

Interstellar medium

Complex multi-phase structure, including relativistic particles and magnetic field, makes simulation of
interstellar medium difficult. In particular, modeling the cold phase of the interstellar medium poses
technical difficulties due to the short timescales associated with the dense gas. In the early simulations, the
dense gas phase is frequently not modeled directly but rather characterized by an effective polytropic
equation of state.[28] More recent simulations use a multimodal distribution[29][30] to describe the gas
density and temperature distributions, which directly model the multi-phase structure. However, more
detailed physics processes needed to be considered in future simulations, since the structure of the
interstellar medium directly affects star formation.

Star formation

As cold and dense gas accumulates, it undergoes gravitational collapse and eventually forms stars. To
simulate this process, a portion of the gas is transformed into collisionless star particles, which represent
coeval, single-metallicity stellar populations and are described by an initial underlying mass function.
Observations suggest that star formation efficiency in molecular gas is almost universal, with around 1% of
the gas being converted into stars per free fall time.[31] In simulations, the gas is typically converted into
star particles using a probabilistic sampling scheme based on the calculated star formation rate. Some
simulations seek an alternative to the probabilistic sampling scheme and aim to better capture the clustered
nature of star formation by treating star clusters as the fundamental unit of star formation. This approach
permits the growth of star particles by accreting material from the surrounding medium.[32] In addition to
this, modern models of galaxy formation track the evolution of these stars and the mass they return to the
gas component, leading to an enrichment of the gas with metals.[33]

Stellar feedback

Stars have an influence on their surrounding gas by injecting energy and momentum. This creates a
feedback loop that regulates the process of star formation. To effectively control star formation, stellar
feedback must generate galactic-scale outflows that expel gas from galaxies. Various methods are utilized to
couple energy and momentum, particularly through supernova explosions, to the surrounding gas. These
methods differ in how the energy is deposited, either thermally or kinetically. However, excessive radiative
gas cooling must be avoided in the former case. Cooling is expected in dense and cold gas, but it cannot be
reliably modeled in cosmological simulations due to low resolution. This leads to artificial and excessive
cooling of the gas, causing the supernova feedback energy to be lost via radiation and significantly
reducing its effectiveness. In the latter case, kinetic energy cannot be radiated away until it thermalizes.
However, using hydrodynamically-decoupled wind particles to inject momentum non-locally into the gas
surrounding active star-forming regions may still be necessary to achieve large-scale galactic outflows.[34]
Recent models [35]explicitly model stellar feedback. These models not only incorporate supernova
feedback but also consider other feedback channels such as energy and momentum injection from stellar
winds, photoionization, and radiation pressure resulting from radiation emitted by young, massive stars.[36]

Supermassive black holes

Simulation of supermassive black holes is also considered, numerically seeding them in dark matter haloes,
due to their observation in many galaxies[37] and the impact of their mass on the mass density distribution.
Their mass accretion rate is frequently modeled by the Bondi-Hoyle model.

Active galactic nuclei

Active galactic nuclei (AGN) have an impact on the observational phenomena of supermassive black holes,
and further have a regulation of black hole growth and star formation. In simulations, AGN feedback is
usually classified into two modes, namely quasar and radio mode. Quasar mode feedback is linked to the
radiatively efficient mode of black hole growth and is frequently incorporated through energy or
momentum injection.[38] The regulation of star formation in massive galaxies is believed to be significantly
influenced by radio mode feedback, which occurs due to the presence of highly-collimated jets of
relativistic particles. These jets are typically linked to X-ray bubbles that possess enough energy to
counterbalance cooling losses.[39]

Magnetic fields
The ideal magnetohydrodynamics approach is commonly utilized in cosmological simulations since it
provides a good approximation for cosmological magnetic fields. The effect of magnetic fields on the
dynamics of gas is generally negligible on large cosmological scales. Nevertheless, magnetic fields are a
critical component of the interstellar medium since they provide pressure support against gravity[40] and
affect the propagation of cosmic rays.[41]

Cosmic rays

Cosmic rays play a significant role in the interstellar medium by contributing to its pressure,[42] serving as a
crucial heating channel,[43] and potentially driving galactic gas outflows.[44] The propagation of cosmic
rays is highly affected by magnetic fields. So in the simulation, equations describing the cosmic ray energy
and flux are coupled to magnetohydrodynamics equations.[45]

Radiation Hydrodynamics

Radiation hydrodynamics simulations are computational methods used to study the interaction of radiation
with matter. In astrophysical contexts, radiation hydrodynamics is used to study the epoch of reionization
when the Universe had high redshift. There are several numerical methods used for radiation
hydrodynamics simulations, including ray-tracing, Monte Carlo, and moment-based methods. Ray-tracing
involves tracing the paths of individual photons through the simulation and computing their interactions
with matter at each step. This method is computationally expensive but can produce very accurate results.

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.[46] the other galaxy is
completely
absorbed, the
distortion will
disappear. The
process typically
takes millions if not
billions of years.

See also
 Big Bang – Description of how the universe began
Bulge (astronomy) – Tightly packed group of stars within a larger formation
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 – Modelling its structure and composition
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 – Spherical component of a galaxy which extends beyond the main, visible
component
Galactic orientation
Galaxy rotation curve – Observed discrepancy in galactic angular momenta
Illustris project – Computer-simulated universes
List of galaxies
Mass segregation (astronomy) – Gravitational process, eg in star clusters
Metallicity distribution function – Distribution within a group of stars of the ratio of iron to
hydrogen in a star
Pea galaxy – Possible type of luminous blue compact galaxy
Recent development (2018): Galaxies with little or no dark matter – Hypothetical form of
matter
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) Archived
(https://web.archive.org/web/20020802193225/http://www.noao.edu/image_gallery/galaxies.
html) 2 August 2002 at the Wayback Machine
Image of Andromeda galaxy (M31) (http://www.noao.edu/image_gallery/html/im0685.htm
l) Archived (https://web.archive.org/web/20021021025020/http://www.noao.edu/image_g
allery/html/im0685.html) 21 October 2002 at the Wayback Machine
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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