Space Technology

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. An image of an ongoing merger of equal sized disk galaxies is shown left. During the merger, stars and dark matter in each galaxy become affected by the approaching galaxy. Toward the late stages of the merger, the gravitational potential, the shape of galaxy, begins changing so quickly that star orbits are greatly affected, and lose any memory of their previous orbit. This process is called violent relaxation. Thus if two disk galaxies collide, they begin with their stars in an orderly rotation in the plane of the disk. During the merger, the ordered motion is transformed into random energy. And the resultant galaxy is dominated by stars that orbit the galaxy in a complex, and random, web of orbits. And this is what we see in elliptical galaxies, stars on random unordered orbits.

Mergers are also locations of extreme amounts of star formation. During a merger some galaxies can make thousands of solar masses of new stars each year, which is large compared to our galaxy which makes about 10 new stars each year. Though stars almost never get close enough to actually collide in galaxy mergers, giant molecular clouds rapidly fall to the center of the galaxy where they collide with other molecular clouds. These collisions then induce condensations of these clouds into new stars. We can see this phenomenon in merging galaxies in the nearby universe. Yet, this process was more pronounced during the mergers that formed most elliptical galaxies we see today, which likely occurred 1-10 billion years ago, when there was much more gas (and thus more molecular clouds) in galaxies. Also, away from the center of the galaxy gas clouds will run into each other producing shocks which stimulate the formation of new stars in gas clouds. The result of all this violence is that galaxies tend to have little gas available to form new stars after they merge. Thus if a galaxy is involved in a major merger, and then a few billion years pass, the galaxy will have very few young stars (see Stellar evolution) left. This is what we see in today's elliptical galaxies, very little molecular gas and very few young stars. It is thought that this is because elliptical galaxies are the end products of major mergers which use up the majority of gas during the merger, and thus further star formation after the merger is quenched.

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.


Adapted from the Wikipedia article Galaxy formation and evolution, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki