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SYFY WIRE Bad Astronomy

Gaia: The sky is littered with undigested galaxies

By Phil Plait
Stars in some of the stellar streams recently discovered using Gaia data superposed on a map of the galaxy. Credit: ESA/Gaia/DPAC

Remember when you were a kid and a friend told you that gum was indigestible, and if you swallowed it the gum would stay in your stomach for seven years?

Yeah, that’s not true. OK, gum is indigestible, but it passes through you just like any other thing your body can’t tear apart and use on the molecular level.

While, this urban legend — like so many others —turns out to be false, on a galactic scale it is true. Kinda. In this case your stomach is the galaxy, and the wads of chewing gum are smaller galactic companions. Our galaxy ate them, and the undigested bits still roam the Milky Way.

OK, enough with the food analogies (as much as I want to make one about a candy bar named after our galaxy). What’s really going on?

The structure of the Milky Way: A flattened disk with spiral arms (seen face-on, left, and edge-on, right), with a central bulge, a halo, and more than 150 globular clusters. The location of the Sun about halfway out is indicated.

Our Milky Way is a spiral galaxy: A central bulge of stars with a flat disk around it, surrounded by a halo of stars stretching out for hundreds of thousands of light years. The Sun sits in that disk, about 26,000 light years from the center, putting it halfway to the edge of the disk, which is 100,000 light years in diameter.

We have a lot of companion objects orbiting us, too. Some are relatively small but decent-sized galaxies like the Small and Large Magellanic Clouds, and some are even smaller and fainter dwarf galaxies. Over 160 globular clusters also orbit the galactic center, each a self-contained ball of a hundred thousand stars (and glorious through even a small telescope).

Here’s a fun fact: We used to have more. But, over its lifetime, the Milky Way has eaten quite a few of these smaller companions, growing in size each time. This can happen if the orbit of the companion takes it too close to the Milky Way, sometimes even passing right through our disk. Stars are small and very far apart, so they rarely physically collide… but the combined gravity of a hundred billion stars in the Milky Way is literally a destructive force. It can rip apart these smaller galaxies and clusters, pulling them apart like, well, chewing gum stretched between your fingers.

Sometimes the companion gets completely torn apart, its stars and gas merging with our own. Sometimes though it may have a dense core which can survive — it’s thought that many globulars clusters are in fact cores of old eaten galaxies like this, spat out of the Milky Way like indigestible gum*.

But digesting a galaxy takes time. A lot more than seven years, of course; more like hundreds of million of years or more. That means that there could be stars moving around our galaxy still on the same paths as their original galaxy or cluster. And in fact we’ve found some of these stellar streams; the Sagittarius Dwarf galaxy stream is the most well-known, but others exist.

Many others: A team of astronomers have just announced the existence of quite a few more. In a series of papers they describe how they found them and their properties, and it’s very cool.

Stars in some of the stellar streams recently discovered using Gaia data superposed on a map of the galaxy. Credit: ESA/Gaia/DPAC

They used Gaia, a European Space Agency satellite that, for several years now, has been mapping the positions, motions, and colors of well over a billion stars in the galaxy. Using a very sophisticated method of parsing the data, they modeled the four-dimensional orbits of the stars (three in space plus time), looking for others stars on similar paths through space. They had to account for all kinds of subtle effects (like the shape and mass distribution of stars in the Milky Way) to make sure they got this right. If they found a stream, for example, one thing they checked for was the abundances of various elements in the stars, under the assumption that if they all formed in the same globular cluster (say) they should have the same relative abundances of those elements.

In all they found over a dozen such streams popping out of the data, which is astonishing. The Milky Way is a glutton!

An interesting thing about these streams is that most of them don’t seem to have any other objects associated with them. If they come from dwarf galaxies or globular clusters, those original objects apparently are long gone. One major exception is a stream they named Fimbulthul: It is clearly coming from the massive globular cluster Omega Centauri! That’s very cool, because it’s been speculated for a long time that Omega Cen may be the leftover core of a dwarf galaxy, and so should have a stream of stars coming off it. This would seem to confirm that.

The orbits of streams of stars in Gaia data as if you are looking down on the center of the Milky Way. The scale is in kiloparsecs (1 kpc = 3,260 light years). Credit: Ibata et al.

Also interestingly, more than half of the streams they found orbit the galaxy retrograde, in the opposite sense as the rest of the stars in the disk. Most globular clusters orbit prograde, in the same sense as stars in the disk, so what gives? The astronomers aren’t sure, but suspect it may be that it’s easier to pick stars orbiting backwards out of a crowded background of stars orbiting forwards, so those streams are easier to spot. Further refinements to their software might clear that up. But if it’s a real effect it’s telling us something about the Milky Way and the objects around it… but what, exactly isn’t known yet.

One other thing I want to mention are the names they gave the streams. Slidr, Sylgr, Ylgr, Fimulthul, Svöl, Fjörm, Gjöll, and Leiptr — all of which were discovered in a recent software upgrade which refined their search method, plus Phlegethon, discovered previously — are names of rivers in Norse mythology, rivers that existed before the world did. How apt! And a lovely thought, given how old some of these streams must be. Many of them very likely were parts of galaxies and clusters long before our Earth formed 4.5 billion years ago.

I’ve written about Gaia many times before, and how it’s revolutionizing our understanding of the galaxy — the distance to Polaris, how far away the Pleiades are, whether the iconic star Albireo is a true binary or not (spoiler alert: it isn’t), and revealing hidden structures in the Milky Way, including stellar streams. I have to admit that when I wrote that last article just a few months ago I wasn’t expecting Gaia data to reveal so many more leftover cosmic cannibal meals.

We’ve lived in this galactic neighborhood for billons of years, and finally, after all that time, we’re really starting to get to know it.

*OK, I lied; I wasn’t done with the food analogies.