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Figures of Eight and Peanut Shells: How stars move at the centre of the Galaxy

Last Updated on Sunday, 01 December 2013 14:38
Published on Wednesday, 27 November 2013 12:22

 

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.

eso milky way x-shape smallAn 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 for a larger image 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

 

Leonor Sierra
University of Rochester
Tel: +1 585 276 6264
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Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
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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

 

 


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., the University gives students exceptional opportunities for interdisciplinary study and close collaboration with faculty through its unique cluster-based curriculum. Its College, School of Arts and Sciences, and Hajim School of Engineering and Applied Sciences are complemented by its Eastman School of Music, Simon School of Business, Warner School of Education, Laboratory for Laser Energetics, School of Medicine and Dentistry, School of Nursing, Eastman Institute for Oral Health, and the Memorial Art Gallery.

The Royal Astronomical Society (RAS, www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

Follow the RAS on Twitter via @royalastrosoc