If there’s one place in the Universe I’d rather not be, it’s sitting in a clump of nearly a dozen galaxies with supermassive black holes in their cores actively gobbling down material and spewing out vast beams of matter and energy at the tippy-top end of the electromagnetic spectrum. 

Normally, I’d say you’d be pretty safe from such a scenario. Getting that many quasars, as they’re called, together in one region of the cosmos would have pretty long odds.

Yet that’s exactly what a team of astronomers found: 11 decently bright quasars all sitting in single area about 40 million light-years across [link to journal paper]. That’s a pretty big chunk of real estate, but quasars are rare enough that finding 11 of them there is incredibly unlikely. The astronomers calculate that the chance of them randomly all being that close together is about 1 in 10⁶⁴ (that’s a 17-sigma discovery, for you mathophiles). In other words, if you flip a fair coin and have it come up heads 200 times in a row, that’s a thousand times more likely than this quasar cluster existing by random happenstance.

So clearly something else is going on. But what?

They found the quasars in data taken using the fourth phase of the Sloan Digitial Sky Survey (or SDSS-IV), which mapped the sky from 2014 – 2020, taking images and spectra of millions of objects. The purpose in part is to map the locations of galaxies on the sky, then use the spectra to measure their redshifts and get their distances. This produces a 3D map of the sky out to billions of light-years! That’s a massively useful database, since the structure and behavior of the early days of the Universe left its imprints in the locations of galaxies now, so we can learn about that terribly ancient history by examining those galaxies.

Every big galaxy has a supermassive black hole at its very center, with millions or billions of times the mass of the Sun. As material falls in its heats up and generates vast amounts of energy, even outshining all the stars in the galaxy. This produces a spectrum that’s pretty easy to spot, so SDSS-IV is perfect to map them. That’s how the astronomers IDed the quasars in the first place. 

They’re all close together on the sky, and also all about 10.7 billion light-years from us (or, to be more accurate, their light took that long to reach us), so we see them as they were when the Universe was about 25% of its current age.

They then turned to a survey of the sky (taken using the immense 8.2-meter Subaru in Hawai’i) called MAMMOTH, which looked for big regions of the Universe that have lots of hydrogen; these are commonly associated with clusters of galaxies. What they then found was even more surprising: there appear to be two clusters, one on each side of the quasar clump, and the clump was in between them, oriented roughly perpendicular to the line connecting them.

What the what?

My first expectation would be to find the quasars inside a galaxy cluster, because a) there are lots of galaxies there, so some of them being quasars is somewhat expected, and 2) galaxy collisions are common in such environments, and that can trigger an infall of material that feeds the black hole and trigger a quasar.

Instead, the quasars are between the two clusters. So weird.

When I saw the graphic showing the quasars, though, I had a second thought: Maybe these were two clusters that collided! That can happen, and when it does the clusters can pass through each other. The galaxies get stirred up a bit but continue on, like two clouds of gnats flying through each other. On the other hand, the gas in between the galaxies of one cluster slams into the gas in the other cluster other, losing momentum and skidding to a halt. That might somehow trigger quasars, maybe kinda sorta.

Sure enough, there does appear to be a lot of gas in between the clusters, where the quasars are. But that doesn’t answer the question of how the quasars all got there in the first place. They’re separated from the cluster centers by roughly 25 million light-years, which is a long walk indeed. I don’t think they were left behind after the collision, since, like the other galaxies, they should sail on. Maybe they were normal galaxies minding their own business when the two clusters collided right where they happened to be, and in the turmoil and lost gas they switched on, becoming quasars. That seems pretty unlikely though.

It’s not at all clear, and while the astronomers do mention a possible collision scenario (which made me happy, since I had the same thought) they don’t have a good explanation for all this. So, ¯\_(ツ)_/¯.

I love a cosmic mystery, and this is a good one. It’ll take a lot more observations to figure it out; perhaps mapping the gas distribution better will yield clues, or deeper images to see if there are normal galaxies in and among the quasars, say. Beats me; I’m just speculating. But for now it’s yet another case of the Universe throwing us a pretty decent curveball from billions of light-years away.

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