This illustration shows the disk of our Milky Way galaxy, surrounded
by a faint, extended halo of old stars. Astronomers using the Hubble
Space Telescope to observe the nearby Andromeda galaxy serendipitously
identified a dozen foreground stars in the Milky Way halo. They
measured the first sideways motions (represented by the arrows) for
such distant halo stars. The motions indicate the possible presence of a
shell in the halo, which may have formed from the accretion of a dwarf
galaxy. This observation supports the view that the Milky Way has
undergone continuing growth and evolution over its lifetime by
consuming smaller galaxies.
Science Credit: NASA, ESA, A. Deason and P. Guhathakurta (University of California, Santa Cruz), and R. van der Marel, T. Sohn, and T. Brown (STScI)
Peering deep into the vast stellar halo that envelops our Milky Way
galaxy, astronomers using NASA's Hubble Space Telescope have uncovered
tantalizing evidence for the possible existence of a shell of stars
that are a relic of cannibalism by our Milky Way.
Hubble was used to precisely measure, for the first time ever, the
sideways motions of a small sample of stars located far from the
galaxy's center. Their unusual lateral motion is circumstantial
evidence that the stars may be the remnants of a shredded galaxy that
was gravitationally ripped apart by the Milky Way billions of years
ago. These stars support the idea that the Milky Way grew, in part,
through the accretion of smaller galaxies.
"Hubble's unique capabilities are allowing astronomers to uncover
clues to the galaxy's remote past. The more distant regions of the
galaxy have evolved more slowly than the inner sections. Objects in the
outer regions still bear the signatures of events that happened long
ago," said Roeland van der Marel of the Space Telescope Science
Institute (STScI) in Baltimore, Md.
They also offer a new opportunity for measuring the "hidden" mass of
our galaxy, which is in the form of dark matter (an invisible form of
matter that does not emit or reflect radiation). In a universe full of
100 billion galaxies, our Milky Way "home" offers the closest and
therefore best site for detailed study of the history and architecture
of a galaxy.
A team of astronomers led by Alis Deason of the University of
California, Santa Cruz, and van der Marel identified 13 stars located
roughly 80,000 light-years from the galaxy's center. They lie in the
Milky Way's outer halo of ancient stars that date back to the formation
of our galaxy.
The team was surprised to find that the stars showed more of a
sideways, or tangential, amount of motion than they expected. This
movement is different from what astronomers know about the halo stars
near the Sun, which move predominantly in radial orbits. Stars in these
orbits plunge toward the galactic center and travel back out again. The
stars' tangential motion can be explained if there is an over-density
of stars at 80,000 light-years, like cars backing up on an expressway.
This traffic jam would form a shell-like feature, as seen around other
galaxies.
Deason and her team plucked the outer halo stars out of seven years'
worth of archival Hubble telescope observations of our neighboring
Andromeda galaxy. In those observations, Hubble peered through the
Milky Way's halo to study the Andromeda stars, which are more than 20
times farther away. The Milky Way's halo stars were in the foreground
and considered as clutter for the study of Andromeda. But to Deason's
study they were pure gold. The observations offered a unique
opportunity to look at the motion of Milky Way halo stars.
Finding the stars was meticulous work. Each Hubble image contained
more than 100,000 stars. "We had to somehow find those few stars that
actually belonged to the Milky Way halo," van der Marel said. "It was
like finding needles in a haystack."
The astronomers identified the stars based on their colors,
brightnesses, and sideways motions. The halo stars appear to move
faster than the Andromeda stars because they are so much closer. Team
member Sangmo Tony Sohn of STScI identified the halo stars and measured
both the amount and direction of their slight sideways motion. The
stars move on the sky only about one milliarcsecond a year, which would
be like watching a golf ball on the Moon moving one foot per month.
Nonetheless, this was measured with 5 percent precision, made possible
in visible-light observations because of Hubble's razor-sharp view and
instrument consistency.
"Measurements of this accuracy are enabled by a combination of
Hubble's sharp view, the many years' worth of observations, and the
telescope's stability. Hubble is located in the space environment, and
it's free of gravity, wind, atmosphere, and seismic perturbations," van
der Marel said.
Stars in the inner halo have highly radial orbits. When the team
compared the tangential motion of the outer halo stars with their
radial motion, they were very surprised to find that the two were
equal. Computer simulations of galaxy formation normally show an
increasing tendency towards radial motion if one moves further out in
the halo. These observations imply the opposite trend. The existence of
a shell structure in the Milky Way halo is one plausible explanation
of the researchers' findings. Such a shell can form by accretion of a
satellite galaxy. This is consistent with a picture in which the Milky
Way has undergone continuing evolution over its lifetime due to the
accretion of satellite galaxies.
The team compared their results with data of halo stars recorded in
the Sloan Digital Sky Survey. Those observations uncovered a higher
density of stars at about the same distance as the 13 outer halo stars
in their Hubble study. A similar excess of halo stars exists across the
Triangulum and Andromeda constellations. Beyond that radius, the
number of stars plummets.
Deason immediately thought the two results were more than just
coincidence. "What may be happening is that the stars are moving quite
slowly because they are at the apocenter, the farthest point in their
orbit about the hub of our Milky Way," Deason explained. "The slowdown
creates a pileup of stars as they loop around in their path and travel
back towards the galaxy. So their in and out or radial motion decreases
compared with their sideways or tangential motion."
Shells of stars have been seen in the halos of some galaxies, and
astronomers predicted that the Milky Way may contain them, too. But
until now there was limited evidence for their existence. The halo
stars in our galaxy are hard to see because they are dim and spread
across the sky.
Encouraged by this study, the team hopes to search for more distant
halo stars in the Hubble archive. "These unexpected results fuel our
interest in looking for more stars to confirm that this is really
happening," Deason said. "At the moment we have quite a small sample.
So we really can make it a lot more robust with getting more fields with
Hubble." The Andromeda observations only cover a very small "keyhole
view" of the sky.
The team's goal is to put together a clearer picture of the Milky
Way's formation history. By knowing the orbits and motions of many halo
stars it will also be possible to calculate an accurate mass for the
galaxy. "Until now, what we have been missing is the stars' tangential
motion, which is a key component. The tangential motion will allow us to
better measure the total mass distribution of the galaxy, which is
dominated by dark matter. By studying the mass distribution, we can see
whether it follows the same distribution as predicted in theories of
structure formation," Deason said.
The Hubble study will appear in an upcoming issue of the Astrophysical Journal.
The science team consists of A. Deason and P. Guhathakurta of
UCO/Lick Observatory, University of California, Santa Cruz, Calif., and
R.P. van der Marel, S.T. Sohn, and T.M. Brown of the Space Telescope
Science Institute, Baltimore, Md.
CONTACT
Donna WeaverSpace Telescope Science Institute, Baltimore, Md.
410-338-4493
dweaver@stsci.edu
Alis Deason
University of California, Santa Cruz, Calif.
831-459-3841
alis@ucolick.org
Roeland van der Marel
Space Telescope Science Institute, Baltimore, Md.
410-338-4931
marel@stsci.edu
