Is M82 exploding? No, but it’s not exactly relaxing on a beach, either.

JWST images of the nearby galaxy show huge tendrils of material blasting away

May 27, 2024 Issue #726

Pic o’ the Letter

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

M82 is a nearby galaxy, about 12 million light-years away, and easily visible in small telescopes (the “M” in the name is a giveaway; that means it’s in the Messier (MEHZ-ee-yay) catalog of nearby bright objects). I’ve seen it quite a few times with my own ‘scope.

That also means that when you point something packing a mirror 6.5 meters (21 feet) across at it, you’re going to see detail.

So yeah, here’s what JWST sees when it looks at M82:

This one is a bit difficult to describe. In a black background, the galaxy is a long smear of white light, but it’s almost hidden by hundreds of red tendrils all pointing perpendicularly away from the galaxy plane, as if the disk of the galaxy is erupting. Because it is.

The galaxy M82 via JWST. Credit: NASA, ESA, CSA, STScI, A. Bolatto (UMD)

Whoa.

M82 is technically a spiral galaxy, even though the spiral arms aren’t obviously evident (they can be seen after heavy processing of infrared images). We see it almost edge on, so it looks like an elongated smudge 

But what the what is with those red tendrils?

Images taken by ground-based telescopes showed them even decades ago, though not in this kind of detail. They look like an explosion, don’t they? For a while, astronomers wondered if the galaxy itself was exploding, with perhaps dozens or hundreds of stars in the core going supernova (I remember reading science fiction author Larry Niven’s books in high school, and he mentioned this idea more than once).

However, there’s no known mechanism that can cause stars to explode all at once like that. Then another idea took hold, one we now know is correct: M82 is a starburst galaxy: It is undergoing a truly massive episode of star birth. It’s making new stars at a rate a dozen or so times faster than the Milky Way.

A wider view shows the underlying galaxy better, though still as a whitish smear. The tendrils can be seen radiating away from the galactic center.

Hubble’s view of M82, also showing the gassy tendrils. Credit: NASAESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (NSF).

That means a lot of those stars will be massive, and luminous. Stars like that drive powerful winds of material outward, so strong that they pick up the gaseous and dusty material between the stars like a snowplow. In the JWST image, the red filter is picking up light from long molecular chains of carbon called polycyclic aromatic hydrocarbons, or PAHs. They’re basically soot, created by stars as they begin to die, and in bucketloads when massive stars explode as supernovae. Normally they float around in interstellar space, but in M82 they’ve been swept up and are getting blown clear out of the galaxy by the newborn stars in the core.

By looking at those tendrils in detail astronomers can get a better understanding of just how those winds work, which in turn reveals the underlying star formation mechanisms. 

This image looks a little fuzzy but is in fact quite detailed. It’s only showing the inner region of M82; here’s a side-by-side view of the Hubble and JWST images, showing the latter’s narrow field of view: 

A side-by-side shot of the two images above, showing the JWST image is only a small portion of the galaxy’s center.

The Hubble image on the left with a box outlining the location of the JWST image on the right. Credits same as above.

As you can see the tendrils all point toward the galaxy’s core, where the starburst is occurring. But why did this burst happen at all. 

M82 is close in the sky to a gorgeous spiral called M81, and likely the two are involved with each other. It’s thought that they made a close pass around 200 million years ago, which jostled M82 pretty hard, gravitationally speaking, causing gas and dust to fall to the galaxy’s core, where it collapsed to form zillions of stars.

The JWST image has more in it than just those tendrils (you can get higher-res version here). Every white dot you see is either a star or a cluster of stars, and right at the center are lots of greenish dots; those are places where there’s lots of iron, so what you’re seeing there are in fact old supernova sites, the expanding debris rich with the heavy element created as the stars exploded.

The dark material is dust, which is actually pretty amazing. JWST sees in the infrared part of the spectrum, and that kind of light can pass through dust more easily than visible light can. In the Hubble image you can see lots of dust, which looks opaque. But you can still see dark dust clouds in the JWST image too, which means they must be thick. Otherwise it’d be transparent to infrared light. It’s unusual to see dust blocking the view in JWST images!

Starburst galaxies aren’t exactly common, but lots of examples are known. M82 is the closest to us, so astronomers study it a lot. Now that we can add JWST’s sharp view of it to those of other observatories, we’ll get to understand this even better, like where the winds originate, and how they interact with the material around it.

Billions of years ago the Milky Way had a much higher star formation rate too. I don’t know if it ever looked quite like M82, but studying it is like a glimpse into our own past, a history of how we got here. It’s one of the most amazing aspects of astronomy; that we can learn about that time so long ago. The more we look outward, the better we see inward.

Et alia

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