A team of astronomers from Germany, the United Kingdom, the United States, France and Japan may have solved the so-called ‘Galactic bar paradox,’ whereby different observations produce contradictory estimates of the motion of the central regions of our Milky Way Galaxy.
When the bar (horizontal in yellow and white) is in its shorter phase on the left, a few spiral arms can be seen near, but not quite firmly connected to the bar. Meanwhile, when the bar is at its longest on the right, there are two stronger spiral arms passing through this time around. These are more clearly connected to either end of the bar, effectively dragging it out and slowing it down. Image credit: Hilmi et al, doi: 10.1093/mnras/staa1934.
The majority of spiral galaxies, like the Milky Way, host a large bar-like structure of stars in their center.
Knowledge of the true bar size and rotational speed is crucial for understanding how galaxies form and evolve, as well as how they form similar bars throughout the Universe.
However, the size and rotational speed of Milky Way’s bar have been strongly contested in the last five years.
While studies of the motions of stars near the Sun find a bar that is both fast and small, direct observations of the Galactic central region agree on one that is significantly slower and larger.
The new study suggests an insightful solution to this discrepancy.
Analysing state-of-the-art galaxy formation simulations of the Milky Way, they show that both the bar’s size and its rotational speed fluctuate rapidly in time, causing the bar to appear up to twice as long and rotate 20% faster at certain times.
The bar pulsations result from its regular encounters with the galaxy’s spiral arms, in what can be described as a ‘cosmic dance.’
As the bar and spiral arm approach each other, their mutual attraction due to gravity makes the bar slow down and the spiral speed up.
Once connected, the two structures move as one and the bar appears much longer and slower than it actually is.
As the dancers split apart, the bar speeds up while the spiral slows back down.
“The controversy about the Galactic bar can then be simply resolved if we happen to be living at a time when the bar and spiral are connected, giving the illusion of a large and slow bar,” said co-lead author Dr. Ivan Minchev, an astronomer at the Leibniz Institute for Astrophysics Potsdam.
“However, the motion of the stars near the Sun remains governed by the bar’s true, much smaller nature, and so those observations appear contradictory,” added co-lead author Tariq Hilmi, a postgraduate research student in the Astrophysics Research Group at the University of Surrey and the Leibniz Institute for Astrophysics Potsdam.
The study was published in the Monthly Notices of the Royal Astronomical Society.
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T. Hilmi et al. 2020. Fluctuations in galactic bar parameters due to bar-spiral interaction. MNRAS 497 (1): 933-955; doi: 10.1093/mnras/staa1934
