A cosmic collision twists a galaxy’s arm

August 21, 2023   Issue #606

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Where I’ll be doing things you can watch and listen to or read about

On August 27, 2023 (this weekend!) I’ll be heading back to Colorado: Specifically, Boulder, to give my “Under Alien Skies” talk at the “Spaced Out 2: Explore the Cosmos” event at the wonderful Chautauqua Auditorium. This is a pretty cool event with talks by astronomers on a bunch of different topics (like the upcoming solar eclipses, the origin of the Universe, and X-ray astronomy). My talk will focus on what it would be like to visit Saturn, describing what you’d actually see and experience if you could physically be there.

It’s a pretty fun talk, and I hope any Coloradan BANners in the area can come. It’ll be fun to be back in Boulder, especially in the same place I gave my TEDxBoulder talk a few years back.

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a description so you can grok it

I love a good galaxy collision (even if it means catastrophe on a very, very large scale). They’re so amazing to look at, the images beautiful, and the science behind them extremely cool.

I, of course, am a scientist and critical thinker, so I try at all times to back up my claims with evidence. BEHOLD.

This is NGC 1532, a galaxy about twice the size of the Milky Way (so, it’s big) and roughly 55 million light-years from us. It’s part of the Fornax Cluster, a group of a few dozen large (and who knows how many small) galaxies. NGC 1532 is nearly edge-on, which gives us an interesting perspective on it: The spiral arms are clearly messed up, with the one going across the center suddenly taking a left-hand turn on the right side — like a bully is literally twisting its arm.

That bully is the smaller fuzzy dude, called NGC 1531, just above the center of NGC 1532. It’s what’s called a dwarf galaxy, and while it’s small(ish) it still has billions of stars in it, and possessed of a decent gravitational pull. Dwarfs like this are commonly found as satellites around bigger galaxies; the Milky Way has two decent-sized ones and dozens of smaller ones.

Sometimes they get too close. That’s what’s happening here; NGC 1531 is crowding NGC 1532, likely passing it not too long ago in cosmic time (maybe some tens of millions of years…?). The smaller galaxy’s gravity yanked on that one errant arm, pulling it up and out of the bigger galaxy’s nominal flat disk.

I think that’s actually easier to see in this other image, taken using a 1.5-meter Danish telescope located in Chile:

This used slightly different filters and the image has lower resolution (meaning it’s a little fuzzier), which ironically makes it easier to see larger structures. The inner part of the big galaxy is redder/yellower, because the stars there are older than in the spiral arms (blue stars tend to die young, leaving longer-lived redder ones behind). The fact that the inner part of the galaxy can be seen beyond that spiral arm (extending a little bit farther downward in the image) implies to me the arm is being pulled up, otherwise the entirety of the arm would be lower down than the inner reddish disk. The 3D structure of this jumps out at me.

The lower arm looks affected, too; it extends way off to the right, farther than you’d expect for a nice, symmetric circular disk. Do you see all the clumpy blue knots in both arms? Those are sites where stars are forming in large numbers, where (I’m assuming based on lots of experience) great huge gas clouds have been disturbed by the interaction between the two galaxies, and are collapsing to form stars in huge numbers. When this happens you get lots of massive stars, which are blue and very luminous, so they dominate the glow of these stellar nurseries. Another sign this collision has galaxy-wide ramifications.

We still don’t understand everything that happens when two galaxies collide. The gravitational interaction itself is complex because the two galaxies have massive objects (stars, clouds) distributed throughout them, making the gravitational fields complicated, and then there are lots of secondary effects (like the above-mentioned star formation, or in some cases lots of material falling to the center to be greedily eaten by the supermassive black hole there) that can in turn affect what’s happening on a galactic scale.

The good news — for astronomers, at least — is that there are plenty of such collisions to observe, so plenty of examples to learn from.

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