How Do Galaxies Form?

The Hubble telescope has shown us incredible sights, including an estimated 100 billion galaxies. That’s definitely not how many are out there, though. Once we have more powerful telescopes, we’ll probably see about 200 billion. So how did all those galaxies get there? How do galaxies form?

1. We don’t know for sure.
Most galaxies formed when the universe was very, very young. For this reason, we don’t really have examples of how galaxies form and so we have to make an educated guess based on computer models.

2. Gas clouds form high mass stars.
Based on simulations, what we think happened is that clouds of gas and dust that were floating out in the early universe began to come together as high-mass stars. When enough of these stars pull together, they become protogalaxies and star collecting gas and dust.

3. As more stars cluster together, the gravity strengthens.
Galaxies are held together by gravity, and it takes a lot of stars to create enough gravity for an entire galaxy to coalesce. Eventually, the gravity becomes so strong that the galaxy begins to spin around a common center of mass. And tada! A fully formed galaxy is born. At least, that’s the basics of what we think happens.
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May 12, 2020
galaxieswere born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form. The other, which has gained strength in recent years, says the young universe contained many small "lumps" of matter, which clumped together to form galaxies.
Jan 27, 2020
Replying to: SaraRayne and ManosMyron on Galactic Formation

In order to see how galaxies formed, we need to see how galaxies have changed over time and we need to find galaxies of different ages so we can see how galaxies change over time. We can utilize our own Milky Way and the Local Group of galaxies as examples of older galaxies, but we need a sample of young galaxies for comparison and to find young galaxies, we need to identify very distant galaxies. The reason distant galaxies = young galaxies is the finite speed of light. If you observe an object 1 million light years away, you are not seeing it as it is today. The light you currently see left the object 1 million years ago. This phenomenon is called lookback time. So, if you want to find a galaxy 5 billion years younger than the Milky Way, you should search for galaxies 5 billion light years away. Then, you can compare those galaxies to the ones you find 10 billion light years away, because those will appear as they were 10 billion years ago.

We can directly observe how galaxies appeared when they were forming if we can find galaxies at very large lookback times. In recent years, astronomers have been using the technique of observing deep fields (like the the Hubble Ultra Deep Field) to pursue the most distant galaxies in the universe which we can observe. This ultra deep field, in particular, has helped answer the question: “How did galaxies look billions of years ago?” The answer appears to be that when galaxies were young, they looked very irregular and ragged. Galaxies with spiral arms like the Milky Way did not appear until about 10 billion years ago. We think that galaxies apparently formed from the bottom up; that is, more than 10 billion years ago, small, irregularly shaped sub-galaxies appear to have collided and merged, leading to the formation of the large spiral and barred spiral galaxies that we see today. Although the Milky Way continues to form new stars today, primarily in its spiral arms, the star formation rates in these sub-galaxies were much higher. Observations suggest that the peak of star formation occurred about 8 billion years ago.

hubble ultra deep field.jpg
Portion of Hubble Ultra Deep Field image, showing young galaxies. Credit: Hubblesite

By comparing local galaxies to distant galaxies, as they appear in the above Hubble Ultra Deep Field Image, and supplementing these observations with computer simulations of the early universe, astronomers believe that galaxies form in roughly the following steps:
  1. The first objects are sub-galaxy sized "pieces."
  2. Several of these pieces coalesce to form a larger mass object.
  3. The gas in the larger galaxy can collapse, increasing the rotation speed of the galaxy.
  4. Stars will rapidly form inside this disk, and their orbits will sort into the familiar spiral structure.
  5. Disk galaxies will continue to evolve by the various interaction processes we saw previously, and major mergers will create elliptical galaxies.
In this general prescription for the evolution of galaxies, we did not fit the Active Galactic Nucleus (AGN) into the scenario. The AGN phase appears to be a short phase in the overall lifetime of a galaxy, and it occurs when the Super Massive Black Hole (SMBH) in the core of that galaxy has enough fuel to power the enormous luminosities these objects emit. Again, using lookback time, we see that quasars* are most numerous about 10 billion years ago. So, the quasar phase appears to be an early phase that perhaps most galaxies went through before settling down as normal galaxies.

Additionally, we must examine the impact of Cold Dark Matter (CDM) on galactic formation.
Semi-Analytic Models (SAMs) are still the best way to understand the formation of galaxies and clusters within the cosmic web dark matter gravitational skeleton, because they allow comparison of variant models of star and supermassive black hole formation and feedback. This touches on the current state of the art in semi-analytic models, and describes the successes and challenges for the best current CDM models of the roles of baryonic physics and supermassive black holes in the formation of galaxies.

