Abell 1142 is a cluster of galaxies located at a distance of 502 million light years (154 Mpc) [1] in the constellation Leo. The cluster is named after George Abell, who basically found clusters of galaxies by staring for extremely long times at photographic plates of the sky and looking for places that contained too many galaxies. The discovery of this cluster was first published in 1958 [2].
Abell 1142 looks a bit strange compared to the typical cluster of galaxies. A gravitationally stable cluster will typically consist of a spherical distribution of galaxies that all orbit around a single central point. The center of the cluster is typically occupied by a very large galaxy, typically a very large elliptical galaxy, that is referred to as the brightest cluster galaxy. This galaxy would have most likely formed from when two or more smaller galaxies merged together. The cluster of galaxies also contains some very hot X-ray emitting gas in between the galaxies and some dark matter that helps to hold everything together.
So that's the typical structure of a relatively stable cluster. However, Abell 1142 stood out right away as a bit weird becuase, based on some people's perscpectives, it had no brightest cluster galaxy [3]. The center of the cluster is just occupied by gas. Other people, however, have indicated that the cluster actually had two brightest cluster galaxies [3]. One of these is actually a pair of interacting galaxies named NGC 3492 [4]. The other is IC 664 [5], which is a lenticular galaxy, or a disk-shaped galaxy with no spiral arms and a really big bulge.
It turns out that Abell 1142 is not really stable at all but seems to consist of two separate clusters of galaxies, one centered on NGC 3492 and the other centered on IC 664, that are in the process of merging [1, 3]. In fact, roughly half of the galaxies seem to be moving at a velocity that is 1500 km/s different from the other half of the galaxies [1].
What is a bit weird about all of this is what is happening with the gas in the center of Abell 1142. That gas in the center is actually cooler than the rest of the gas in the cluster [1]. In a stable cluster of galaxies, cool gas would be expected to sink to the center of the cluster in the same way that cool gas would sink in the Earth's atmosphere, but given that Abell 1142 actually consists of two clusters merging together, the explanation of how the cool gas got into the center is a bit more messy and complicated, and quite frankly, I don't think anyone has really figured this out yet.
What's even weirder is that a lot of elements heavier than hydrogen and helium have accumulated in the center of Abell 1142 [3]. These are elements that could only form either inside stars or in nuclear processes that take place when stars die and eject their outer gas layers. The gas near the centers of many other clusters of galaxies contain lots of heavy elements, but those clusters also contain brigtest cluster galaxies in their centers, and those galaxies contain stars that people thought could have been the source of those heavy elements. In contrast, because the center of Abell 1142 does not contain a brightest cluster galaxy, it doesn't have the stars that could have produces all of the heavy elements found there.
Therefore, Abell 1142 specifically is potentially proof that the heavy elements in the gas within the center of a cluster do not need to have come from primarily the cluster's brightest galaxy [3]. Those elements could have come from the other galaxies as well and, over time, made their way into the center of the cluster [3]. However, like I said before, it's not quite clear how the cold gas at the center of Abell 1142 got there in the first place, and one possibility is that it blob of cold gas used to be associated with the brightest cluster galaxy in one of the two smaller clusters that have merged to form Abell 1142 [3], although it's not really clear to me (and I suspect it's not clear to anyone else either) how that could have happened. I look forward to reading about whether people can figure out exactly how the center of Abell 1142 formed or whether they can find another cluster like this one.