From SKY & Telescope: “Galactic Archaelogy in the Milky Way Halo”

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Sky & Telescope

March 10, 2016
Monica Young

Milky Way surrounded by halo of 13 old stars

Thirteen lonely stars in the outer reaches of the Milky Way may hold clues to our galaxy’s formation. New research shows that they might be part of a shell-shaped relic that marks an ancient run-in with a dwarf galaxy.

In the galactic disk, gas and stars circle the center in orderly orbits, but the same can’t be said of the cloud of stars that surround the Milky Way. Theorists think most of these so-called halo stars were born in a multitude of dwarf galaxies that were later devoured by our own. Now these stars follow orbits that take them in and out of our galaxy’s center rather than around it. At such great distances, these stars move in slow motion, just as Pluto moves far more slowly in its orbit than flighty Mercury, so their orbits “remember” their ancient origins.

Digging Up Relics in Milky Way’s Halo

Three years ago, Alis Deason (then at University of California, Santa Cruz) led a team in measuring the motions of halo stars across the sky. The team took advantage of a Hubble Space Telescope program that was observing the Andromeda Galaxy.

NASA Hubble Telescope

Andromeda Galaxy

Picking out a baker’s dozen of foreground stars that lie in front of Andromeda, Deason and colleagues watched their sideways motion over a period of five to seven years.

Though exact measures of distance aren’t available, these stars are about 65,000 light-years from the center of the Milky Way: square in our galaxy’s halo, which extends out some 300,000 light-years.

“Watching the motion of stars across the face of galaxies is analogous to watching human hair grow on the surface of the Moon as seen from Earth,” says Puragra Guha Thakurta (University of California, Santa Cruz). Yet, he notes, it’s doable.

But the measurements from Deason’s team only gave stellar motions in two dimensions. This year, Emily Cunningham (University of California, Santa Cruz) took on the mantle, leading an effort to collect each star’s spectrum, looking for the shift in spectral lines that would reveal their motions along our line of sight.

Cunningham’s team confirms that these 13 stars defy expectations: they’re not traveling along paths that take them straight into or out of our galaxy. Most likely, they’re part of a shell of stars that are piling up as they turn around in their in-and-out orbits.

“If we are correct in our interpretation of a shell, this shell would be a relic from a past accretion event,” says Cunningham. This shell would be all that’s left of a dwarf galaxy, or perhaps even a group of dwarfs, that fell into the Milky Way’s gravitational grasp several billion years ago.

The alternative is that the stars are actually forming out there, in the farthest corners of the Milky Way. But as James Bullock says, “It’s hard to imagine stars forming out there in regions of such low density.” Moreover, he adds, “cosmological models predict that shells like this should be out there.”

The Future of Galactic Archaeology

Ultimately, though, 13 stars is a pretty small sample with which to dig up our galaxy’s history. That’s why Cunningham, Deason, and colleagues are planning a much larger sample in a project known as HALO7D, which will contain information on hundreds of halo stars once observations are complete.

HALO7D: Looking at and Through the Milky Way – Raja GuhaThakurta
Access the mp4 video here .

Bullock, whose simulations have outlined our galaxy’s formation history, is excited to see the results of this and other studies. “We aim to figure out what kinds of smaller galaxies fell into the Milk Way (How big were they? What kind of stars were they made of?), and also when those galaxies fell in.”

But even the grandest cosmic implications can’t prevent more prosaic considerations. “We are completely at the mercy of the weather,” says Cunningham. Cloudy nights disrupted plans to finish collecting data last spring, so observing will continue this spring and next, with final results one to two years from now.

See the full article here .

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Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”