Galaxy Formation

Blog Author: Will Pepi

Topic: Galaxy Formation

Galactic Origins

This blog is aimed to give an overview of galaxy formation, and provide answers to some common and naturally asked questions. 

How Do Galaxies Form?

Due to the grand time scale of galaxy behavior, much of what is known about galaxies, especially early galaxies and galaxy formation, is theoretical. While we can learn a lot simply from observing, much of what we know about galaxy formation, interaction, and evolution is only visible through computer simulations. The most widely accepted theory is that, before galaxies existed, the entire Universe was made up of clouds that were initially remarkably uniform and stable, but developed gravitational over-densities over time, setting in motion the clumping of the clouds that existed throughout the universe. Out of this clumping, the current large scale, fractal-like structure of the universe began to form. After the gas consolidated into the regions that now make up clusters and superclusters of galaxies, the gas continued to condense to eventually form individual galaxies and stars. There are three basic types of galaxies: spiral, elliptical, and irregular, each with their own type of formation and evolution. To learn about different galaxy types in further detail, click here

(1) Spiral galaxies are disc shaped with arms spiraling towards the high star-density center “bulge.” The Milky Way, and other

Comolli, Lorenzo. M31: The Andromeda Galaxy. 2013. NASA, Apod.nasa.gov. NASA. Web. 07 Dec. 2016.

spiral galaxies most likely form out of a spherical protogalactic cloud. Star formation begins while the galaxy is still a sphere, explaining why there are a small number of stars that exist in the halo of spiral galaxies—a spherical region around a galaxy loosely populated with almost exclusively old stars. As star formation continues the cloud has sufficient mass to rotate, the same way a star does when it is being born. As a result of this rotation and conservation of angular momentum, the spherical clouds collapses into a disk. Due to the continued rotation, this disk forms as a spiral around a very star-dense spherical center region. Star formation occurs in the disk, which is why Halo stars tend to be older than disk stars. All the stars in the disk orbit with the same velocity due to the presence of dark matter, making outer stars orbit faster than they "should". Almost all spiral galaxies have a supermassive blackhole at their centers, but theories about how exactly these supermassive blackholes form are still widely disputed.

Evolution of Elliptical Galaxies. N.d. Daily Galaxy. Daily Galaxy.com. Web. 07 Dec. 2016. 

(2) Elliptical galaxies can form under very similar conditions to spiral galaxies, however, the gas in the cloud is thought to be higher density than those that yield spiral galaxies. The result is that the somewhat spherical cloud never collapses and star formation continues to occur so the galaxy forms as an elliptical galaxy. A cloud that has a slow rotation can also yield an elliptical galaxy, because the cloud never has enough angular momentum for it to collapse into a disk. Another way elliptical galaxies can form is through interactions with other galaxies. For example, when two spiral galaxies collide (our own Milky Way is on course to collide with the spiral galaxy Andromeda) the gravitational interactions greatly disrupt the spiral structure and, after a galactic dance, the result is an elliptical galaxy. Elliptical galaxies are also theorized to form by accretion of small clumps of stars until eventually it has enough mass to be considered a galaxy.

Hubble Space Telescope image of the interacting Antennae 

Galaxies, NGC 4038 and NGC 4039 (credit: ESA/Hubble & NASA)

(3) Irregular galaxies are essentially any galaxy that doesn’t fit into the other two, broad, categories. There are many observed galaxies that do not fit either classification and so are just labeled irregular. These galaxies are often very small and are host to a lot of star formation because they are normally made of mostly dust and gas. Most irregular galaxies star out as some other type of galaxy and become irregular through gravitational interactions with other galaxies.

What Were the First Galaxies Like?

How the very first galaxies formed is still a disputed and active question in Astronomy. It is difficult to determine, through observation, whether the first galaxies simply formed out of gas and dust, or if they formed as a result of accretion of smaller, already formed things like star clusters. The James Webb telescope will allow astronomers the ability to observe at even longer infrared wavelengths than Hubble can observe, allowing us to look at radiation from when the universe was only a few hundred million years old. Observing these extremely distant galaxies from the when the universe was still very young will hopefully give more insight into how the very first galaxies formed. 

How does Dark Matter Effect Galaxy Formation?

Dark matter is some unknown material that makes up about a fourth of the Universe. We know very little about it, but we do know that it has gravitational effects. (It gravitational effects are a big reason why astronomers are able to know for certain that it exists) Dark matter is tremendously important in the formation of galaxies. Computers models indicate that the reason galaxies even begin to form is due to dark matter being gravitationally attracted, and clumping together. Dark matter seems to act as a sort of framework on which visible matter exists and interacts. So galaxy formation and evolution is intrinsically related to, and to some extent dictated by dark matter. Follow this link to see a simulation of how dark matter effects the structure of the universe.

How did the Universe Change When Galaxies Began Forming?

On a very large scale, galaxies completely transformed the architecture of our Universe. Before galaxies, the universe was an extremely organized primordial soup of particles—essentially just a lot of dust. But due to the small gravitational irregularities that triggered a chain reaction to eventually lead to objects as large as galaxies to form, the Universe, on a galactic scale, has taken on a somewhat fractal structure.

Bibliography

Origins Education Forum

http://origins.stsci.edu/faq/galaxies.html

What Are Irregular Galaxies?

John P. Millis, Ph.D Space/Astronomy Expert - http://space.about.com/od/Galaxies/a/Irregular-Galaxies.htm

http://chronicle.uchicago.edu/060713/darkmatter.shtml

Millennium Simulation Project

http://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/

Galaxies

http://www.astronomynotes.com/galaxy/s10.htm