Updated7:26 p.m. Eastern
HONOLULU?Agalactic tourist has drifted from its hometown into the neighborhood of theMilky Way, marking the first clear evidence of a dwarf galaxy entering theso-called Local Group of galaxies.
Thetraveling galaxy is the fastest known galaxy in this region and could sweepthrough the Local Group without so much as a ?rub? with the natives, hurlingback out into empty space.
Thisstellar system, called Andromeda XII, could be the faintest dwarfgalaxy ever discovered and may have the lowest mass ever measured, said aco-author of the study, Jorge Penarrubia of the University of Victoria inBritish Columbia.
Newobservations show the galaxy is a first-time guest in the Local Group and sohas yet to be pushed and pulled by the group?s giants?the Milky Way andAndromeda. The Local Group contains about 40 galaxies, most of which are smallsatellites called dwarf galaxies that are gravitationally bound to the giantsof the group, Andromeda(also called M31) and the Milky Way.
Forinstance, a pass through the Local Group can strip a dwarf galaxy of up to halfof its mass. Galactic guests also lose some of their dark matter frominteractions with local-group members. Dwarf galaxies are thought to containaround 50 to 500 times the dark matter found in stars.
?AndromedaXII may be the first galaxy of the local group ever observed that has not yetbeen disrupted by the strong gravity of the Local Group,? said Penarrubia said.
The newentrant has preserved traits from when it formed and will thus provide animportant window into galaxy formation. The finding, presented here at ameeting of the American Astronomical Society (AAS), will shed light on galaxyformation and even the distribution of dark matter.
Scientistspredict that small galaxieslike this one should continue to fall into the Local Group, yet until now nonehas been spotted. Dubbed the Olympian Galaxy after the twelve Olympians in theGreek Pantheon, Andromeda XII was first spotted in October 2006 in a wide-fieldsurvey.?
ScottChapman of the University of Cambridge, Institute of Astronomy, and hiscolleagues used the Keck II Observatory operated atop Mauna Kea to observe 49stars in the region of Andromeda XII, finding that eight were members of thenew dwarf galaxy. From their observations, they estimated the galaxy?s orbit,speed and dark-matter content. They found the galaxy has a highly eccentricorbit and is moving at a swift pace through the Local Group.
?OtherLocal Group members are thought to have extreme orbits, including Leo I,Andromeda XIV and Andromeda XI,? Chapman said, ?but Andromeda XII really standsout as a contender for a new entrant into the Local Group.? He added, ?Theothers have likely already been seriously harassed by Andromeda and the MilkyWay.?
Its lowmass and high speed suggest the galaxy will be a short-term guest. The systemis moving at a staggering 345 miles per second (556 kilometers per second)toward Earth, or 174 miles per second (280 kilometers per second) towardAndromeda.
?It'sfalling into M31 on a plunging orbit (rather than circular) so it is basicallyat the speed one would expect for something that arrived at the edge of M31'sgravity influence and started to free fall toward the center,? Chapman said.?
The universeis not old enough for Andromeda XII to have started its life in the dense LocalGroup and to now be on its second trip through our system. Plus, by tracing outits past orbit, the astronomers estimate Andromeda XII likely formed about375,000 light-years from the center of the Andromeda galaxy before free-fallinginto the star-dense environment of the Local Group.
?Eventuallyit will merge with Andromeda or the Milky Way,? Chapman told SPACE.com.?It's hard to say which or at what time at this point without a betterconstrained orbit.?
Anotherpossibility is, ?that Andromeda XII could sail right through the Local Groupand continue out into empty space ?passing strangers in the night,?? Chapmansaid.
Theresearch will be published in an upcoming issue of the Astrophysical Journal.
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