Big galaxies get large by two main methods: they are born out of the gravitational collapse of a lot of matter, and they grow by eating or merging with smaller galaxies. The gravitational collapse phase happened very early in the history of the Universe, so early that we have trouble seeing the first galaxies that formed. However, we’re assisted by the fact that the largest galaxies don’t form in isolation, but are born in galaxy clusters. An observation of a very early cluster shows that the gravitational collapse part of galaxy growth stopped before the time we’re witnessing, leaving a very modern-looking galaxy cluster 9.9 billion light-years away. My latest for The Daily Beast has the story:

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. [Read more…]