If the Milky Way were ever indicted for gobbling dwarf galaxies, it would have a defence: other giant galaxies are doing the same thing.
Two teams of astronomers independently spotted a dwarf in distress orbiting NGC 253, a giant spiral galaxy 11 million light-years from Earth and the largest member of the Sculptor group of galaxies. The dwarf spans roughly 20,000 light-years and its stars add up to some 10 million times the mass of the sun – 5000 times less than the stellar mass of the Milky Way.
The new-found galaxy is a ghostly type known as a dwarf spheroidal, whose stars are widely separated from one another. The first such galaxy ever found, in 1938, was initially dismissed as a fingerprint or defect on a photographic plate.
Dozens of these galaxies orbit the Milky Way, but dwarf spheroidals are so dim that until recently no one had seen any beyond the Local Group, the collection of galaxies that includes our own.
But unlike most others of its type, the new dwarf is very elongated, with its long axis pointing towards the giant galaxy. This suggests the giant’s gravitational pull is stretching the dwarf by tugging more strongly on its near side than its far side.
“It looks like it’s being ripped apart by the larger galaxy,” says Aaron Romanowsky of San Jose State University in California, a member of one of the teams that discovered the new galaxy.
Astronomers caught our galaxy tearing apart a dwarf galaxy in the constellation Sagittarius in 1994. The latest discovery is the first time a giant galaxy beyond the Local Group has been caught disrupting a dwarf spheroidal, whose stars will join the predator galaxy.
“It is similar to the Sagittarius dwarf,” says Elisa Toloba of Texas Tech University in Lubbock, a member of the other team that discovered the galaxy.
Although the two teams reach similar conclusions, they used radically different instruments to discover the dwarf. Toloba and her colleagues deployed the giant 6.5-metre Magellan telescope in Chile, while Romanowsky’s group used 18 and 30-centimetre telescopes owned by amateur astronomers.
Image: © 2015 R. Jay GaBany (Cosmotography.com), Zachary Jennings (University of California, Santa Cruz) and National Astronomical Observatory of Japan