A galactic collision makes for a weird and beautiful galaxy

May 15, 2023   Issue #564

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Pic o’ the Letter

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

Sometimes galaxies come in weird shapes.

Really weird shapes. Like VV 689:

OK, wut.

First, it looks like a squid, or maybe an octopus, with that bulbous body on top of the more wispy tentacles. It also reminds me of a Melkot from the original Star Trek series.

But if you look more closely you can see clues as to what’s going on. Ignoring the fainter material around them, in the “body” you can see what look like two spiral galaxies, if distorted. The one on the left looks kinda sorta normal, while the one on the right looks elongated, like someone grabbed it by the arms and pulled it apart.

And hey, that’s what happened, more or less.

Sometimes, galaxies collide. It actually happens pretty often, and any big galaxy today probably got that way by colliding and merging with smaller galaxies long ago. Heck, the Milky Way has a lot of as-yet not completely undigested galaxies still swirling around inside it.

When a big galaxy eats a much smaller one the big one is relatively unscathed, but the little one can be torn apart by tidal forces; basically, gravity gets stronger the closer you are to something, so as the smaller galaxy gets close enough to the bigger one, the gravity of the big one is much stronger on one side of the small galaxy than the other. This stretches it, and can tear stars away. Eventually the tiny galaxy gets stretched into a long stream of stars that loops around inside the bigger one, eventually dissolving completely, digested.

Got a friend who’s into squid-shaped galaxies? This issue is free so please share it!

But if both galaxies are about the same size then both get taffified by tides. It gets more complicated, too, if they pass by each other first, missing each other by a small distance. Both can have long streamers pulled out of them, called tidal tails, that curve around due to their motion. These can be huge, much longer than the galaxies originally.

Sometimes gas clouds in the tidal tails collide, and they collapse under their own gravity and form stars. Lots of stars. A small fraction of the stars are massive, luminous, and blue, and this makes the tidal tail look blue (redder stars are generally speaking much fainter so we don’t see them as well).

Now look again at VV 689. You can see both galaxies flung out a pair of tidal tails, and the pattern is very nearly symmetric. The tails at the upper and lower right are decidedly blue and emanate from the galaxy on the right, which I’m guessing had a lot more gas in it to form stars with.

I’m actually amazed at the detail. They’re about 700 million light-years from us. That’s a long walk — the light we see from it left the pair before the Cambrian Explosion, if that helps — so it’s neat to see so much going on here.

This odd object was discovered decades ago but mostly unexplored. But it was found by citizen scientists in the Galaxy Zoo project — something I’ve written about many times. The software displays images of galaxies and allows anyone who signs up (after some brief and simple training) to tag and note characteristics about the objects. It’s fun and easy, and you never know, you might something extraordinary, like VV 689.

Bill Keel is a professional astronomers and part of Galaxy Zoo; he targets the most interesting galaxies found (decided by participants’ votes) with Hubble, which is how we got this spectacular shot of VV 689. The image is actually a combination of Hubble, Sloan Digital Sky Survey, and DECam observations. Quite a mix.

I suggest you grab the highest-res version (3,663 x 2,653 pixels!) from the ESA Hubble Flickr page. It’s pretty danged cool, and makes for a good desktop background.

Et alia

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