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JWST’s view of the magnificently dusty skeletal arms of the spiral galaxy M51
Creepy, or beautiful? Why not both?
September 5, 2023 Issue #613
Pic o’ the Letter
A cool or lovely or mind-bending astronomical image/video with a description so you can grok it
M51 is a nearby spiral galaxy that we happen to see face-on. As is the case with a lot of nearby galaxies, the exact distance isn’t well known; it’s roughly 20 – 30 million light-years away. Different methods yield different values for its distance, and it’s not far enough away, ironically, for the expansion of the Universe to be strong enough to use redshift reliably.
Still, it’s close enough that it can be seen with binoculars from a dark site, or with a small telescope from mildly light-polluted areas. Despite that I was never able to see it through the eyepiece with my own telescope when I was growing up, frustratingly. I eventually did when I had access to bigger ‘scopes and darker sites, which was a wonderfully moving experience. Seeing another galaxy’s spiral arms with your own eyes, even faintly, is pretty danged amazing.
Now imagine what you’d see if you take a truly huge 6.5-meter telescope, launch it into space, and aim it at M51 for a little while.
Oh wait, you don’t have to imagine it. I can show it to you.
HOLY FLURKING WOW.
That ridiculously spectacular image is, of course, from JWST. In this case, taken by its Mid-Infrared Instrument, or MIRI. These are infrared colors with wavelengths much longer than the eye can see. This image is a combination of two observations using different filters, specifically 5.6 microns (shown in teal) and 7.7 microns (in yellow and red). The longest wavelength our eyes can detect is about 0.75 microns, so these are out there, electromagnetic spectrally speaking.
The 5.6-micron image is broadband, meaning it can see a wide range of wavelengths, so it sees things that emit light in a broad range of wavelengths: stars. If you look carefully, you’ll see that individual stars look teal, confirming that conclusion. The center of the galaxy has lots of old, red stars, and these give off a lot of infrared light, so they look teal in the image. I know, that’s weird, but when you take observations in light we can’t see you have to make some adjustments to how you think of colors. We might display things as blue that would actually look red to our eyes in real life; it’s all infrared anyway and we need to pick some way to show it! We tend to use blue for shorter wavelengths and red for longer in images, which at least makes sense since blue has a shorter wavelength than red.
Anyway, the 7.7-micron filter is also broadband, but the longer wavelengths are preferentially emitted by long molecules of carbon called polycyclic aromatic hydrocarbons, or PAHs… or what astronomers tend to just call galactic dust. PAHs are created when massive stars start to die, blasting huge quantities of carbon into space (which is what Betelgeuse did recently which caused it to fade dramatically). Exploding stars make them, too.
These molecules collect into huge clouds that trace the magnificent spiral arms in galaxies like M51, and that’s pretty evident in this image! The cause of the spiraling is complicated and, in fact, still argued about amongst astronomers, though it’s likely to be due to something like a moving cosmic traffic jam — that’s been the assumed hypothesis for decades.
Whatever the cause, dust (as well as gas clouds) piles up in the arms. These can collapse to form stars, and when that happens the stars warm up the dust, making it glow more fiercely. So in the MIRI image the redder dust is where it’s relatively cold and just floating in space, while the yellower regions are where stars are actively forming.
So, as you’d expect, the dust that appears yellow in the image makes up the major arms of the galaxy, and the redder stuff lies in between the arms. The feathery, webbed structure to it is incredible, just astonishingly beautiful. It reminds me of delicately rotted wood in a fallen tree, which is perhaps a bit of a jarring analogy but hey, I see what I see.
You can actually see this dichotomy between the arms and the space between better in the JWST NIRCAM image, which looks at infrared light with shorter wavelengths:
The filters used in this image run from 1.1 microns out to 4.4. Hydrogen glows at many of these wavelengths, so gas clouds are emphasized here. Gas tends to clumps up more in the arms, and glows when it’s warmed by active star formation (which happens in the arms) so the arms really pop here (seen in red). Again, old stars appear bluer in this image, and those are prevalent in the galaxy’s center.
If you compare the two images, the yellower dust in the MIRI image overlays on the obvious major arms in the NIRCAM image, just as claimed. Science! There’s also a composite image of the two, if you’re interested in seeing what it looks like when they’re combined. Even better, there’s a page with a slider tool so you can go back and forth between them. Very cool.
Before JWST, near-infrared images taken by Hubble had the highest resolution ever observed. But JWST has a much bigger mirror and its instruments are designed from the start to work best in IR, while Hubble is more tuned to visible light, the kind we see. So these show much more detail! If you go to the main page for these observations there are links to huge versions, 8,100 x 2,700 pixels in size, you can download. I suggest you do. They’re absolutely jaw-dropping, and you can get a sense of the huge job astronomers have trying to make sense of the intense and complicated structures galaxies display for us.
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