From Sky and Telescope: “Milky Way’s New Neighbor: A Giant Dwarf”

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Sky & Telescope

April 27, 2016
John Bochanski

Astronomers have discovered a “feeble giant”: one of the largest dwarf galaxies ever seen near the Milky Way.

Ever since astronomers discovered our universe’s accelerating expansion, tension has rippled between theory and observations, especially in studies of our galaxy’s neighborhood.

The standard model of cosmology, which suggests that dark energy and “cold” dark matter govern the universe’s evolution, predicts many more small galaxies near the Milky Way than what we’ve observed so far. Dwarfs should be the building blocks of larger galaxies like our own, so the lack has puzzled astronomers — are they not there, or are we just not seeing them?

Observations have closed in on theory in recent years with the advent of large surveys such as the Sloan Digital Sky Survey and the Dark Energy Survey, where observers have begun to identify hard-to-find dwarf galaxies. Dozens of dwarfs have been spotted over the last 15 years. But theory suggests perhaps even hundreds more have yet to be discovered.

SDSS Telescope at Apache Point, NM, USA
SDSS Telescope at Apache Point, NM, USA

Dark Energy Icon
Dark Energy Camera,  built at FNAL
NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, ChileCTIO Victor M Blanco 4m Telescope interior
DECam, built at FNAL; the NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile

Now, the list of known dwarfs has just added one of its largest members: Crater 2 [no image available]. You’d think large dwarfs would be easy to find, but this one’s stars are spread out and easily entangled with the stars of the Milky Way. It took a sensitive survey to pick out the small galaxy hidden behind the galaxy’s stars.

A New Dwarf Galaxy

Gabriel Torrealba (University of Cambridge, UK) led a team that discovered the Crater 2 dwarf galaxy in survey data collected at the Very Large Telescope in Chile.

ESO/VLT at Cerro Paranal, Chile
ESO/VLT at Cerro Paranal, Chile

The team used specialized software to spot over-crowding among stars, searching for dim stellar clumps. But identifying a clump isn’t enough. Only Crater 2 contained red giant stars and horizontal branch stars — both old, evolved stars that mark an ancient stellar population separate from the youthful Milky Way disk.

Torrealba and colleagues estimate that Crater 2 lies 391,000 light-years from Earth. That makes it one of the most distant dwarf galaxies known. It’s also one of the largest: at 6,500 light-years across, it comes in fourth among our galaxy’s neighbors, after the Large and Small Magellanic Clouds, and the torn-apart Sagittarius dwarf galaxy. Moreover, it’s incredibly diffuse, its stars spread out over several square degrees. So despite its size, Crater 2 is much fainter than those Milky Way companions, nearly 100 times fainter than Sagittarius and almost 10,000 times fainter than the LMC.

Dwarf Galaxy Groups

The discovery of Crater 2 may help unlock an ongoing puzzle in the Milky Way’s evolution. As astronomers began to discover dwarf galaxies en masse in large sky surveys, it soon became clear that some dwarfs cluster in their orbits. Crater 2 is no exception: the team estimated that the dwarf’s orbit lines up with those of the Crater globular cluster, as well as the Leo IV, Leo V and Leo II dwarf galaxies.

Dwarf Galaxies with Messier 101  Allison Merritt  Dragonfly Telephoto Array
Dwarf Galaxies with Messier 101 Allison Merritt Dragonfly Telephoto Array

U Toronto Dunlap Dragonfly telescope Array
U Toronto Dunlap Dragonfly telescope Array

While not a definitive association, similar orbits suggest that these objects might form a group that fell together into our galaxy’s gravitational well. Astronomers have recently found similar groups near the Large Magellanic Cloud, suggesting that our galaxy’s halo might have formed through many such group captures.

Large Magellanic Cloud. Adrian Pingstone  December 2003
Large Magellanic Cloud. Adrian Pingstone December 2003

As sky surveys continue to enable discoveries of dwarf galaxies such as Crater 2, the gap between theory and observations continues to narrow, clarifying our understanding of the Milky Way’s evolution. The future is bright for the study of these dim galaxies, thanks to surveys such as the Large Synoptic Sky Survey (LSST) on the horizon. LSST will push to even fainter magnitudes and may finally resolve the discrepancy between theory and observation.

LSST/Camera, built at SLAC
LSST Interior
LSST telescope, currently under construction at Cerro Pachón Chile
LSST/Camera, built at SLAC; LSST telescope, currently under construction at Cerro Pachón Chile

G. Torrealba et al. The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater. Accepted for publication in Monthly Notices of the Royal Astronomical Society.

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.”