The Milky Way is a fairly hefty galaxy, consisting of more than 100 billion stars, along with plenty of gas, dust, and dark matter. Yet it’s puny compared with the real giants: the biggest galaxies in the Universe might be hundreds of times the Milky Way’s mass, and correspondingly larger in size.
These huge galaxies live in clusters: associations of hundreds or thousands of galaxies spanning millions of light-years across. Galaxy clusters are the largest objects in the cosmos that are held together by their own gravity. Not only that, they are a rich environment for galaxies, hot plasma, and dark matter. In fact, it seems that the hugest of galaxies can only be born in clusters: they require a lot of food in the form of gas and smaller galaxies.
So, if we want to understand galaxies, we need to study their environments. The Milky Way is the second-largest member of the boringly-named Local Group, along with the Andromeda Galaxy and more than 50 smaller objects. Since it’s much smaller than a galaxy cluster, the Local Group must have been formed from a relatively small amount of gas, in a cosmic sense at least: there wasn’t enough raw material to make thousands of galaxies.
By contrast, the Coma Cluster is a large galaxy cluster about 320 million light-years away. It has more than 1,000 members, including ten bright spiral galaxies visible to backyard astronomers with small telescopes. The real heavyweights, though, are two giant elliptical galaxies known as NGC 4874 and NGC 4889. (“NGC” stands for “New General Catalogue”, a late-19th century attempt to list as many galaxies, nebulas, and star clusters as possible. Astronomers sometimes call names like NGC 4889 “license-plate numbers”.)
Giant elliptical galaxies lack pretty spiral arms. Instead, they are blobs of reddish stars without any particular structure. Though they look boring, they’re important for showing how many galaxies look at middle age: they’ve exhausted their supply of gas that can be used to make new stars. Spiral galaxies harbor younger, bluer stars. In the Coma Cluster and similar galaxy clusters, spirals live toward the edge, while the monster elliptical galaxies sit in the center.
The Coma Cluster, like the Local Group, is the end product of billions of years of cosmic evolution. For example, based on the ages of stars, we know the Milky Way is about 13.2 billion years old. We can’t see exactly how these objects looked when they formed, but the Universe itself can provide a kind of archeology.
As you’re reading this, it takes light about a billionth of a second to travel from the screen to your eyes. Sunlight requires around 8 minutes to reach Earth, and light from bright stars like Betelgeuse in the night sky can travel for decades or centuries. In other words, looking at the sky is literally looking back in time. That means the Coma Cluster we see isn’t how it appears now, but how it looked about 320 million years ago.
Even 320 million years is fairly short in cosmic terms, so astronomers hunt for even farther objects. The problem: the farther away a galaxy is, the harder it is to see. To observe a galaxy cluster, astronomers have to identify many galaxies at the same distance and be really sure they’re actually part of the same group instead of just a random association.
That’s why the announcement of the object known as JKCS 041 is exciting. (Yeah, another kind of license plate number.) At 9.9 billion light-years away, it’s the farthest galaxy cluster yet discovered. Astronomers first found it in 2006, but the distance is so large that it took many years of data to be sure all 19 large galaxies were really a cluster. That meant measuring the distance of each galaxy individually to determine that they were equally far from Earth and close enough together on the sky to be bound by gravity.
With that many bright galaxies, chances are there are a lot more in JKCS 041 too faint to see. That means the cluster is a good example of what the Coma Cluster may have looked like when it was much younger.
What’s interesting is how mature the galaxies in the cluster look, even 9.9 billion years ago. Like the Coma Cluster, the youngest stars are in galaxies toward the cluster’s edge, with very few new stars being formed in the central galaxies. In fact, JKCS 041 looks a lot like other clusters much closer to Earth!
That information tells us a lot about the evolution of galaxy clusters, if it turns out to be typical of others. Once star formation stops in a galaxy, the only way for it to get bigger is by merging with or eating smaller galaxies. JKCS 041 already looks like it has reached that point.
Galaxy mergers can go on as long as there are galaxies sufficiently close to each other. For example, astronomers expect the Milky Way to merge with Andromeda 5 billion years from now (give or take), making a giant elliptical galaxy once things quiet down again. As an early cluster, JKCS 041 will likely see its biggest galaxies grow by nomming on others. Its most valuable contribution to our knowledge, though, may be in showing that early galactic growth is fast, and that giant galaxies are born early.