Chow Down! Milky Way gobbles its closest known neighbor

Astronomers have discovered that the Milky Way is shredding a tiny galaxy into elongated streams of stars and claiming them for itself. At 42,000 light-years from our galaxy’s center, the distorted body stands as the Milky Way’s closest galactic neighbor. This marks the second time that the Milky Way has been found guilty of cosmic cannibalism.

Although the 10-billion-year-old Milky Way formed its basic structure long ago by capturing and merging myriad smaller galaxies, the new finding provides fresh evidence that our galaxy is still snacking on small-fry satellite galaxies. In an upcoming Monthly Notices of the Royal Astronomical Society, Rodrigo A. Ibata of the Strasbourg Observatory in France and his colleagues describe the newly discovered dwarf galaxy, dubbed Canis Major for the constellation in which it resides.

Both theoretical models and observations have indicated that the outskirts of large galaxies, such as the Milky Way, grow by gravitationally capturing gas and stars from their smaller galactic neighbors. But in consuming the newly found dwarf galaxy, the Milky Way is adding material to its starlit disk rather than to the outlying regions, Ibata notes.

His team’s computer simulations reveal that the Milky Way’s disk has been feeding on the dwarf galaxy–tearing out streamers of stars and wrapping them around the Milky Way–for about a billion years and will do so for another billion.

The Canis Major dwarf is a lightweight galaxy, today weighing only as much as a billion suns, or about one-fiftieth as much as the disk of the Milky Way. So far, it may have contributed as much as 1 percent of the disk’s mass.

The dwarf appears to be the origin of a vast ring of stars that circles our galaxy, Ibata says. It was during a search for the source of the ring that Ibata’s team came across a galaxylike clumping of stars.

Finding such a stellar concentration posed a formidable challenge because astronomers had to peer through our galaxy’s veil of dust.

Infrared radiation penetrates the Milky Way’s dust, so Ibata’s team relied on a recently completed near-infrared atlas of some 300 million stars.

By tracing out the infrared signals of bright stars along the Milky Way’s vast outer ring, the researchers discovered the dwarf galaxy, which is below the plane of the Milky Way.

Astronomer Heidi Jo Newberg of the Rensselaer Polytechnic Institute in Troy, N.Y., says that she would need to see more data to be convinced that the clump of stars found by Ibata’s team indeed constitutes a galaxy. But “if they have really found a dwarf galaxy in Canis Major, it is very exciting and almost certainly the progenitor of the ringlike structure,” adds Newberg.

A year ago, Newberg notes, astronomers were convinced that the Milky Way was devouring just one galaxy, which Ibata’s team found in 1994 (SN: 4/9/94, p. 228). The new discovery “opens the door to the possibility that there may be more out there for us to find,” she says.

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