Galaxy Mergers and The Formation of Elliptical Galaxies
The most massive galaxies in the sky are giant elliptical galaxies. Their stars are on orbits that are randomly oriented within the galaxy (i.e. they are not rotating like disk galaxies). They are composed of old stars and have little to no dust. All elliptical galaxies probed so far have supermassive black holes in their center, and the mass of these black holes is correlated with the mass of the elliptical galaxy. They are also correlated to a property called sigma which is the speed of the stars at the far edge of the elliptical galaxies. Elliptical galaxies do not have disks around them, although some bulges of disk galaxies look similar to elliptical galaxies. One is more likely to find elliptical galaxies in more crowded regions of the universe (such as galaxy clusters).
Astronomers now see elliptical galaxies as some of the most evolved systems in the universe. It is widely accepted that the main driving force for the evolution of elliptical galaxies is mergers of smaller galaxies. These mergers can be extremely violent; galaxies often collide at speeds of 500 kilometers per second.
Many galaxies in the universe are gravitationally bound to other galaxies, that is to say they will never escape the pull of the other galaxy. If the galaxies are of similar size, the resultant galaxy will appear similar to neither of the two galaxies merging, but would instead be an elliptical galaxy. An image of an ongoing merger of equal sized disk galaxies is shown left.
In the Local Group, the Milky Way and M31 (the Andromeda Galaxy) are gravitationally bound, and currently approaching each other at high speed. If the two galaxies do meet they will pass through each other, with gravity distorting both galaxies severely and ejecting some gas, dust and stars into intergalactic space. They will travel apart, slow down, and then again be drawn towards each other, and again collide. Eventually both galaxies will have merged completely, streams of gas and dust will be flying through the space near the newly formed giant elliptical galaxy. M31 is actually already distorted: the edges are warped. This is probably because of interactions with its own galactic companions, as well as possible mergers with dwarf spheroidal galaxies in the recent past - the remnants of which are still visible in the disk populations.
In our epoch, large concentrations of galaxies (clusters and superclusters) are still assembling.
While scientists have learned a great deal about ours and other galaxies, the most fundamental questions about formation and evolution remain only tentatively answered.
Read more about this topic: Galaxy Formation And Evolution
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