An ethereally glowing and seriously weird-looking disk galaxy

I do love it when Hubble takes a peek at a disk galaxy

May 9, 2024 Issue #719

Pic o’ the Letter

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

I do love an odd galaxy. They come in all shapes and sizes, from smeared-out irregulars to huge cotton-ball-shaped ellipticals to grand design spirals.

Then there are those that are just weird, like NGC 6684. 

In a black background sprinkled with stars and a few small, distant galaxies, NGC 6684 is seen as a tilted fuzzy ring around a thick rectangular glow, almost like a TIE fighter form Star Wars. It has an extended fuzzy halo around it.

NGC 6684, seen by Hubble. Credit: ESA/Hubble & NASA, R. Tully

NGC 6684, shown here in an image by Hubble Space Telescope, is relatively nearby to us, at 35 million light-years. It’s classified as an S0 galaxy, which means it’s a disk galaxy like the Milky Way but shows little or no evidence of spiral arms. It’s also called a ring galaxy, for obvious reasons. Not only that it’s a barred ring galaxy, because of that long rectangularish structure inside the ring, making the galaxy look a bit like Darth Vader’s TIE fighter.

The bright nucleus is interesting; I found some reference to this also being a Seyfert galaxy, which means it has a supermassive black hole in its heart that’s eating voracious amounts of matter. As that material piles up outside the black hole’s event horizon it heats up and glows ferociously, creating the bright point dead center. Galaxies like this are called active.

When I first saw this image I was taken aback by how cool it is, and how beautiful. But then I started wondering… why don’t we see any real detail? No stars, no gas clouds, no dust lanes. One reason is that S0 galaxies don’t have a lot of gas and dust, sometimes none at all. Usually this is because they’re members of a cluster of galaxies. There’s material in clusters between the galaxies, thinly strewn gas, and if a galaxy in the cluster is barreling at high speed through this junk all the gas and dust inside the galaxy gets stripped away by a process called ram pressure. As I’ve written many times before, it’s the same thing that happens when you roll down the windows of your car on the highway to clear the air because the dog farts.

Not elegant, I know, but the imagery is vivid, and the physics pretty similar.

The second reason this image looks smooth and relatively featureless is because of the filters used. The observations were made as part of a project to get distances to some of the nearest galaxies to us in the Universe (out to roughly 35 million light years). There are lots of ways to do this, and in this case the astronomers were using the “tip of the red giant branch” method. As stars age and die, they swell up and cool off, becoming red giants. There’s a theoretical limit to how bright they can get, so if you can see these in distant galaxies, you can compare how bright they appear versus how luminous they actually are, and get the distance (basically, applying math to the idea that bright things get dimmer with distance). 

To look for these red stars the astronomers used red filters (one that actually lets red light through, and another that’s in the near infrared, just outside what our eyes can detect). At those wavelengths the light is dominated by old red stars, and we don’t see much gas or dust. This makes the galaxy look smooth — the stars are fairly evenly distributed.

I poked around and didn’t find much info on this galaxy in the literature, and no high-resolution images besides this one. That’s too bad! I’m curious what it looks like in bluer wavelengths; is it still smooth? How bright is the ring and the nucleus in other colors?

Still, if you have to have only one sharp-eyed image of this gorgeous nearby galaxy, there are worse ones to have than ones using Hubble.

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