Map Of The Milky Way Shows Our Galaxy To Be A Cannibal
Study Shows the Milky Way is Out to Lunch
September 24, 2003 --
Chicken Little was right. The sky is falling.
of stars stripped from the nearby Sagittarius dwarf galaxy are
streaming through our vicinity of the Milky Way galaxy, according
to a new view of the
local universe constructed by a team of astronomers from the University of
Virginia and the University of Massachusetts.
volumes of data from the Two-Micron All Sky Survey (2MASS),
project to survey the sky in infrared light led by the
University of Massachusetts,
the astronomers are answering questions that have baffled scientists for
decades and proving that our own Milky Way is consuming one
of its neighbors in a dramatic
display of ongoing galactic cannibalism.
study, to be published in the Dec. 20 issue of the Astrophysical
Journal, is the first
to map the full extent of the Sagittarius galaxy and show
visually vivid detail how its debris wraps around and passes through our
Sagittarius is 10,000 times smaller in mass than the Milky Way, so it is
out, torn apart and gobbled up by the bigger Milky Way.
clear who’s the bully in the interaction,” said
Steven Majewski, U.Va. professor of astronomy and lead author on the paper
describing the results.
model images made to show the interaction in 3-D, available
the Milky Way appears as a flattened disk with spiral arms, while Sagittarius
is visible as a long flourish of stars swirling first under and then
over and onto the Milky Way disk.
people had infrared-sensitive eyes, the entrails of Sagittarius
would be a prominent fixture sweeping across our sky,” Majewski
at human, visual wavelengths, they become buried among countless intervening
and obscuring dust. The great expanse of the Sagittarius system has
been hidden from view.”
any more. By using infrared maps, the astronomers filtered
away millions of foreground stars to focus on a type of star
M giant. These
large, infrared-bright stars are populous in the Sagittarius galaxy
in the outer Milky Way. The 2MASS infrared map of M giant stars
analyzed by Majewski
and collaborators is the first to give a complete view of our galaxy’s
meal of Sagittarius stars, now wrapping like a spaghetti noodle around
the Milky Way. Prior to this work, astronomers had detected only
a few scattered pieces
of the disrupted Sagittarius dwarf. Even the existence of Sagittarius
was unknown until the heart of this nearest satellite galaxy of the
Milky Way was discovered
by a British team of astronomers in 1994.
“We sifted several thousand interesting stars from a catalog of half a
co-author Michael Skrutskie, U.Va. professor of astronomy and principal
investigator for the 2MASS project. “By tuning our maps of the sky to the ‘right’ kind
of star, the Sagittarius system jumped into view.”
“This first full-sky map of Sagittarius shows its extensive interaction
with the Milky Way,” Majewski said. “Both stars and star clusters
now in the outer parts of the Milky Way have been ‘stolen’ from Sagittarius
as the gravitational forces of the Milky Way nibbled away at
its dwarf companion. This one vivid example shows that the Milky Way grows by
eating its smaller neighbors.”
used to view galaxy formation as an event that happened in
the distant past,” noted David Spergel, a professor of astrophysics at
Princeton University after viewing the new finding. “These observations
reinforce the idea that galaxy formation is not an event, but an ongoing process.”
study’s map of M giants depicts 2 billion years of Sagittarius stripping
by the Milky Way, and suggests that Sagittarius has reached
a critical phase in what had been a slow dance of death.
slow, continuous gnawing by the Milky Way, Sagittarius has
been whittled down to the point that it cannot hold itself
together much longer,” said
2MASS Science Team member and study co-author Martin
Weinberg of the University of Massachusetts. “We are
seeing Sagittarius at the very end of its life as an intact
this mean we are at a unique moment in the life of our galaxy?
Yes and no.
possible, astronomers appeal to the principle that we are
not at a special time or place in the universe,” Majewski
over the 14 billion-year history of the Milky Way it is unlikely that we would
to catch a brief event like the death of Sagittarius,
we infer that such events must be common in the life of big spiral galaxies
like our own. The Milky Way
probably dined on a number of dwarf galaxy snacks
in the past.”
the other hand, Majewski and his colleagues have been surprised
by the Earth’s
proximity to a portion of the Sagittarius debris.
only a few percent of its 240 million-year orbit around the
Milky Way galaxy does our Solar System pass through the path
of Sagittarius debris,” Majewski
said. “Remarkably, stars from Sagittarius
are now raining down onto our present position
in the Milky Way. Stars from an alien galaxy
near us. We have to re-think our assumptions
about the Milky Way galaxy to account for this
new findings will help astronomers measure the total mass
of the Milky
Way and Sagittarius
dark matter in these systems.
“The shape of the Sagittarius debris trail shows us that the Milky Way’s
unseen dark matter is in a spherical distribution,
a result that is quite unexpected,” Weinberg
observations provide new insights into the nature of the
mysterious dark matter,” said Princeton’s Spergel. “Either
our galaxy is unusual or the dark matter has richer properties
than postulated by conventional
was a joint project of the University of Massachusetts and
the Infrared Processing
Institute of Technology.
National Aeronautics and Space Administration,
and the National Science Foundation
funded the project. Additional funding
for the Sagittarius study with 2MASS came from
the David and Lucile Packard Foundation
and the Research Corporation.
Fariss Samarrai, (434) 924-3778, Dr. Steven Majewski, (434) 924-4893,
Dr. Michael Skrutski, (434) 924-4328, Dr. Martin Weinberg,