Dark matter preserved the primordial fluctuations in cosmological density on galaxy scales that were wiped out in baryonic matter by momentum transport (viscosity) as radiation decoupled from baryons in the first few hundred thousand years after the big bang. The growth of dark matter halos started early enough to result in the formation of galaxies that we see even at high redshifts z > 6. Dark matter halos provide most of the gravitation within which stable structures formed in the universe. In more recent epochs, dark matter halos preserve these galaxies, groups, and clusters as the dark energy tears apart unbound structures and expands the space between bound structures such as the Local Group of galaxies. Thus we owe our existence and future to dark matter.

Cold dark matter theory including cosmic inflation has become the basis for the standard modern CDM cosmology, which is favored by analysis of the available cosmic microwave background data and large scale structure data over even more complicated variant theories having additional parameters. Most of the cosmological density is nonbaryonic dark matter (about 23%) and dark energy (about 72%), with normal baryonic matter making up only about 4.6% and the visible baryons only about 0.5% of the cosmic density. The fact that dark energy and dark matter are dominant suggests a popular name for the modern standard cosmology: the ―double dark matter theory as Nancy Abrams and Joel R. Primack proposed in their recent book about modern cosmology.

Optical (dots) and radio (triangles) rotation curve data for the Andromeda galaxy M31. superimposed on the M31 image from the Palomar Sky Survey (from Vera Rubin)

The physical nature of dark matter remains to be discovered. The two most popular ideas concerning the identity of the dark matter particles remain the lightest supersymmetric partner particle, also called supersymmetric weakly interacting massive particles (WIMPs), and the cosmological axion. These are the two dark matter candidate particles that are best motivated in the sense that they are favored by other considerations of elementary particle theory.

Supersymmetry remains the best idea for going beyond the standard model of particle physics. It allows control of vacuum energy and of otherwise unrenormalizable gravitational interactions, and thus may allow gravity to be combined with the electroweak and strong interactions in superstring theory. Supersymmetry also allows for grand unification of the electroweak and strong interactions, and naturally explains how the electroweak scale could be so much smaller than the grand unification or Planck scales (thus solving the ―gauge hierarchy problem). It thus leads to the expectation that the supersymmetric WIMP mass will be in the range of about 100 to about 1000 GeV.

Axions** remain the best solution to the CP problem of SU(3) gauge theory of strong interactions, although it is possible that the axion exists and solves the strong CP problem but makes only a negligible contribution to the dark matter density.

Many other particles have been proposed as possible dark matter candidates, even within the context of supersymmetry. An exciting prospect in the next few years is that experimental and astronomical data may point toward specific properties of the dark matter particles, and may even enable us to discover their identity. There are good opportunities for detecting the dark matter particles in deep underground experiments, producing them at the Large Hadron Collider at CERN, detecting their annihilation products, and exploring the possibility that the dark matter is warm by studying small scale structure.

In order to get order-of-magnitude better constraints than presently available, and a possible detection of non-cosmological-constant dark energy, better instruments will probably be required both on the ground and in space, according to the Dark Energy Task Force. The National Academy Beyond Einstein report recommended the Joint Dark Energy Mission (JDEM) ( See: as the first Beyond Einstein mission. It also recommended that JDEM be conceived as a dual-purpose mission, collecting a wide range of data that will shed light on galaxy formation and evolution as well as on the nature dark energy.

* Quasar stands for quasi-stellar radio source. Quasars got that name because they looked starlike when astronomers first began to notice them in the late 1950s and early 60s. But quasars aren’t stars. They’re now known as young galaxies, located at vast distances from us, with their numbers increasing towards the edge of the visible universe. How can they be so far away and yet still visible? The answer is that quasars are extremely bright, up to 1,000 times brighter than our Milky Way galaxy. We know, therefore, that they’re highly active, emitting staggering amounts of radiation across the entire electromagnetic spectrum.

Because they’re far away, we’re seeing these objects as they were when our universe was young. The oldest quasar, currently, is J0313-1806. Its distance has been measured as 13.03 billion light-years, and therefore we see it as it was just 670 million years after the Big Bang.

Astronomers now believe that quasars are the extremely luminous centers of galaxies in their infancy. After decades of intense study, we have another term for these objects: a quasar is a type of active galactic nucleus, or AGN. There are actually many different types of AGNs, each with their own tale to tell. The intense radiation released by an AGN is thought to be powered by a super-massive black hole at its center. The radiation is emitted when material in the accretion disk surrounding the black hole is superheated to millions of degrees by the intense tidal friction generated by the particles of dust, gas and other matter in the disk, sometimes entire stars, colliding countless times with each other.

The inward spiral of matter in a supermassive black hole’s accretion disk – that is, at the center of a quasar – is the result of particles colliding and bouncing against each other and losing momentum. That material came from the enormous clouds of gas, mainly consisting of molecular hydrogen, which filled the universe in the era shortly after the Big Bang.

