AIP: A Solution to the Galactic Bar Paradox

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bystander
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AIP: A Solution to the Galactic Bar Paradox

Post by bystander » Wed Aug 26, 2020 5:29 pm

Cosmic dance: A Solution to the Galactic Bar Paradox
Leibniz Institute for Astrophysics, Potsdam | 2020 Aug 25
galaxy-simulation-snapshots[1].png
Snapshots of a Milky Way galaxy simulation. The bar seen in the center and the
spiral arms rotate with different rates. If they are disconnected, the bar shows
its true and smaller structure (left). Every time they meet, the bar appears
longer and its rotational speed lower (right). Credit: T. Hilmi/Univ of Surrey

The very heart of our Milky Way harbours a large bar-like structure of stars whose size and rotational speed have been strongly contested in the last years. A new study has found an elegant solution to the discrepancies found in different observational studies, using the fact that the bar and spiral arms move at different rotational velocities, encountering each other about every 80 Million years. As the faster-rotating bar approaches a spiral arm, it appears to be much longer and their ongoing mutual attraction due to gravity periodically varies both their rotational speeds.

While studies of the motions of stars near the Sun suggest the bar is small and rapidly rotating, direct observations of the Galactic central region find it to be significantly longer and slower. An international team of scientists ... has found a solution to this apparent discrepancy. The team looked at the most recent stages of the Milky Way evolution. Analysing state-of-the-art galaxy formation simulations of the Milky Way, the scientists now showed that both the bar’s size and its rotational speed fluctuate in time, causing the bar to appear up to twice as long and 20 percent faster at certain times.

These bar pulsations result from its regular encounters with the Galactic spiral arms, in what can be described as a “cosmic dance”. Spiral arms are density waves within our Galaxy and move at a similar velocity as the Sun. A full rotation around the center of the Milky Way takes about 220 million years, while the central bar needs only about 60 million years. As the faster rotating bar approaches a spiral arm, their mutual attraction due to gravity makes the bar slow down and the spiral arm speed up. Once connected, the two structures move as one and the bar appears much longer than it actually is. As the dancers split apart, the bar speeds up while the spiral arm slows back down. ...

Galactic Bar Paradox Resolved in Cosmic Dance
Royal Astronomical Society | 2020 Aug 25

Fluctuations in Galactic Bar Parameters Due to Bar-Spiral Interaction ~ Tariq Hilmi et al
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