Two teams have announced the detection of a distended galactic satellite associated with NGC 4449, a faint but actively star-forming dwarf galaxy in Canes Venatici, near the Big Dipper. Astronomers first reported a suspicious stream of stars near NGC 4449 in 2007, but the new papers — one in the February 9th Nature and the other appearing in an upcoming issue of the Astrophysical Journal Letters — definitively map the object. NGC 4449 and its buddy, which is also a dwarf galaxy, are about 12 million light-years away and are both members of the Local Group, the collection of about 50 galaxies and dwarf galaxies to which the Milky Way belongs.
The satellite, NGC 4449B, is somewhat S-shaped and looks like it’s recently had a close encounter with NGC 4449. (By recently, I mean within the last billion or so years.) NGC 4449B is in the midst of being torn apart by NGC 4449, much as the Sagittarius dwarf galaxy is suffering a prolonged rending by the Milky Way. In fact, NGC 4449B may be causing its big brother’s starburst activity.
In the composite image at right, created by amateur astronomer R. Jay GaBany from images taken with the Subaru Telescope on Mauna Kea, the satellite appears as a dim swath of red stars. The stars are old enough that they’ve stopped fusing hydrogen in their cores. GaBany collaborated with the ApJL team to confirm the satellite’s existence and is a coauthor on their paper.