How did Galaxies Form

After recombination there were obviously pockets of density enhancements around which material accumulated

The exact mechanism for this is unclear and most of the mass is probably darkmatter. Gravitational coalescence of this dark material then built the galaxies we observe today. Below is an N-body supercomputer simulation of this process.

• Galaxy Formation (?)

These eventually grow to form galaxies. Unfortunately, there is a rather wide range of different galaxy types, shapes and densities which means the formation process was not simple. Also most all galaxies today are embedded in some larger scale structure. The formation of these structures is unclear but here are a couple of possibilities:

• Large Scale Structure
• Formation of a single Galaxy Cluster

Structure formation could have either occurred from fragmentation of very large regions into smaller regions or from the gravitational coalesence of sub-units into successively large structures. Both formation scenarios lead to a highly clustered Universe with structure on many different size scales.

Individual Galaxy Types:

• Elliptical Galaxies These systems have little rotation and are essentially a big ball of stars swarming around in isotropic orbits. Today, these galaxies are mostly gas free and no longer form stars.

• Sprial Galaxies These are huge rotating, flattened systems of stars, gas and dust. They still have ample amounts of gas and hence are still forming stars today. Other Examples:
  • M33
  • M81
  • NGC 6946

• Irregular Galaxies These are smaller mass systems that lack symmetric structure. Their intrinisic shapes are not well known.

• Nearly Invisible Galaxies Over the last 10 years a brand new class of galaxies has been discovered. These galaxies seem to have significant lower densities than more "normal" kinds of galaxies.

So many questions about galaxy formation and evolution certainly remain. Among them are:

• How come some galaxies rotate and others don't?
• How come some galaxies form stars over a long period of time while others formed the bulk of their stars very early?
• How come there is apparently a significant range in average mass density for galaxies?

For our purposes a galaxy is just a large collection of gas which is gravitationally bound. This gas eventually clumps to make stars. Without the formation of stars in galaxies, the Universe would have never made elements heavier than Helium.

Hence, the formation of planets with life depends upon the initial formation of galaxies, which is largely an unknown process.

Galaxies represent essentially collection sites for the material made in the early Universe to coalesce into big balls of gas which essentially ignite the fires of fusion in their centers thus becoming stars.

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The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu