How The Milky Way was Shaped by Galactic Cannibalism (Script) (Patreon)

When I was a young graduate student I got to use one of the giant telescopes at the Las Campanas observatory in the Atacama Desert in Chile. I was traveling with a much more experienced astronomer from Europe, but it was also his first time in the southern hemisphere observatory. He went outside for a weather check and came back looking annoyed to report two gigantic clouds in the sky.  I went to look  - it was the most crystal clear night I’d ever seen. Was my colleague suffering altitude sickness? Then I realized - there WERE two clouds on the sky - two hazy blobs just off the dust-streaked center of the Milky Way band. I laughed out loud - this wasn’t water vapour - this was lunch - the Milky Way’s lunch. Find a dark night in the southern hemisphere and you’ll see too: the several billion stars of the large and small magellanic clouds in their slow death spiral towards the Milky Way. My colleague needn’t have worried - we did some great observing that night. But astronomers watching the resulting collision in around 2 billion years might have more cause for concern.

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When we scan the heavens with giant telescopes like those on Las Campanas, we see galactic cannibalism everywhere. We see moments that appear frozen on the human timescale, but are really snapshots of the incredibly violent process of galaxy formation. This is how all galaxies are made. We can piece together a pretty good understanding of this process from countless snapshots. Looking into the distance means looking into the past, so it’s possible to stitch together a frankenstein flipbook of galaxy evolution.

But what  about our home galaxy? It would be nice if we could learn the Milky Way’s true history and final fate. And, in fact, we can. Since that night on Las Campanas, enormous surveys have tracked the positions and motions of more than a billion Milky Way stars. We’re now able to calculate a detailed and dynamical map of the Milky Way. We can predict its future mergers with the Magellanic clouds and Andromeda - but perhaps more astonishing, we can reconstruct its past.  But before we get to this, let’s review what we know about the Milky Way of the present, and of galaxy evolution in the general sense.

The Milky Way is a pretty typical barred spiral galaxy. Our sun is in the disk, on a minor outcropping of one of the spiral arms. It orbits in the same direction as all the disk stars once every 230 Million years. In the center we have the bulge envelopes - an elongated spheroid of stars that all orbit randomly at different angles. All of this is surrounded by the halo - also spheroidal, but twice the diameter of the 100-thousand light year wide disk. This thing is sprinkled with wayward stars and ancient, dense mini-galaxies called globular clusters. But mostly the halo is made of dark matter, which also suffuses the disk and halo and constitutes 80% of the Milky Way’s mass.

The entire galaxy is beautiful and intricately structured. Weird to think that it built itself into this through a life of such violence. So let’s look at what we know about galaxy evolution based on all the other galaxies in the universe.

We actually talked about this process recently in our episode on the galactic habitable zone. The first galaxies collapsed from very slight over-dense regions in the hot hydrogen and helium gas that filled the universe after the Big Bang. It’s hard to see the galaxies of the first billion years or so. Galaxies were still growing, and most are too small to see at those great distances. The most distant galaxy known as of the filming of this episode was discovered in April this year. It shines out from a young universe, only 350 million years after the big bang. Back then, galaxies were raging storms of star formation due to the abundance of gas back then. We only see the brightest of those first galaxies. But based on the flipbook that we’ve assembled of the following several billion years we have a clear picture:

Galaxies assemble from the bottom-up, which just means that small clumps form first, and then merge into larger clumps. So early galaxies were messy and mostly small, and formed stars furiously. These clumps fell together and spun each other up into whirling disks, and their violent convulsions settled into the density waves that we see as spiral arms. The new spiral galaxies continued to gobble up wispy irregular galaxies to grow that disk.

So that’s how the Milky Way got to its current form. But how can we possibly reconstruct the details of such a chaotic process? The stars from every previous merger are mixed all across the Milky Way disk or through the halo by now. Teasing out the Milky Way’s history is sometimes called stellar archaeology. Perhaps galactic forensics comes is a better description - teasing out the evidence of violent encounters from evidence that in some cases has been carefully buried.

Let’s review the evidence. Item 1: Stars that join the Milky Way at the same time, like during a merger, should have similar properties. For example, stars that form from the gas of the same galaxy tend to have similar amounts of heavy elements in them, as the gas that formed them was enriched by the same number of supernovae and other explosions. We can measure the heavy element abundance - also called metallicity - by looking for the dips and spikes in a star’s spectrum that result from specific elements sucking up or producing light at specific wavelengths. Stars with similar metallicities may have formed around the same time and even in the same part of the galaxy, even if they’re now very far apart. That could indicate that they came from the same merger event, or were produced in the same burst of star formation triggered by that event.

Spectra on their own aren’t enough to tell if any two stars came from the same snack. So Evidence item number 2: If those stars came from the same merger, they should also have similar orbits. If their orbital speeds are even slightly different, they may have drifted to opposite sides of the galaxy by now. But there are other orbital properties we can try to match. For example, how stretched out, or eccentric, their orbits are. And the orientation of their orbits relative to the galactic disk.

Let’s see what our forensic investigation has told us so far. A lot, actually. For example, astronomers have identified the last truly gigantic merger that happened to the Milky Way around 10 billion years ago. This merger was so large - around 50 billion Suns worth of matter - that it must have reshaped the galaxy and can be thought of as the birth of the “modern” Milky Way.

The galaxy the Milky Way consumed has been dubbed ‘Gaia-Enceladus’ - named for the Gaia Space Telescope which was used to discover the event by identifying star from this devoured galaxy in the Milky Way’s halo. The “Enceladus” part is for the Greek titan of that name, and has nothing to do with the moon of Saturn. Gaia is able to identify the stars from this merger because of the satellite’s incredible ability to pinpoint stellar positions. That enables the telescope to detect tiny motions, which in turn allows astronomers to reconstruct detailed orbits of Milky Way stars. The stars from Gaia-Enceladus move on highly elongated orbits in the inner halo of the galaxy, but with a slight ‘backwards’ bend to the orbit, a clear indication that these stars were not born in our galaxy. We’ve also found a group of 13 globular clusters with matching orbital properties and spectra that were probably once part of Gaia-Enceladus.

We see more evidence for this past violence in the disk of the Milky Way. That disk has two parts - there’s the thin disk, which is a few hundred lightyears thick, and is the main star factory of the Milky way. It’s home to the spiral arms and the big, bright star forming clouds and our sun. We did an episode on why galaxies become flat disks  - but the TLDW is that giant gas clouds tend to collapse into thin disks, and then produce stars that share that geometry.

Surrounding the thin disk we have - you guessed it - the thick disk, which extends a few thousand light years above and below its more slender counterpart. Not all spiral galaxies have a thick disk, which suggests something special happened to the Milky Way to create it. It’s made of stars, not gas, and these stars orbit just a little faster on orbits that are more inclined than the thin disk. That causes them to rise above and drop below the Milky Way disk, leading to the aforementioned thickness. Those stars are different in other  ways - for example, they tend to have few heavy elements. That suggests they formed before the thin disk stars - around 9 billion years ago, plus or minus a billion years.

We already have a perfectly consistent explanation for the origin of the thick disk. It may have been formed during the merger with Gaia-Enceladus. While the stars of Gaia Enceladus got mixed into the halo, the crazy gravitational pulls of the two galaxies slamming into each other kicked up the orbits of many of the stars in the Milky Way’s original thin disk to create the thick disk. Meanwhile, the fresh gas from the merger replenished and reformed the thin disk, and triggered a new round of star formation.

Since its very large and thickening breakfast 10 billion years ago, the Milky Way has only snacked lightly. But we can trace that history also. When a dwarf galaxy gets too close to the Milky Way it gets pulled thin as tidal forces cause its near-side to move faster than its rear. It gets drawn out into a lengthening tidal stream, and ultimately can be wrapped around the galaxy multiple times. In the end it disperses into the Milky Way’s halo.

Our galaxy has dozens of known streams. Some of the littlest ones are still close together so it’s easy for astronomers to pick out the little stripe of stars, like GD-1: a globular cluster that’s in the process of being pulled apart. On the other hand, some of the biggest, like the Helmi stream, contain tens of millions of stars and wrap in a full loop around the Milky Way.

Perhaps the biggest of all the snacks the Milky Way has had since the Gaia Enceladus breakfast is the Sagittarius Dwarf Spheroidal Galaxy which first fell in about 5 billion years ago. Since then, it’s wrapped all the way around the galaxy, going up and over the poles and coming back down to punch through the disk three times.  Most importantly, the massive core is still relatively intact and moving together, so every billion years or so when it punches through the Milky Way’s disk its gravity hits like a hammer. Actually, more like a drum stick.

As the mass of the galaxy approaches, it pulls the disk of the Milky Way up, but then when it passes through it pulls it back down. Like beating a drum or plucking a guitar string, the stars oscillate up and down in the galactic plane, making a very faint ripple through the disk. As best we can tell, the three passes that the core of the Sagittarius Dwarf have made through the disk correspond to three episodes of star formation in the Milky Way, and one of these passes even happens to line up with the formation of our sun and solar system 4.5 billion years ago. Now, we’re not saying the Sagittarius Dwarf galaxy is fully responsible for the existence of our solar system, but it’s certainly possible that it helped.

All together, we know of up to 7 mergers in the history of the galaxy and at least 42 distinct streams surrounding the Milky Way, and as we continue to study the data from the Gaia mission we’re likely to find more.

This, finally, brings us to the present, as the Milky Way prepares to have its dinner and biggest meal yet. The Milky Way’s two brightest satellite galaxies, the Large and Small Magellanic Clouds, are currently making their first pass. Already, we see them being pulled apart- the Magellanic Clouds have massive tails that make a great loop all across the ‘bottom’ half of the galaxy, a 600,000 lightyear long tail of gas called the ‘Magellanic Stream.’

And while the Magellanic clouds themselves are only about 1% the mass of the Milky Way themselves, the entire stream may be 10 billion solar masses due to the enormous amount of dark matter it contains, making this easily the biggest merger the Milky Way has seen since Gaia Enceladus. Then that merger happens the Milky Way will get a fresh infusion of gas, probably triggering another bout of star formation in about 2 billion years. The accompanying supernova waves may not be the best thing for life on Earth, but we do have 2 billion years to get ready for that.

All of the Milky Way’s past mergers have been ‘minor’, meaning that the Milky Way was always significantly more massive than its meal. That’s going to change with the final merger of our local group of galaxies - when Andromeda and the Milky Way collide. This is a major merger, because Andromeda is a full-blown spiral galaxy in its own right. In fact it’s around twice as massive as the Milky Way. We’ve gone into the gory detail of this collision in a previous episode. Go ahead and watch that one if you want to see our galaxy get a taste of its own medicine.

For now let’s enjoy these short few billion years of being the biggest and hungriest kid in the playground, as we gobble up any galaxies foolish enough to stray into the Milky Way’s patch of space time.