Its generally said that discoveries in science tend to be at the thin hairy edge of what you can do always at the faintest limits you can see, the furthest reaches, the lowest signals. That can be trivially true because stuff thats easy to find has already been discovered. But many times, when youre looking farther and fainter than you ever have, you find things that really are new and can (maybe!) be a problem for existing models of how the Universe behaves.

Astronomers ran across just such thing recently. Hubble observations of a distant galaxy cluster revealed an arc of light above it. Thats actually the distorted image of a more distant galaxy, and its a common enough sight near foreground clusters. But the thing is, that galaxy shouldnt be there.

This picture is a combination of two images taken in the near-infrared using Hubble. The cluster is the clump of fuzzy blobs in the center left. The small square outlines the arc, and the big square zooms in on it.

The cluster is unusual. Its at a distance of nearly 10 billion light years away. Clusters have been seen that far away, so by itself thats not so odd. The thing is, its a whopper: the total mass in all those galaxies combined may be as much as a staggering 500 trillion times the mass of the Sun, making this by far the most massive cluster seen at that distance.

But that arc First, things like this are seen pretty often near clusters. Theyre gravitational lenses: the gravity from the cluster bends the light from a more distant galaxy in the background, bending its shape into an arc. See Related Posts below for lots of info and cool pictures on these arcs. In this case, Ill note the shape of the arc implies the biggest galaxy in the cluster, the one right below the small square, is doing most of the lensing.

But heres the problem: the galaxy whose light is getting bent has to be on the other side of the cluster, and that cluster is really far away. Note only that, the galaxy has to be bright enough that we can see it at all. Combined, this should make an arc like this rare. Really rare.

So rare, in fact, that it shouldnt be there at all! The astronomers who did this research worked through the physics and statistics, and what they found is that the odds of seeing this arc in this way are zero. As in, what the heck is it doing there at all?

Now we have to be careful here. What we have is one observation of one arc, and it happens to be behind an extraordinarily massive cluster. Its hard to extrapolate exactly what this means. Maybe galaxies formed more vigorously than we thought in the early Universe, so there are more than we might suppose. Maybe its a huge coincidence, with a bright galaxy behind a massive cluster. Maybe the galaxy in the cluster doing most of the heavy lifting is surrounded by more than the usual amount of matter, making it an even stronger lens. Interestingly, using the arc itself, astronomers calculated the mass of that one big galaxy is something like 70 trillion times the mass of the Sun, making it bigger than most entire clusters at that distance!

If you get one weird thing happening, you might be able to shrug it off as coincidence. But two? In this case the existence of the arc at all coupled with the huge mass of this galaxy and cluster make me think theres more going on here than we see. Still, its not clear what it might be.

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