We are living through our Solar System’s twentieth, 223-250 million-year orbit of the Milky Way near the inner edge of the ephemeral spiral feature known as the Orion Arm or, less poetically, the Local Arm . The ghostly spiral arm is a concentrations of gas and dust where stars form, produced by disturbances within the Milky Way, or on occasions by a jolt from outside, such as a supernova. Life-threatening asteroids and comets are more frequent when a planetary system is crossing one of the galaxy’s spiral arms, where potentially deadly, dusty gas clouds congregate, stacking up in the equivalent of a hydrostatic jump.
The Galactic Bar Paradox
We also happen to be living at a time when the Milky Way’s central bar and spiral are connected, giving the illusion of a large and slow bar. Embedded at the center the center of the Milky Way’s spiral arms, the bar is 27,000 light-years long and cuts across the heart of the central region where the supermassive black hole resides.
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.”” says Ivan Minchev of the Leibniz Institute for Astrophysics Potsdam (AIP), about new research that sheds light on a mysterious and long-standing conundrum at the very heart of our galaxy, the so-called “Galactic bar paradox,” whereby different observations produce contradictory estimates of the motion of the central regions of the Milky Way.
The image below from a Milky Way galaxy simulation shows the bar in the center and the spiral arms are thought to rotate at different speeds. 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
The Milky Way’s bar size and rotational speed have been strongly contested in the last 5 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 majority of spiral galaxies 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.
The new study, by an international team of scientists led by Tariq Hilmi of the University of Surrey and Minchev, provides a solution to this discrepancy. Analyzing 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, creating the illusion of the bar to appear up to twice as long and rotate 20 percent faster at certain times.
Spiral Arms Create an 80-Million-Year Cosmic Dance
The bar pulsations, reports the Royal Astronomical Society, result from its regular encounters with the Galactic 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.
Recent observations have confirmed that the inner Milky Way spiral arm is currently connected to the bar, which happens about once every 80 million years according to the simulations. Data from the forthcoming 3rd data release of the Gaia mission will be able to test this model further, and future missions will discover if the dance goes on in other galaxies across the universe.
Source: T Hilmi et al. Fluctuations in galactic bar parameters due to bar–spiral interaction, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa1934