As matter in a quasar/black hole’s accretion disk heats up, it generates radio waves, X-rays, ultraviolet and visible light. The quasar becomes so bright that it’s able to outshine entire galaxies. But remember that quasars are so far away from us that we only observe the active nucleus, or core, of the galaxy in which they reside. We see nothing of the galaxy apart from its bright center. It’s like seeing a distant headlight at night: you have no idea of which type of career truck you are looking at, as everything apart from the headlight is in darkness.

Quasars can emit up to a thousand times the energy of the combined luminosity of the 200 billion or so stars in our own Milky Way galaxy. A typical quasar can be 27 trillion times brighter than our sun!

Astronomers believe that most, if not all, large galaxies went through a so-called “quasar phase” in their youth, soon after their formation. If so, they subsided in brightness when they ran out of the steady stream of matter needed to feed the accretion disk surrounding their supermassive black holes. After this epoch, galaxies settled into quiescence, their central black holes starved of that steady flow of material to feed on. The black hole at the center of our own galaxy has been seen to flare up briefly, however, as passing material strays into it, releasing radio waves and X-rays. It’s conceivable that entire stars or even clusters of stars could be torn apart and consumed as they cross a black hole’s event horizon, the point of no return.

When these massive visible jets of energy are perpendicular to our view, we see a radio galaxy. If they’re at an angle, we see a quasar. And when we’re staring right down the barrel of the jet, that’s a blazar. It’s the same object, seen from three different perspectives.

It must be pointed out, however, that our knowledge of galaxy evolution – from youthful quasar to quiescent middle-aged galaxy – is far from complete. Galaxies often provide us with exceptions, and as an example we need look no further than our own Milky Way. We now know that some 3.5 million years ago there was a gigantic explosion known as a Seyfert flare at the center of our galaxy. It was apparently centered on Sagittarius A*, the Milky Way’s own central supermassive black hole, producing two huge lobes of superheated plasma extending some 25,000 light years from the north and south galactic poles. These huge lobes are called Fermi bubbles and are still visible today at gamma and X-ray wavelengths (very high frequency electromagnetic emissions).

** Axions - The axion is a pseudo-Nambu-Goldstone boson. It appears after the spontaneous breaking of the Peccei-Quinn symmetry***, which was proposed to solve the strong-CP (charge conjugation parity symmetry) problem. Other pseudo-Nambu-Goldstone bosons, postulated in some extensions of the standard model of particle physics, are called axion-like particles (ALPs) if they share certain characteristics with the axion, in particular a coupling to two photons. Thus far, axion and ALP searches have been unsuccessful, indicating that their couplings have to be extremely weak. However, axions and ALPs could be responsible for some observable effects in astrophysics and cosmology, which can also be exploited to constrain the parameter space of these particles. We focus on limits coming from cosmology, which is an optimal field for studying axions and ALPs. In particular, we first investigate the possibility of a primordial population of axions and ALPs arising during the earliest epochs of the universe. The importance of this analysis lies on the fact that axions and ALPs are ideal dark matter candidates because of their faint interactions and their peculiar production mechanisms. Finally, we consider the consequences of the decay of such a population on specific cosmological observables, namely the photon spectrum of galaxies, the cosmic microwave background, the effective number of neutrino species, and the abundance of primordial elements. Our bounds constitute the most stringent probes of early decays and exclude a part of the ALP parameter space that is otherwise very difficult to test experimentally.
See: ;

*** The proximity of the Peccei-Quinn scale to the scale of supersymmetry breaking in models of pure gravity mediation hints at a common dynamical origin of these two scales. To demonstrate how to make such a connection manifest, we embed the Peccei-Quinn mechanism into the vector-like model of dynamical supersymmetry breaking a la IYIT. Here, we rely on the anomaly-free discrete Z4R symmetry required in models of pure gravity mediation to solve the mu problem to protect the Peccei-Quinn symmetry from the dangerous effect of higher-dimensional operators. This results in a rich phenomenology featuring a QCD axion with a decay constant of O(10^10) GeV and mixed WIMP/axion dark matter. In addition, exactly five pairs of extra 5 and 5* matter multiplets, directly coupled to the supersymmetry breaking sector and with masses close to the gravitino mass, m3/2 ~ 100 TeV, are needed to cancel the Z4R anomalies.





Thus, from pure mechanics, we have ventured into the dark matter and dark energy as galactic building blocks over time as found and charted by Vera Rubin among others. Galaxies change over time and the farther back we look we can see simpler galactic models and see them form in their five main steps. Again, I am astounded to realize that I am star stuff, as Carl Sagan termed us all, looking out on the universe from whence we all came and I can see beauty in every direction in every age of development.
Hartmann 352
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