A massive galaxy cluster with massive galaxy collisions

September 11, 2023   Issue #615

Subscribers are the gravity that keeps my social cluster bound 

Pic o’ the Letter

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

Who doesn’t love a cool galaxy cluster?

Galaxies are immense structures, usually at least tens of thousands of light-years across (our Milky Way is 120,000) and have billions of stars, if not over a trillion.

As big as they are, I know something bigger: clusters of galaxies, which can have from dozens up to many hundreds of full-size galaxies in them. Like star clusters, they’re held together by their own gravity; each individual galaxy contributes to the overall gravitational field, holding them all in sway as they orbit around their mutual center of mass. And that mass is huge, up to a quadrillion times the mass of the Sun.

And, as you may come to expect when I have an intro to something like this, they’re just spectacular to look at.

That is the cluster Abell 3754, imaged by DECam and processed by Roberto Colombari, a telecommunication engineer, amateur astronomer, and amateur image processor. He’s really good at putting images together; I’ve written about his work many times before.

DECam is a huge camera on the 4-meter Victor M. Blanco telescope in Chile. It’s purpose is to map lots and lots (and lots) of galaxies, getting their distances and helping astronomers try to figure out what the heck dark energy is (spoiler: ¯\_ (ツ)_/¯). BTW DECam is a 520 megapixel camera, so yeah. Kinda big.

The galaxy cluster Abell 3754 is about 200 million light-years away, so a bit of a hike. I’m not sure how many galaxies are in the cluster; I poked around the journals but didn’t see a definitive count. Certainly hundreds.

Everyone likes a huge cosmic collision, so why not share this post with a friend? It’s free, so they can just take a look and enjoy this scientific disaster movie with you!

Despite its distance the cluster eats up a decent amount of real estate on our sky, easily the size of the full Moon. That means it’s at least 2 million light-years wide, and probably much larger.

The image is pretty mind-blowing. I had to shrink it both in size and filesize to be able to put it in the newsletter, so if you want the whole shebang Roberto has a 7,800 x 6,000 pixel version on his site.

The image looks a little washed out to my eye, so I wasn’t surprised to find out it was taken in only two filters; “true color” images, ones that more closely mimic how our eyes see, use three filters. The two filters used here let through blue-green light (shown in blue) and near-infrared (just outside the reddest light our eyes can see; shown in red). So the color depth isn’t as rich as you might expect, but that’s not the point of this image! Astronomers use filters to select for information they need to determine things like what kinds of stars are in a galaxy, not (usually) how pretty the image is. And in most cases the image is pretty anyway.

A lot of things jumped out at me. The first was the honkin’ big galaxy to the upper left. That’s IC 4329, a giant elliptical and the biggest galaxy in the cluster. You can see that it’s surrounded by huge rings of light, and I do mean huge; that edge-on spiral next to it (which I’ll get to in a sec) is the size of the Milky Way! Those rings are actually shells of stars, likely ejected when IC 4329 collided with other galaxies. As a biggish galaxy falls in, the energy of its motion can be converted via gravity into a huge push on stars around it, blasting them out of the galaxy at high speed like shrapnel. You get a huge pulse of stars flung out, creating a shell of them. This has likely happened multiple times, which is why there are several shells. They’re pretty common around elliptical galaxies, occurring in about 10% of them.

The edge-on galaxy is IC 4329A, likely a spiral similar to the Milky Way. But only a little similar: a big difference is that IC 4329A is blasting out X-rays, just pouring them out of its center. It’s one of the brightest X-ray sources on the sky despite being so far from us. This is what’s called an active galaxy. Every big galaxy has a supermassive black hole in its center, and sometimes these gobble down matter. This stuff piles up into an accretion disk, and gets incredibly hot, so hot it can outshine entire galaxies. In this case that black hole is a true glutton. When we see a galaxy edge-on we are looking through a lot of gas and dust in the disk — you can see that as the dark material splitting it in two — yet those X-rays are still screaming out.

That’s a lot of power. Just the center of that galaxy is glowing with the energy of a trillion times the energy of the Sun. A trillion. Yegads.

Another obvious feature in the cluster is that weird mess over on the right. That’s NGC 5291, another victim of a cosmic collision, a train wreck between two galaxies (the interloper is likely that weird earlobe-shaped seashell thingy below it). Gas can be flung out as well as star from the collision, and you can see long clumpy “tidal tails” that really do look like something violently thrown out from the galaxies. This material formed stars, including massive, hot, blue stars, which are so bright their light dominates, making the material look blue. The collision may have happened over 350 million years ago, and so many stars formed from it that the clumps are classified as dwarf galaxies in their own rights. Wow. 

And of course there are galaxies galore everywhere you look in this image. If you grab the high-res version you’ll see a lot of amazing things, including thousands of small background galaxies. But that’s the point: The more of these things we see, the better we can map the Universe, and hopefully the better we can understand it. That’s the purpose of DECam, and, in a very real way, the purpose of astronomy itself.

Et alia

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