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 is being
presented today at the American Astronomical Society meeting in Atlanta, Georgia.

The team members presenting the report at the meeting include Megan DeCesar, an
undergraduate student at Penn State University in University Park, Pennsylvania;
Patrick Durrell, a research associate at Penn State; and John Feldmeier, a
research associate at Case Western Reserve University in Cleveland, Ohio, and a
National Science Foundation Astronomy and Astrophysics Postdoctoral Fellow.
Other team members include Robin Ciardullo, professor of astronomy and
astrophysics at Penn State; and Denise Hurley-Keller, research associate at Case
Western Reserve University and a National Science Foundation Astronomy and
Astrophysics Postdoctoral Fellow.

To find these curious objects, the team used the 100-Megapixel CCD camera of the
Canada-France-Hawaii Telescope (CFHT), which can image a 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 we
could 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.”

[NOTE: Images supporting this release are available at
http://www.astro.psu.edu/users/pdurrell/M81/pr.html ]