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Formation of Galaxies

The first galaxies are believed to have formed about 800 million years after the big bang, when the reionization of matter had taken place. It is uncertain if the galaxies were formed in giant clouds of hydrogen, or if (relatively) smaller lumps of gas congregated into larger structures. Research points to the latter. The problem with this theory of galactic merging is that most galaxies don't look like they have merged together with smaller ones. Astronomers know that galaxies that are in the middle of the merging process usually look like train wrecks. Perhaps galaxies regain their shape relatively quickly due to the galaxy's angular momentum and the large halos of dark matter that seem to surround them.

Today, several (13-14) billion years after the big bang, astronomers have found signs that some galaxies are still forming - galaxies collide to create even larger galaxies. There is yet another theory on how galaxies formed:

Stream-Fed Galaxies

The problem with the theory of merging galaxies is that the predicted stellar birth rate does not match what is seen in the early universe, even in galaxies that are not in the process of merging. This, in combination with the fact that most galaxies do not have spherical shapes suggests that something else is at play.
Astrophysicists at the Hebrew University of Jerusalem have formulated a new theory (published in the journal "Nature", Jan, 2009) on how galaxies formed: intense, cold streams of gas (mostly hydrogen) originating from the dark matter halo provide the forming galaxy with gas. “These gas streams follow the filaments of the ‘cosmic web’ that defines the large-scale structure of matter in the universe, filaments that feed the dark-matter halos in the first place. These cold gas streams penetrate through the dark-matter halo of each galaxy and the hot gas that fills it and reach the center, where they become a rotating disk. These disks, each subject to its own, local, gravitational forces, break into a few giant clumps in which the gas converts into stars very efficiently” – explained the researchers.
According to their model the rate of newly born stars can be explained, and matched with the rate observed.

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Supermassive Black Holes

As the galaxy forms and evolves, so does the center of the galaxy. At the heart, most galaxies evolve black holes that feed on the gas, and stars surrounding it. It grows bigger and bigger and may reach a mass billions of times more than our sun. At a point in the evolution of a galaxy the supermassive black hole will be feeding so much that a bright accretion disc will evolve around it. This disc may become millions or even billions of degrees warm. The disc will then become enormously luminous, outshining the rest of the galaxy by a factor as large as a thousand! During this phase, the feeding supermassive black hole puts its' host galaxy in the AGN - Active Galactic Nuclei category, as a quasar. The AGN seems to hold the power to both trigger, and shut down the birth of stars in its' vicinity.
The mass of a supermassive black hole seems to be in proportion to the mass of the galactic bulge and the mass of the galaxy as a whole. So, when the supermassive black hole has reached its' limit, it shuts down - the disc stops receiving gas and thereby eventually vanishes. The black hole will still be there, but it will be dormant. A galaxy being a quasar seems to be a stage in the evolution of galaxies, and it is possible that a supermassive black hole lurks at the heart of every galaxy.

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Next: Active Galactic Nuclei.


Above: An illustration of a collision between two galaxies. The Collision process is a way for galaxies to grow and evolve.

This illustration is available as a print (5000x3000 pixels, 300 dpi), and as a PSD-document so that it can be customized according to your own desire.


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