The James Webb Space Telescope released a stunning image of the Cartwheel galaxy.

It shows what appears to be the aftermath of a cosmic hit and run.

The galaxy was once most likely a spiral, much like our home galaxy, the Milky Way.

But something, probably a small compact galaxy smacked a dead on, sending out a powerful

shockwave that obliterated its spiral structure and set off a wave of star formation that

continues to this day.

The result is a dramatic example of destructive creation.

The cartwheel is located about 440 million light years away in the constellation sculptor.

It's about 150,000 light years in diameter, so a little bit larger than the Milky Way.

It was discovered by Fritz Zwicky back in 1941.

Even though he never had images as sharp as Hubble's or Webb's, Zwicky described the Cartwheel

as having "one of the most complicated structures awaiting its explanation on the basis of stellar

dynamics."

In other words, he wanted to know just what the hell happened to this galaxy!

He wasn't alone, of course.

Lots of astronomers have been wondering the same thing.

That's why the Cartwheel has been observed extensively, and not just in visible light,

but in X-rays, ultraviolet, and even in the infrared with the Spitzer telescope.

But being a small telescope, it was hard for Spitzer to get a really good look at the interior

of the Cartwheel.

So composite images like this have varying levels of detail, and we can see that there's

still some information that's being hidden by all of the dust that was stirred up in

the galaxy.

But now with Webb, we can see through much of the intervening dust at high resolution.

Now we can see individual stars and star forming regions in incredible detail.

This image is a composite of Webb's Near Infrared Camera and its Mid-Infrared Instrument.

The colors were chosen such that the shorter wavelengths are blue and the longer wavelengths

are red.

So together they give us like a pink rosé Cartwheel because of the colors that the imaging

team just happened to use.

But if we consider the images from the two instruments separately, we can begin to see

different structures within the galaxy.

NIRCam shows us where the stars, the star forming regions, and star clusters are located,

while MIRI shows the cooler dust lanes and how they make up the overall structure of

the galaxy.

And notice that in the MIRI image, the colors were once again remapped to bring blue into

the shorter end of the mid-infrared spectrum.

The Cartwheel's weird shape makes it a rare type of galaxy called a ring galaxy.

It's thought that ring galaxies, like the Cartwheel, used to be a flat spiral disc galaxy

that underwent a head on, or nearly head on collision with a smaller, compact galaxy.

The collision sends out a shockwave that rips through the galaxy, setting off a tsunami

of star formation.

Since the shockwave expands radially, the result is a giant ring of newly formed stars.

Now, exactly how that happens depends on a number of variables.

There's the masses of the colliding galaxies, the amount of gas and dark matter between

them, their radii, their impact velocities, the sizes and orientation of the impactor

and many others.

And that's why groups like Renaud at al did a lot of work in simulating the creation of

a Cartwheel-like galaxy.

Interestingly, they found that before the collision, the approaching galaxy strips out

a lot of gas from the target, and it effectively shut down that galaxy's star formation.

Galaxies affected this way are described as "quenched" because they no longer have the

gas needed to create new stars.

Obviously, the target galaxy doesn't stay quenched for very long, but it's still pretty

weird to think that the galaxy wouldn't be able to produce very many stars for a few

million years before impact.

The impact creates a shockwave that expands outward at a fairly constant speed of 120

kilometers per second.

And this is what forms the main ring.

But the impact also creates a vertical structure that's kind of reminiscent of the way water

rises after an impact.

In a way, the nuclear region ends up getting displaced vertically from the main ring.

Meanwhile, the ring is expanding outward, creating new stars along the way.

But the ring doesn't last all that long.

Most of the star formation in the ring takes place during just the first 100 million years.

After 110 million years, the ring is barely noticeable.

But as the ring passes by, clumps of gas flows through the spokes down to the nuclear region,

where it kicks off a second wave of star formation in the inner ring.

The Cartwheel's ring is estimated at being between 200 and 300 million years old, so

that would be at least twice as old as what the simulation predicts.

But the simulations weren't trying to recreate the actual Cartwheel, specifically.

They were just trying to understand those weird orbital dynamics of those head on collisions.

Still, the result does underscore that the Cartwheel's ring is a transitory phenomena,

and we should not expect it to last for much longer.

Now, with so many massive star clusters forming, there is undoubtedly going to be a number

of supernovae!

In fact, Supernova 2021afdx was detected in November of last year.

supernova of a massive star.

Supernovae go off in dusty galaxies all the time,

but we really can't see them when there's so much dust in the way.

So Webb's ability to see through so much of this dust means it's going to be able to detect

far more supernovae than we can right now.

Observations taken with the Chandra X-ray Observatory reveal the location of several

ultra-luminous X-ray sources.

