Milky Way a galactic cannibal
By Fariss Samarrai
Little was right. The sky is falling. Thousands 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), a
major 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 in visually vivid detail how its debris wraps
around and passes through our Milky Way. Sagittarius is 10,000
times smaller in mass than the Milky Way, so it is getting stretched
out, torn apart and gobbled up by the bigger Milky Way.
clear whos the bully in the interaction, said Steven
Majewski, U.Va. professor of astronomy and lead author on the
paper describing the results.
In model images made to show the interaction in 3-D, 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
said. But at human, visual wavelengths, they become buried
among countless intervening stars 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 called
an M giant. These large, infrared-bright stars are populous in
the Sagittarius galaxy but uncommon 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 galaxys 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.
sifted several thousand interesting stars from a catalog of half
a billion, said 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.
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
studys 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 system.
this mean we are at a unique moment in the life of our galaxy?
Yes and no.
Whenever possible, astronomers appeal to the principle that
we are not at a special time or place in the universe, Majewski
said. Because over the 14 billion-year history of the Milky
Way it is unlikely that we would just happen 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 Earths proximity to a portion of the Sagittarius
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.
have to re-think our assumptions about the Milky Way galaxy to
account for this contamination.
new findings will help astronomers measure the total mass of the
Milky Way and Sagittarius galaxies, and probe the quantity and
distribution of the invisible dark matter in these systems.
our galaxy is unusual, Spergel said, or the dark matter
has richer properties than postulated by conventional models.