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  • [♪ INTRO]

  • With so many galaxies getting ripped apart or smashing into each other, you might think

  • of our Milky Way as a comparatively peaceful place.

  • Well, sorry about that.

  • Because it's actually a cannibal.

  • We've seen evidence of the Milky Way devouring other galaxies before,

  • and now, it seems like we've found another one.

  • In this month's issue of the Monthly Notices of the Royal Astronomical Society, scientists

  • announced evidence suggesting that, 8 to 11 billion years ago, the Milky Way devoured

  • a neighborhood they call the Sausage Galaxy.

  • I mean if you're gonna eat a galaxy, it might as well be a sausage galaxy.

  • According to an international team of astronomers, some stars hanging out in the Milky Way's

  • halo, a sort of spherical cloud full of stuff like old stars,

  • serve as a record of this event.

  • Or, more specifically, their orbits do.

  • These orbits are super long and narrow, oriented

  • radially with respect to the center of the Milky Way.

  • These stars also have a noticeably different chemical makeup than other ones nearby, and

  • the directions of their orbits go backwards.

  • So the team concluded that they probably had to have come from a different galaxy.

  • Since plots of these stars's velocities relative to others in the area look vaguely

  • sausage-shaped, the team dubbed the starsGaia Sausageand the galaxy they came

  • from theSausageGalaxy.

  • It almost definitely didn't look like a sausage itself, though.

  • Although that would be awesome, because it would totally team up with that

  • galaxy that looks like a fried egg.

  • Anyway. Just call me when they find a galaxy that looks like poptarts; I'll be there for that.

  • When this galaxy collided with the Milky Way, it likely caused our disk to puff up, or it

  • might've even broken part of it up and forced it to reform.

  • Essentially, the Sausage's guts got scattered around the inner Milky Way, adding to our

  • galaxy's bulge.

  • You know, when you have too many sausages.

  • Telescopes like the Hubble have collected a lot of beautiful images of galactic collisions,

  • but we don't have a time machine or a camera outside of the Milky Way

  • to see them happen to us.

  • So, the team had to use computer models to check if the observed stellar positions and

  • the trajectories could be produced by this hypothetical Milky Way/Sausage Galaxy interaction.

  • The simulations used data from both the Sloan Digital Sky Survey and the ESA's Gaia satellite,

  • which is mapping stars in our galaxy and how they move through space.

  • And they concluded that a galaxy roughly 5% the mass of the Milky Way with an extremely

  • eccentric orbit could produce these leftover stars with weird orbits.

  • Of course, this wasn't the first time the Milky Way ate another galaxy,

  • and it won't be the last.

  • But the Sausage hypothetically contributed the bulk of the stars in the inner stellar

  • halo, which is a helpful thing to know about.

  • In another paper, the team of astronomers also identified at least eight globular clusters,

  • which are dense, spherical clusters of stars, that might have originally belonged to the

  • Sausage galaxy, too.

  • But since the paper hasn't completed the peer-review process yet,

  • we can't really say that for sure.

  • Meanwhile, while the U.S. was blasting off fireworks last week, another team was getting

  • ready to share findings about different light shows on Jupiter.

  • Specifically, auroras, which happen when electrons get accelerated by Jupiter's magnetic field.

  • The process is similar to how some auroras form on Earth, but on Jupiter, some of the

  • planet's moons actually help shape what these shows look like.

  • A study published last Thursday in the journal Science shed a little more light on the subject

  • and also showed that these interactions are more complicated than we thought.

  • Jupiter has upwards of 60 moons, but the most famous are those discovered by Galileo at

  • the start of the 17th century: Io, Europa, Ganymede, and Callisto.

  • All of them orbit within Jupiter's magnetosphere, or the region ruled by its magnetic field.

  • And they're close enough to the planet that they form sort of an obstacle to any charged

  • particles trying to follow Jupiter's super strong magnetic field lines.

  • Basically, this causes a special kind of magnetic wave to form, called an Alfvén wave.

  • It accelerates electrons toward Jupiter's atmosphere and causes additional auroral emissions.

  • This leaves a noticeable footprint in Jupiter's atmosphere, showing up as bright spots in

  • both the north and south hemispheres.

  • Astronomers have previously used the Hubble to study these features, but now that we have

  • the Juno spacecraft orbiting Jupiter's poles, we have a much better vantage point.

  • In this new paper, one team used Juno's Jovian Infrared Auroral Mapper, or JIRAM,

  • to reveal the hidden complexity within these footprints.

  • In September 2017, JIRAM set its sights on where models

  • expected Io's footprint to show up.

  • Since Io is the closest of the Galilean moons to Jupiter, it leaves the largest footprint.

  • And one did show up there, but it was a lot more than just a big spot.

  • Images revealed a trail of swirling vortices in both hemispheres, which sometimes split

  • into two wing-shaped arcs.

  • The main spot was located where models predicted, but secondary shadows stretched for a thousand

  • kilometers or so on each side, each about one Io diameter away from each other.

  • While the pattern is clearly something reminiscent of fluid dynamics, like air swirling off a

  • plane wing, the team doesn't have a specific model that explains how Io's path through

  • Jupiter's magnetic field creates it.

  • They suggest the Alfvén waves might be broken into smaller waves with different travel paths,

  • which could lead to something so turbulent looking.

  • But they're not positive yet.

  • Still, the fact that something as small as Io can have such a big influence on Jupiter

  • is pretty cool.

  • The team also captured images of Ganymede's footprint, revealing never-before-seen double

  • shadows of sorts, with two identical spots roughly 170 kilometers apart.

  • This might be due to the fact that Ganymede is the only moon to have its own magnetic

  • field, but again, more data would help.

  • The good news is that Juno has at least another three years of work to do, so we'll have

  • time to make more observations.

  • And someday, we'll hopefully have a better idea of how Jupiter's magnetic field reacts

  • to such a complex system.

  • We've also observed Enceladus's auroral footprint on Saturn, so we know that this

  • isn't something unique to Jupiter.

  • In fact, these auroral footprints could be found where there's any electrically-conducting

  • moon orbiting inside a planet's magnetosphere, or even a planet inside a star's.

  • So one day, we might look back on these Juno observations as a major step toward understanding

  • a much bigger picture.

  • Thanks for watching this episode of SciShow Space!

  • Besides influencing Jupiter's auroras, it turns out Io has a lot more to brag about,

  • like, the fact that it might have an underground magma ocean.

  • You can find out all about that in an episode we did on Io.

  • [♪ OUTRO]

[♪ INTRO]

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天の川最後の大食いに出会う。ソーセージ銀河 (Meet the Milky Way's Last Big Meal: The Sausage Galaxy)

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    林宜悉 に公開 2021 年 01 月 14 日
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