How Did Galaxies Form?

It is a given fact that our technology today cannot see back to the birth of galaxies. So how do we study the early stages of galaxy evolution? In order to study the early stages, scientists have created theoretical models, which more and more evidence is being found to support. To understand the models we must first assume two concepts.

• When the universe was only a few million years old, hydrogen and helium gas filled all of space.

• When the universe was young matter was not uniformly distributed; there were small variations in densities.

Assuming the above ideas and adding laws of physics, we can begin to understand how denser regions in the young universe became the galaxies of the past and present. We can gather that the regions of space that had greater densities expanded just like the rest universe, but soon slowed due to the extra gravitational pull. After a few billion years the expansion stopped and began to reverse. The matter present in these regions contracted and formed protogalactic clouds. This is likely what formed our very own Milky Way.

Our mathematical models show that protogalactic clouds started to contract and cool; this contracting led to the formation of the galaxy’s first stars. These first generation stars were very massive and had very short life spans. When the lives of these stars end in massive supernovae explosions, shockwaves are sent into the surrounding protogalactic cloud. The shockwaves heat the surrounding gas which slows down the rate of collapse for the protogalactic cloud, this in turn results in the formation of the disc shape which is characteristic of most galaxies.

Some spiral galaxies have the same basic structure as the Milky Way. A disk population of stars, for example, that orbit the galactic center in a pretty flat plane, and a spheroidal population of stars with more randomly oriented orbits. In the spheroidal population, it consists of stars that were born before the galaxy's rotation became organized. That is why they have orbits oriented in many different ways around the galactic center. In the disk population, it consists of stars born after the gas in the protogalactic cloud settled into a rotating disk. That is why they all have similar orbits around the center of the galaxy.

While this idea assumes that galaxies are formed in slightly enhanced densities, it offers no explanation where these enhancements came from. One of the more difficult questions to answer in astronomy is where did these density enhancements originate? Also, this idea explains the origin of spiral galaxies but fails to explain why some galaxies are elliptical and why some are irregular.

 

What Is The Role Of The Central Black Hole?

At the center of galaxies there appears to be something known as a Super Massive Black Hole, which is a black hole with a mass millions to billions of times that of our sun. Studies show that orbital speeds of stars as well as gas clouds in the center of galaxies show that the masses of super massive black holes is around 1/500 of the mass of a galaxy’s bulge. It is believed although not proven that all galaxies contain super massive black holes.

It is believed that super massive black holes may have played a role in galaxy formation. For a long time astronomers have suspected that galaxy evolution goes along with the formation of super massive black holes. The reason being because matter falling into a super massive black hole causes quasars. Quasars exist early in the life of galaxies, and were very common early in time when galaxies were growing swiftly. Although we do not understand this process, we have some theories as how super massive black holes formed. Some astronomers believe that black holes came first out of gas at the center of a protogalactic cloud and that their energy output regulated the growth of the galaxy around them. While others believe that clusters of neutron stars which came from extremely dense starbursts at the centers of young galaxies some how combined to form a super massive black hole. Whatever the case may be super massive black holes remain a mysterious aspect of astronomy to this day.