A real problem with astronomy is knowing where stuff is. By that I mean we don't always have distance information on an object, so we don't know if it's close by or really far away. This gets even worse when you have two objects near each other in the sky, or even overlapping. If one is big and one small, it's easy to think that the small one is part of the big one. And sometimes that's really really really not the case.
Let me introduce you to an interesting little example of this: the cosmic object LGGS J004527.30+413254.3. That stands for Local Group Galaxy Survey and the object's coordinates in the sky, so let's call it J0045 before I wear out my keyboard.
The Local Group is a small collection of nearby galaxies, of which our Milky Way is one. Another is the Andromeda Galaxy, a magnificent spiral that is very close in size and shape to our own. The two of us dominate the group, in fact.
J0045 was (re)found by a team of astronomers interested in looking for a very rare object … in fact it's so rare it's never been seen in the wild: A red supergiant star being orbited by a neutron star or black hole. Red supergiants are massive stars that are near the ends of their lives and have swollen up to huge dimensions. If they have one of these compact objects orbiting them, material can be drawn off the star, fall onto the neutron star or black hole, heat up to ridiculous temperatures, and emit X-rays.
If such an object exists, it's easiest to look for one in our own neighborhood, so they searched the LGGS database and found J0045. It sits in the outskirts of the Andromeda Galaxy, the quieter suburbs, if you will. Not exactly where you'd expect a gigantic star (they tend to be very close to active star-forming gas clouds, and there aren't any close by to J0045), but still interesting. In optical light (the kind we see) it kinda matched the characteristics they were looking for, so the next step was to check X-ray data and see if there was anything there.
And there was! The orbiting Chandra X-ray Observatory showed a source essentially on top of the position of J0045. Nice! Looking good.
… until they looked more carefully. The X-ray data didn't match a red supergiant getting slowly eaten by überdense compact dead star. Instead, it looked more like what you'd expect if J0045 was actually what we call an active galaxy: a distant galaxy with a supermassive black hole in its center. If that black hole is gobbling down matter, it will shine very brightly in optical and X-ray light. A coincidental overlap of an active galaxy and Andromeda wouldn't be too surprising. Andromeda is huge and eats up a lot of real estate on the sky. Statistically speaking, you expect to see about 200 background active galaxies in that same area of sky.
So which was it? Something in Andromeda, or behind it? There's a good way to find out: Take a spectrum. By breaking the light up into thousands of individual colors, important information can be gleaned, like the distance to the source.
So they did. And they got a surprise: J0045 is not local. Like really not local: It's about 2.7 billion light-years away, a thousand times farther away than Andromeda!
And that, in turn, means J0045 is photobombing Andromeda. Awesome.
But holy yikes, that's a long way off. This is like seeing what looks like a bush in your neighbor's yard across the street, then finding out it's a mountain 30 kilometers away!
And there's more. The light from J0045 wasn't steady, but instead got brighter and fainter over a somewhat regular period. Closer examination indicated this object may be very special indeed: It looks like two supermassive black holes orbiting each other. Binary supermassive black holes!
To be clear, it's not 100% certain that's what J0045 is, but the evidence is pretty interesting. If so, it's most likely two galaxies colliding. When this happens, it's a cosmic train wreck. The huge gravity of each galaxy rips the other one up. We know all big galaxies have supermassive black holes in their cores, and the collision can disturb a huge amount of gas and dust, sending it down into the centers where those black holes can eat it. When the material heats up it gets incredibly bright, creating the active galaxies.
If this is indeed the case, the changing brightness means the two black holes are orbiting each other. They may be pretty close together, too, as close as 30 to 80 billion kilometers apart. That's only a few times bigger than our solar system! On a galactic scale, that's scraping each others' edges.
If so, they may also be giving off gravitational waves strong enough to be detectable using various methods starting to come online (like using pulsar timing, which is something I've been meaning to write about for a while; I've made myself a note to do so).
And no matter what, the fact that J045 is shining through the Andromeda galaxy is helpful, because it allows us to study the junk floating around in our neighbor. It selectively absorbs some of the light from J0045, and that tells us what it's made of. This discovery is a win all around.
And it's also just amazing. Two galaxies collide, two monster black holes orbiting each other, both gulping down huge amounts of material and causing them to shine, lying in the sky just so that we see them overlapping a spiral neighbor … and all this was found because astronomers were looking for something else!
As something of a photobomber myself, I know they tend to be happy accidents. This may be the happiest of all of them.