Hors d’Oeuvre for the Milky Way

A delicate formation in the southern sky provides new clues to the galaxy's formation

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Stars in night sky, centre of Milky Way Galaxy

Galaxies never stop eating. A massive swirl like the Milky Way — with its 120,000-light year width and its 300 billion stars — exerts a whole lot of gravity. That means anything — including smaller, globular clusters or even entire dwarf galaxies — will get sucked in. Most astronomers believe major galaxies like ours assemble themselves in this way, with a constant gobbling increasing their mass over billions of years. But with the 14-billion year old Milky Way having long since reached its full adult size, we don’t get to see it eating as much as it used to.

A new paper, soon to be published in the Astrophysical Journal Letters, however, has found evidence that our galaxy may still be in the process of digesting a small meal — one that, at least in cosmic terms, occurred comparatively recently in its history. The finding is significant because it illustrates both how changeable even the most fixed structures in the heavens are, and how creative astronomers can be in teasing out evidence of that fact.

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The study, conducted by a team of three astronomers from Yale University, relied on observations made by the Sloan Digital Sky Survey (SDSS), a massive and ongoing eight-year effort to map the heavens using a wide angle telescope at the Apache Point Observatory in New Mexico. The SDSS has so far produced three-dimensional maps of more than 930,000 galaxies, though the most detailed ones are, obviously, of the Milky Way, simply because we live in the thick of it.

One feature the SDSS detected in the southern sky, roughly in the direction of the constellation Triangulum, would be easy to miss by most people, but caught the astronomers’ eyes immediately. It was a thin, almost filamentous structure that the researchers knew could be what’s known as a galactic stream. As galaxies ingest star clusters, they pull those smaller formations apart, sucking them in and stretching them out to bands or strands. Big formations like large dwarf galaxies get pulled, taffy-like, into thick bands. Smaller formations like mere globular clusters may be just as long but a lot thinner.

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To determine what they were looking at, the Yale astronomers had to study the images through a series of filters that would selectively eliminate dust in the vicinity of the strand as well as gas, more dust and other material that lay in the vast stretch of space between the formation itself and observers on Earth. Cleaning up the image that way revealed a lot.

The composition of the strand appeared to be consistent with exactly what you’d expect to find from the remains of old, metal-poor stars that were born back in the galaxy’s own formative days. It measured about 245 light years wide and 17,930 light years long. That, believe it or not, is small, nowhere near big enough to be the remnants of a dwarf galaxy with its billion or so stars, but just right for a globular cluster, with tens of thousands to a million.

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“Our discovery is more of a light snack than a big meal for the Milky Way,” said Marla Geha, one of the co-authors of the paper. “Studying the digestion process in detail is important because it gives us new insight into how all galaxies form and evolve.”

It could also reveal something about the gravitational physics that hold the Milky Way together and shape it today. If gravity drives the tides of the galaxy, stellar streams can serve as a sort of dye marker, flowing with the currents and tracing their directions. Ultimately, like the dye marker too, they will disperse and become subsumed, leaving no mark of their passing. But the Triangulum, at least, is teaching us some lessons before it goes.

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