These are likely neutron stars and black holes and binary systems that are accreting material

from their companions.

As matter falls in, they form an accretion disk, which heats up to hundreds of millions

of degrees and radiates enormous amounts of X-rays.

Now we have similar phenomena here in the Milky Way, but with the Cartwheel producing

so many massive binary star systems, it's just producing X-rays on a whole other scale.

And now we have this incredible infrared dataset from Webb!

Now, remember, this image is a composite of NIRCam and MIRI.

The NIRCam image is covering a wavelength range from about 0.9 to 4.4 microns and get

colored blue, green, yellow and red.

It looks similar to the Hubble image in that it shows us a concentration of younger, bluer

stars.

But it's also showing us the glowing dust from newborn stars as well as the reddish

glow of older stars.

And you can see in the ring where it's just littered with pockets of star formation.

And NIRCam also lets us see those newly formed clusters in the inner ring as well.

Now, the thing about young stars is that they generate very strong, stellar winds and that

allows them to blow away much of the dust that immediately surrounds them.

NIRCam can then peer through the remaining dust to reveal their presence.

But what really blows my mind are NIRCam's, images of the spokes!

At visible wavelengths, the spokes appear almost ghostly in comparison, but this just

goes to show that we can't trust our eyes all the time either.

The dust is opaque to visible light.

In the Milky Way,

we only can see dust lanes visually when they're seen in silhouette against a bright background.

But there's nothing very bright behind the Cartwheel, so it never seemed intuitive to

me that we'd actually be seeing dust that's blocking our view.

I always assumed we were just looking through the galaxy into intergalactic space.

Well, Webb shows us this isn't true.

NIRCam sees through the dust, reveals star formation taking place in the spokes as well.

It gets even better with MIR, which images the cartwheel from 7.7 to 18 microns.

That takes us well out into the mid-infrared and detect the thermal emission.

But we don't see as many stars in this part of the spectrum because they're not particularly

bright at these wavelengths.

But we do see the warm glow of hydrocarbons, organic compounds, silica dust and polycyclic

aromatic hydrocarbons or PAH's.

PAH's are essentially soot.

Now, the thing about dust is that it's created by dying stars as they belch out huge quantities

of complex organic molecules as they age.

So it could be that this is relatively new dust that was expelled by dying stars in the

wake of the shockwave.

On the other hand, it could be that the dust was already there before the collision, got

plowed by the shockwave, and has been spiraling back down along the spokes ever since.

Meanwhile, back in the core, MIRI reveals that the inner ring seems to be connected

to the nucleus by another set of spokes.

So it's kind of like a Cartwheel within a Cartwheel!

Now, all of this is thanks to a head-on collision with another galaxy.

But which galaxy was it?

Well, there are two companion galaxies right next to the cartwheel dubbed G1 and G2 and

G1 is a blue, almost Magellanic Cloud-like galaxy with vigorous star formation.

G2 is a diffuse yellow compact spiral with older, fainter stars and relatively little

dust.

Now, the difference between these two companions is really noticeable in the MIRI imagery.

G1 shows plenty of dust, while G2 all but disappears.

So who done it?

Well, on the one hand, G1 does have a lot of star formation going on.

Just like the Cartwheel.

Coincidence?

Maybe.

But G2 is lacking all of that dust and has older stars, which you could also expect to

see happen in the aftermath of a collision where the gravity from the larger galaxy strips

out the smaller galaxy's

mass.

However, neither of these two galaxies are likely to be the culprit because they're both

really close to the Cartwheel.

And that means they'd have to be moving pretty slow to be this close 200 million years after

the collision.

It's hard to imagine there would be enough energy getting transferred into the Cartwheel

this way.

However, there is a third companion galaxy outside of Webb's field of view, and this

galaxy, called G3, is linked to the Cartwheel by long plume of hydrogen gas.

We can't see this gas in optical or even in the infrared, but radio observations can trace

out the contours of this hydrogen.

The plume is pretty much the smoking gun evidence that G3 was in fact the bullet.

The cartwheel is a beautiful mess, but it's also a laboratory for understanding the dynamics

of galaxy interactions, star formation, and stellar motions on the largest scales.

And now with Webb, we can study the flow of matter within these colliding galaxies like

never before.

Now, these images were made available as part of Webb's Early Release Science Program.

And the idea here is to take as many images of scientifically interesting targets as possible

and just get them out there into the public so that astronomers can look at these datasets

and better map out the interiors of the cartwheel and go back and simulate how the collision

could have actually produced the galaxy.

There will be a bunch of papers coming soon and we'll be talking about their results here.

And by the way, we are going to be talking with the imaging team to see just how these

images were, in fact, created in the first place!

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