Galaxy Clusters
Published on Oct 15, 2006 at 11:25 pm.
1 Comment.
Filed under galaxies.
My last post was about galaxies and the Hubble galaxy classification system. Today, I’m posting more about galaxies. This time, I’ll talk about clusters of galaxies.
Galaxies are seldom found isolated. The vast majority of galaxies are found in clusters and groups. Galaxy clusters come in all sizes and concentrations. Some are very rich clusters, with many galaxies crammed fairly close together. Others are sparse, with galaxies far from one another. Most have dozens to hundreds of galaxies. A few contain very few large galaxies, and a handful of smaller dwarf galaxies. These tiny clusters are called groups. Our own galaxy is part of the small cluster with only three or four large galaxies and a dozen or so smaller ones. 
It is called the Local Group (real original name, huh?). At the top of this post is a picture of a cluster of galaxies in Hercules, and here is a picture of a small group of galaxies, perhaps not too dissimilar from our own Local Group.
One interesting thing about these groups and clusters is the distrubution of Hubble type galaxies in them. Small groups tend to contain spiral galaxies as the larger members. Sparse galaxy clusters often have lots of spiral galaxies, too. But the rich clusters are very rich in elliptical galaxies. They have spirals in them, for sure, but the ellipticals dominate. In fact, the interior members of rich clusters are almost always elliptical galaxies or S0 galaxies. The spirals tend to congregate in the outer portions of these rich clusters. 
In fact, the centers of the larger galaxy clusters tend to contain giant elliptical galaxies, called cD galaxies, that are absolutely monstrous in size. These giant ellipticals can be two to three million lightyears across — as big as the entire Local Group! One such giant elliptical is M87, shown here.
Again, we see that there is something to the Hubble classification system. There is something physically going on with galaxies that governs their appearance. That is why classifying them by their appearance has hung on as a common way to talk about them, even though it was a first attempt at organizing the data.
There’s another interesting thing about the richer galaxy clusters. The rich galaxy clusters also have a lot of very hot gas within the cluster between the galaxies. 
Most gas of this concentration is located inside galaxies, but in rich galaxy clusters, it is between the galaxies. Now, that is interesting, because as I said, these clusters often are rich in elliptical galaxies, and ellipticals, as I said yesterday, don’t have much gas in them. Here is an X-ray photo of the center of the Virgo cluster taken by the ROSAT X-ray telescope. This is interesting because this gas must be extremely hot in order to give off X-rays like this.
So, what’s going on? Well, the key is in realizing just how close galaxies are in clusters. OK, they are often millions of lightyears apart, but these galaxies are also millions of lightyears across. That means that they are often only a few galaxy diameters away from each other. At this close distance, the are close enough to be able to run into one another over time. This is particularly true in galaxy clusters. Galaxies collide with one another all the time. In fact, we now believe that galaxy collisions are key to the structure of galaxies, and even to building galaxies. When spiral galaxies collide, the collision can is not the violent affair that comes from asteroids colliding with planets. While galaxies are fairly close to one another within a galaxy cluster, stars are very far away from one another within galaxies. In fact, two galaxies can directly collide with one another and pass right through each other! The stars would typically never collide with one another, just pass on by. However, the interstellar medium, the gas between the stars, is another matter altogether. The gas of one galaxy can slam into the gas of the other galaxy, and this heats the gas and alters its motion. I might blog more later on galaxy collisions and give more details. But, suffice it to say that the end result of two large galaxies colliding can be to strip the gas from those galaxies. Without the interstellar medium, these galaxies can not form new stars, and they will become elliptical galaxies. And the hot gas will be left in the space between the galaxies. This is likely where the hot intergalactic gas in clusters comes from. Galaxy collisions can even result in mergers of galaxies. That would explain the giant elliptical galaxies in the middle of the larger and richer galaxy clusters.
-Astroprof
(Images courtesy of NASA, HST, ROSAT)
Kambria Jaye on October 5, 2011 at 12:57 pm: 1
That’s real good.
I’m actually very interested
in the old myth’s of Hercules.
And this is some good information.