An
artist’s impression showing how the Milky Way galaxy would look seen
from almost edge on and from a very different perspective than we get
from the Earth. The central bulge shows up as a peanut shaped glowing
ball of stars and the spiral arms and their associated dust clouds form a
narrow band. Credit: ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt. Click here
for a larger image
Two months ago astronomers created a new 3D map of stars at the
centre of our Galaxy (the Milky Way), showing more clearly than ever the
bulge at its core. Previous explanations suggested that the stars that
form the bulge are in banana-like orbits, but a paper published this
week in Monthly Notices of the Royal Astronomical Society suggests that
the stars probably move in peanut-shell or figure of eight-shaped orbits
instead.
The difference is important; astronomers develop theories of star
motions to not only understand how the stars in our galaxy are moving
today but also how our galaxy formed and evolves. The Milky Way is
shaped like a spiral, with a region of stars at the centre known as the
“bar,” because of its shape. In the middle of this region, there is a
“bulge” that expands out vertically.
In the new work Alice Quillen, professor of astronomy at the
University of Rochester, and her collaborators created a mathematical
model of what might be happening at the centre of the Milky Way. Unlike
the Solar System where most of the gravitational pull comes from the Sun
and is simple to model, it is much harder to describe the gravitational
field near the centre of the Galaxy, where millions of stars, vast
clouds of dust, and even dark matter swirl about. In this case, Quillen
and her colleagues considered the forces acting on the stars in or near
the bulge.
As the stars go round in their orbits, they also move above or below
the plane of the bar. When stars cross the plane they get a little push,
like a child on a swing. At the resonance point, which is a point a
certain distance from the centre of the bar, the timing of the pushes on
the stars is such that this effect is strong enough to make the stars
at this point move up higher above the plane. (It is like when a child
on the swing has been pushed a little every time and eventually is
swinging higher.) These stars are pushed out from the edge of the bulge.
The resonance at this point means that stars undergo two vertical
oscillations for every orbital period. But what is the most likely shape
of the orbits in between? The researchers showed through computer
simulations that peanut-shell shaped orbits are consistent with the
effect of this resonance and could give rise to the observed shape of
the bulge, which is also like a peanut-shell.
Next month the European Space Agency will launch the Gaia spacecraft,
which is designed to create a 3D map of the stars in the Milky Way and
their motions. This 3D map will help astronomers better understand the
composition, formation and evolution of our Galaxy.
“It is hard to look back into the past of our galaxy and know what
was there, but simulations can give us clues,” explained Quillen. “Using
my model I saw that, over time, the resonance with the bar, which is
what leads to these peculiarly shaped orbits, moves outwards. This may
be what happened in our Galaxy.”
“Gaia will generate huge amounts of data – on billions of stars,”
said Quillen. This data will allow Quillen and her colleagues to finesse
their model further. “This can lead to a better understanding of how
the Milky Way might have evolved into the shape it has today.”
Quillen explained that there are different models as to how the
galactic bulge was formed. Astronomers are interested in finding out how
much the bar has slowed down over time and whether the bulge “puffed up
all at once or slowly.” Understanding the distributions of speeds and
directions of motion (velocities) of the stars in the bar and the bulge
might help determine this evolution.
“One of the predictions of my model is that there is a sharp
difference in the velocity distributions inside and outside the
resonance,” Quillen said. “Inside – closer to the galactic centre – the
disk should be puffed up and the stars there would have higher vertical
velocities. Gaia will measure the motions of the stars and allow us to
look for variations in velocity distributions such as these.”
To be able to generate a model for the orbits of stars in the bulge,
Quillen needed to factor in different variables. She first needed to
understand what happens at the region of the resonance, which depends on
the speed of the rotating bar and the mass density of the bar.
“Before I could model the orbits, I needed the answer to what I
thought was a simple question: what is the distribution of material in
the inner galaxy?” Quillen said. “But this wasn’t something I could just
look up. Luckily my collaborator Sanjib Sharma was able to help out.”
Sharma worked out how the speed of circular orbits changed with
distance from the galactic centre (called the rotation curve). Using
this information, Quillen could compute a mass density at the location
of the resonance, which she needed for her model.
Quillen was also able to combine the new orbit models with the speed
of the bar (which is rotating) to get a more refined estimate of the
mass density 3000 light years from the Galaxy centre (about one eighth
of the distance from the centre of the Galaxy to Earth), which is where
the edge of the bulge is.
And there is not long now to wait now for Gaia to start collecting
data. Gaia's launch is set for 0912 GMT on December 19, and will be
streamed live on the ESA Portal.
Media contacts
Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
rm@ras.org.uk
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
rm@ras.org.uk
Further information
Quillen’s co-authors in this paper are Sanjib Sharma, Sydney
Institute for Astronomy, Australia; Ivan Minchev, Astronomy Institute of
Potsdam, Germany; Yu-Jing Qin, Shanghai Astronomical Observatory,
China; and Paola Di Matteo, Paris-Meudon Observatory, France.
The new work appears in “A Vertical Resonance Heating Model for X- or
Peanut-Shaped Galactic Bulges”, Monthly Notices of the Royal
Astronomical Society, Alice C. Quillen, in press.
A preprint of the paper can be seen at http://arxiv.org/pdf/1307.8441.pdf
GAIA mission: http://sci.esa.int/gaia/
Image and movies
An artist’s impression showing how the Milky Way galaxy would look
seen from almost edge on and from a very different perspective than we
get from the Earth. The central bulge shows up as a peanut shaped
glowing ball of stars and the spiral arms and their associated dust
clouds form a narrow band. Credit: ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt. http://www.eso.org/public/usa/images/eso1339a/
Two movies of N-body simulations, one showing a bar that buckles, the
other without buckling. These movies show face-on and edge-on views of
barred galaxies. Both movies show that the peanut shape becomes more
extended as the bar slows down. Credit: Ivan Minchev.
With buckling: http://astro.pas.rochester.edu/~aquillen/mytalks/ivan_gS0_peanut.mp4
Without buckling: http://astro.pas.rochester.edu/~aquillen/mytalks/ivan_gSa_peanut.mp4
Without buckling: http://astro.pas.rochester.edu/~aquillen/mytalks/ivan_gSa_peanut.mp4
Notes for editors
The University of Rochester (www.rochester.edu) is one of the leading
private universities in the United States. Located in Rochester, N.Y.,
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