New Dwarf Neighbor Moves In Next to Milky Way
This map shows the stars within the dwarf galaxy Andromeda XII found to have fallen for the first time into the Local Group of galaxies.
Credit: Scott Chapman/University of Cambridge

Updated 7:26 p.m. Eastern


HONOLULU?A galactic tourist has drifted from its hometown into the neighborhood of the Milky Way, marking the first clear evidence of a dwarf galaxy entering the so-called Local Group of galaxies.


The traveling galaxy is the fastest known galaxy in this region and could sweep through the Local Group without so much as a ?rub? with the natives, hurling back out into empty space.


This stellar system, called Andromeda XII, could be the faintest dwarf galaxy ever discovered and may have the lowest mass ever measured, said a co-author of the study, Jorge Penarrubia of the University of Victoria in British Columbia.


New observations show the galaxy is a first-time guest in the Local Group and so has yet to be pushed and pulled by the group?s giants?the Milky Way and Andromeda. The Local Group contains about 40 galaxies, most of which are small satellites called dwarf galaxies that are gravitationally bound to the giants of the group, Andromeda (also called M31) and the Milky Way.


For instance, a pass through the Local Group can strip a dwarf galaxy of up to half of its mass. Galactic guests also lose some of their dark matter from interactions with local-group members. Dwarf galaxies are thought to contain around 50 to 500 times the dark matter found in stars.


?Andromeda XII may be the first galaxy of the local group ever observed that has not yet been disrupted by the strong gravity of the Local Group,? said Penarrubia said.


The new entrant has preserved traits from when it formed and will thus provide an important window into galaxy formation. The finding, presented here at a meeting of the American Astronomical Society (AAS), will shed light on galaxy formation and even the distribution of dark matter.


Galactic travelers


Scientists predict that small galaxies like this one should continue to fall into the Local Group, yet until now none has been spotted. Dubbed the Olympian Galaxy after the twelve Olympians in the Greek Pantheon, Andromeda XII was first spotted in October 2006 in a wide-field survey.?


Scott Chapman of the University of Cambridge, Institute of Astronomy, and his colleagues used the Keck II Observatory operated atop Mauna Kea to observe 49 stars in the region of Andromeda XII, finding that eight were members of the new dwarf galaxy. From their observations, they estimated the galaxy?s orbit, speed and dark-matter content. They found the galaxy has a highly eccentric orbit and is moving at a swift pace through the Local Group.

?Other Local Group members are thought to have extreme orbits, including Leo I, Andromeda XIV and Andromeda XI,? Chapman said, ?but Andromeda XII really stands out as a contender for a new entrant into the Local Group.? He added, ?The others have likely already been seriously harassed by Andromeda and the Milky Way.?


Free-falling galaxy


Its low mass and high speed suggest the galaxy will be a short-term guest. The system is moving at a staggering 345 miles per second (556 kilometers per second) toward Earth, or 174 miles per second (280 kilometers per second) toward Andromeda.


?It's falling into M31 on a plunging orbit (rather than circular) so it is basically at the speed one would expect for something that arrived at the edge of M31's gravity influence and started to free fall toward the center,? Chapman said.?


The universe is not old enough for Andromeda XII to have started its life in the dense Local Group and to now be on its second trip through our system. Plus, by tracing out its past orbit, the astronomers estimate Andromeda XII likely formed about 375,000 light-years from the center of the Andromeda galaxy before free-falling into the star-dense environment of the Local Group.


?Eventually it will merge with Andromeda or the Milky Way,? Chapman told ?It's hard to say which or at what time at this point without a better constrained orbit.?


Another possibility is, ?that Andromeda XII could sail right through the Local Group and continue out into empty space ?passing strangers in the night,?? Chapman said.


The research will be published in an upcoming issue of the Astrophysical Journal.


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