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Sometimes a story of astronomy reads like a lift pitch for a Hollywood blockbuster.
"So you have a nice normal spiral galaxy, just, just doing it, when BAM! A smaller galaxy passes through like a ball! Chaos, fireworks, explosions everywhere! And then, after the departure of the second galaxy, the first begins to change shape, to turn into a big ring, then the stars start to die, and now there is a gigantic ring of gazillers with black holes and other strange objects. problem and vomit the rays of death!
Art imitates life, I guess. Because it happened for real.
Meet the galaxy AM 0644-071. It's been a long time … and it turned out that a lot of things went through it.
It's a ring galaxy, a kind of particular galaxy that was once a spiral very similar to the Milky Way, but a very bad thing happened. A smaller and more compact galaxy passed through it during a galactic collision. The physics is pretty cool, and I'll explain in a moment, but what happens is that it creates something like a compression wave, where the material closest to the center of the target galaxy moves outward. After a while, you get this ring of stars and gas in the periphery of the galaxy and very little material remains near the nucleus.
This is not without effect on all this material! The gas clouds were quite scattered throughout the galaxy, probably along the spiral arms. But when the galaxy went through, all that material was thrown out. The gas clouds collided, which means that they collapsed and formed stars. Because all this has happened on the expanding ring, we now see a wave of star formation coming out of the center of the galaxy.
That's why AM 0644 looks blue! When clouds form stars, many of them are massive and hot, so they shine blue. In addition, they do not live long; these massive stars are passing through their fuel in a million or tens of millions of years. When this happens, they explode, leaving behind either an extremely dense ball called a neutron star or a black hole.
In many cases, these stars would have been in binaries, orbiting another star. Once the massive star has exploded and left behind the compact object, it could draw matter to the remaining star. This material falls to the black hole or neutron star and forms a disc. Friction can make this record incredibly hot, and it will shine (an irony of the Universe: black holes are the darkest objects that exist, but the material around them can become so hot that they are part of the brightest objects in the Universe). If the temperature gets high enough, like millions of degrees, the discs will emit high-energy X-rays. Not only that, but bundles of material and energy paired can be thrown away from the disk, crossing the space.
And that's exactly what we see! Here is the same galaxy, but this time as observed the Chandra X-ray Observatory:
You see this ring of points on the right? They are extremely bright X-ray sources, almost certainly black holes and neutron stars created after the collision that actively eat their companions. You can see that they form the shape of the ring! Here are the two superimposed images:
Boom. Yep. It's really cool. You see the ghosts of stars that are born and died because of a galactic collision, and howl now in the night by emitting light that we can not see except with sophisticated equipment orbiting the Earth.
I love this kind of things.
Now, why did he form this ring? I promised to explain, so that's it.
To be honest, until now I have never really understood it; I knew ring galaxies (they are extremely fresh and beautiful) but I have never really dug in their training mechanism. In the article on the amendment to X-ray sources, AM 0644, they are related to the first article to explain them (published in 1974), and reading it was as if a light bulb lit up my head.
Think of it this way (this is similar to the explanation in the article): Imagine our solar system in space. If another star were to cross our system perpendicularly to Earth's orbit, moving rapidly and passing right next to the Sun on its path, what would happen?
Well, if the star moved very fast, the Earth would feel for a short moment a stronger pull towards the center of the solar system. That would pull it a bit, reducing the orbit. But the speed at which the Earth rotates around the Sun depends on the gravity of the Sun, which depends on the mass of the Sun. The second star would add to the gravity felt by the Earth, so that the speed of the Earth would increase as the orbit decreases.
But once the star passes, gravity becomes normal again. However – and this is the key part – the Earth still has this extra orbital speed. The gravitational force of the Sun alone is not sufficient to balance this centripetal acceleration (or centrifugal force, if you prefer), so that the Earth moves outward. Finally, it will move into an elliptical orbit, but this orbit will be much larger than it was before.
What happens to other planets? The calculation is a little odd, but in the end, Venus gets more velocity than the Earth, so it moves even faster when the second star passes. The mercury becomes even more, so it throws itself even faster. Mercury catches up with Venus, who catches up with Earth … and while all pile up on the same path, at a more or less equal distance from the Sun.
Now, imagine not only a few planets, but millions of stars. Billions all at different initial distances from the center of their galaxy, forming a flat disk like our solar system. Now, a galaxy marauding through them, creating one of these ripples expanding. All stars (and gas clouds!) End up moving outward, catching up, forming a huge ring. The exact shape of the ring depends on a lot of things, including the mass of galaxies, the off center of the collision, the angle of impact, and so on. But the bottom line is that the stars pile up as they move outward, forming the ring.
It's amazing. It explains why these ring galaxies exist, but also why they are rather rare; you need specific conditions to make them. But these X-ray observations show us that we are on the right track because we expected to see black holes and / or neutron star binaries on the ring. That's what we do, that is, it supports the idea that these galaxies are made.
Rather wild. The Universe is an interesting place! I'm glad we live here.
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