Colossal 'fossil' structures have been detected lurking on the outskirts of our galaxy

From Earth's vantage point in one of the Milky Way's spiral arms, the structure of our galaxy is pretty difficult to reconstruct.

That's because gauging the distance to something in space when you don't know its intrinsic brightness is really, really hard. And there are a lot of objects in the Milky Way whose brightness is unknown to us. This means that sometimes, we can totally miss huge structures that you'd think should be right under our noses.

A new set of such enormous structures has now been unveiled at the outer regions of the Milky Way disk: massive, spinning filaments with unclear provenance. Astronomers will be conducting follow-up surveys to try and solve the mystery.

The discovery came about thanks to the European Space Agency's Gaia space observatory, a project to map the Milky Way in three dimensions with the highest precision yet.

Gaia orbits the Sun with Earth, in a looping orbit around the Sun-Earth L2 Lagrangian point, a gravitationally stable pocket of space created by the interactions between the two bodies.

From there, it carefully studies stars in the Milky Way over an extended period, watching to see how the positions of stars seem to change against more distant stars. This provides a parallax, which can be used to calculate the distances to the stars.

While this can be done from here on Earth, atmospheric effects can interfere with the measurements. From its position in space, Gaia has an advantage, which it has been using to great effect. Since its 2013 deployment, the space telescope's data have revealed a number of structures and stellar associations we had no idea about.

The new structures were identified by a team led by astronomer Chervin Laporte of the University of Barcelona in Spain in data from the latest release, made in December of last year, with improved parallax precisions. The same data also showed previously known structures with much higher clarity than we'd seen before.

"We report the discovery of multiple previously undetected new filaments embedded in the outer disk in highly extincted regions," the researchers wrote in their paper.

"Some of these structures are interpreted as excited outer disk material, kicked up by satellite impacts and currently undergoing phase-mixing ('feathers'). Due to the long timescale in the outer disk regions, these structures can stay coherent in configuration space over several billion years."

Such spinning filaments at the galaxy's outskirts are not unexpected. According to simulations, interactions between the Milky Way and its satellite galaxies could produce such structures. The Milky Way has a swarm of satellites currently in orbit (maybe).

But there's a problem: the sheer number of the filaments found by Laporte and his colleagues vastly supersedes those seen in such simulations, which means we need another explanation.

One possibility is that the filaments are remnants of tidal spiral arms that were excited at various times by interactions with satellites; galactic fossils, in other words.

Another possibility is that they are the crests of distortions of the Milky Way disk which occurred due to collisions with other galaxies. The Milky Way has a history of collisions with other galaxies, which can cause perturbations in the galactic disk, so it's not an unreasonable supposition.

Such collisions, the researchers believe, could send disturbances propagating through the galactic disk like ripples on a pond.

The next step will be to conduct follow-up observations, to try and determine which of these scenarios is the most likely.

"Typically this region of the Milky Way has remained poorly explored due to the intervening dust which severely obscures most of the galactic midplane," Laporte said.

"While dust affects the luminosity of a star, its motion remains unaffected. We were certainly very excited to see that the Gaia motions data helped us uncover these filamentary structures! Now the challenge remains to figure what these things exactly are, how they came to be, why in such large numbers, and what they can tell us about the Milky Way, its formation and evolution."

The research has been published in the Monthly Notices of the Royal Astronomical Society: Letters.

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