Infant Galaxies Discovered in Nearby Galaxy Group

A team of astronomers has discovered objects in the nearby M81 Galaxy Group that may be small, young galaxies that have formed only recently. If confirmed, these objects would be among the nearest examples of tidal dwarf galaxies—small galaxies formed from gas expelled by a larger galaxy due to gravitational interactions with other galaxies in the group. A report of their work was presented at a meeting of the American Astronomical Society in Atlanta, Georgia.

The team members include undergraduate student Megan DeCesar, research associate Patrick Durrell, and professor Robin Ciardullo at Penn State, and Case Western Reserve University research associates John Feldmeier and Denise Hurley-Keller who are both National Science Foundation Astronomy and Astrophysics Postdoctoral Fellows.

To find these curious objects, the team used the 100-Megapixel CCD camera of the Canada-France-Hawaii Telescope (CFHT), which can image an area of the sky 1.5 times the area of the Full Moon. In their search of the “empty” regions between the galaxies M81 and NGC3077, the team found two small clumps of bright blue stars. These clumps, each of which are over 3000 light years across, have never been observed before and may reflect the initial burst of star formation in newly formed galaxies.

The M81 group of galaxies, named for its largest member, lies about 12 million light years from us. Like the Local Group of galaxies, home to the Milky Way, the M81 group consists of a few large spirals and a few dozen smaller dwarf galaxies. However, the large spirals in the M81 group have passed close to each other recently—about 200 to 300 million years ago according to computer simulations. This close encounter caused large streams of hydrogen gas to be ripped out into intergalactic space. The clumps of young blue stars discovered by DeCesar and her collaborators lie inside these regions of stripped gas. Says Durrell, “These stars have almost certainly been formed inside the gas clouds. It gives us a close-up view of how star formation can occur when two galaxies pass close to each other.”

What these new objects truly are is unclear. They may well be tiny young galaxies which have just begun to form stars, each with less than 0.1 percent as many stars as our own Milky Way galaxy. Alternatively, these stars may not be bound to any galaxy, and may be destined to live their lives in intracluster space. What is clear, however, is that these stars did not form inside any of the large galaxies. Because hot blue stars live only a short time, their presence implies that the clumps cannot be more than 100 million years old, which is extremely young compared to most galaxies. Moreover, the new stars must have formed outside of the larger galaxies because the M81 group gas was stripped from its parent galaxies during the interaction over 200 million years ago. Hurley-Keller adds, “If these stars had formed inside the large galaxies, and then simply got ripped out, they would have expired long ago, before wecould detect them.”

If the young star clumps are indeed “infant” dwarf galaxies, they would be among the closest examples of tidal dwarf galaxies. But even if these objects are not dwarf galaxies, their existence still proves that star formation can take place without a parent galaxy if there is a sufficient amount of gas around to make the process work. Says Feldmeier, “We have plenty of evidence that large galaxy clusters have stars outside the galaxies, but in this nearby, less-dense group, we are seeing stars actually form in intergalactic space.”

Patrick Durrell, Robin Ciardullo, and Barbara K. Kennedy