[Note: Yesterday’s article was on a bummer topic, so here’s a unicorn chaser for you. Because of all that, I’ve also made this issue freely available to everyone as well.

Look, I love me a galactic collision. The scale is enormous, with two vast beasts approaching each other over millions of years, mutual gravity pulling both into weird shapes, then sometimes merging to form a single bigger galaxy. The science is cool, and the images lovely.

But there’s another kind of collision, where a smaller galaxy barrels through a bigger one. If it happens off to the side you can get all kinds of weird shapes, but sometimes, rarely, the smaller one aims true, and hits the bigger dead center. What that happens you still get something weird, but also spectacular: a ring galaxy. These are rare, and usually have a small clump of stars in the center surrounded by one or two rings some thousands of light-years farther out. The Cartwheel Galaxy is the iconic example.

Sure, but then there’s LEDA 1313424. What looks like a spiral galaxy with, admittedly, some extra structure, turns out to be far more than that: It’s a ring galaxy with nine rings total! And the image of it is so spectacularly beautiful I actually gasped out loud when I saw it.

Feast your brains on this:

LEDA 1313424, as seen by Hubble. Click to galactinate for the incredible 3,800 x 3,600 pixel image. Credit: NASA, ESA, Imad Pasha (Yale), Pieter van Dokkum (Yale) 

HOLY WOW!

This image is a combination of two Hubble images, one using a blue-green filter (475 nanometers, shown in cyan) and the other deep red (814 nm, shown in orange) [link to journal paper]. Usually, images taken in two filters like this tend to smooth out the structure in spiral galaxies, since it downplays the dust and the clumpy hydrogen gas that fills the arms, but in this case there’s a lot to see, and it’s amazing.

First, you can see the inner rings looking yellowish-red. The center of a spiral galaxy, called the bulge, tends to be made up of older stars, since the gas used to make stars ran out long ago. Blue stars are massive and don’t live long, so all that’s left in the bulge are the lower-mass and longer-lived redder stars. You can see the central elliptical glow of these older stars, surrounded by quite a few rings of red stars.

The star-forming gas of a spiral lies in the disk surrounding the central bulge, and here you do see the bluer stars, indicating recent star birth. There’s both spiral and ring structure here. And if you look much farther out, to the upper right, there’s another, fainter ring, much broader than the inner ones. 

So what’s going on?

When the smaller galaxy (sometimes called the bullet) screams through the center of the bigger galaxy (the target), the gravitational interaction creates waves that move through the target’s disk, not too dissimilar to waves you see in a pond when you toss a rock in it. In this case these are density waves, expanding waves that compress the material they pass through. If there’s gas, it gets squeezed and sometimes swept up by the wave, which triggers lots of star birth as the gas clouds collapse. This is really obvious in the Cartwheel Galaxy.

Ring galaxies are rare because so few collisions are not only head-on but hit the bulls-eye of the target (the astronomers who took these images nicknamed LEDA 1313424 the Bulls-Eye Galaxy). The ring-forming stage doesn’t last long compared to the age of the galaxy, too, so we don’t see it often. Also, they’re usually far away from us and small, so they’re hard to study. On top of all that, most have only one ring, with some having two. None has been seen with more… yet this one has nine. Nine.

LEDA 1313424 is fairly close to us as galaxies go, about 540 million light-years, so it appears in our sky big enough to study with Hubble. That may also be why we see so many rings; if it were a lot farther away they might blur together, and we’d see fewer.

This galaxy is about the same size as the Milky Way, 100,000 or so light-years across, except for that outer ring, which is a staggering 230,000 light-years out. That’s a long way.

Ring galaxies like this are important to study for a few reasons. The ring structure is sensitive to the galaxy structure before the collision, so it helps us understand how galaxies are built. Also, a dead-center collision is easier to model analytically (that is, using equations of physics starting from simple principles like how gravity works, how stars move, and so on). The astronomers studying the galaxy used some pretty simple assumptions to predict how the rings would look, and got decent fits to the actual rings seen. That gives more confidence that the equations are correct, and makes it easier to modify them for more complex interactions (like off-center impacts).

So which galaxy was the bullet? There are several seen in the image that could be the culprit. But my eyes were drawn to the one directly to the left of the big spiral’s core. It’s blue, which indicates recent star formation, and patchy as well which you’d expect if it got its bell rung by passing through a bigger galaxy. Turns out it’s at about the same distance as LEDA 1313424, and moving away from it at about 2.5 million kilometers per hour. It’s very likely our bullet. 

That would make the pair about 130,000 light-years apart, and would time the collision to about 56 million years ago. Galaxies live for many billions of years, so again these rings are fairly ephemeral.

One other thing. There’s another kind of galaxy called a giant low-surface brightness galaxy. These are very large spirals, but unlike the Milky Way or the way LEDA 1313424 looks now, these are very faint and difficult to see. One example is called Malin 1, a galaxy I’ve had my eye on for many years. Malin 1 is an immense 500,000 light-years across, far larger than our own galaxy, and in fact so big that our ideas of how galaxies form have a difficult time explaining it. 

But it’s been thought that ring galaxies can eventually form these giant low-surface brightness galaxies. As the rings expand they make the galaxy bigger, and spread the stars and gas so much that the brightness of the galaxy fades. Some of the characteristics of LEDA 1313424 are similar to those of the fainter giant galaxies, so this supports the idea that one forms the other. That’s super cool. It also means we have a better understanding of how galaxies evolve over time.

That’s usually the case. We sometimes see weird, extreme galaxies that seem to defy explanation, but then it turns out they have a formation channel and we just haven’t figured it out yet. In this case though it looks like we have!

And wow, what a beauty. I hope more galaxies like this one are found. It would really help the science, and I never get tired of looking at magnificent galaxies like it.

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!