ESA: Milky Way's Warp Caused by Galactic Collision

[bystander]

Milky Way's Warp Caused by Galactic Collision, Gaia Suggests
ESA | Space Science | Science & Technology | Gaia | 2020 Mar 02

Astronomers have pondered for years why our galaxy, the Milky Way, is warped. Data from ESA's star-mapping satellite Gaia suggest the distortion might be caused by an ongoing collision with another, smaller, galaxy, which sends ripples through the galactic disc like a rock thrown into water.

The warped disc of our galaxy, the Milky Way. Credit: Stefan Payne-Wardenaar; Magellanic Clouds: Robert Gendler/ESO

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Since the late 1950s, astronomers have known that the Milky Way's disc – where most of its hundreds of billions of stars reside – is not flat but somewhat curved upwards on one side and downwards on the other. For years, they debated what is causing this warp. They proposed various theories including the influence of the intergalactic magnetic field or the effects of a dark matter halo, a large amount of unseen matter that is expected to surround galaxies. If such a halo had an irregular shape, its gravitational force could bend the galactic disc.

With its unique survey of more than one billion stars in our galaxy, Gaia might hold the key to solving this mystery. A team of scientists using data from the second Gaia data release has now confirmed previous hints that this warp is not static but changes its orientation over time. Astronomers call this phenomenon precession and it could be compared to the wobble of a spinning top as its axis rotates.

Moreover, the speed at which the warp precesses is much faster than expected – faster than the intergalactic magnetic field or the dark matter halo would allow. That suggests the warp must be caused by something else. Something more powerful – like a collision with another galaxy. ...

Evidence of a Dynamically Evolving Galactic Warp ~ E. Poggio et al

• Nature Astronomy (online 02 Mar 2020) DOI: 10.1038/s41550-020-1017-3

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<<Wood warps when the moisture content in the wood changes unevenly. Think of it this way: You have a 2×4 that gets wet. As it dries, one part of the board dries faster than the other, causing the drier area to shrink faster. That causes stress on the wood and can cause it to warp or bend.

While some types of wood have a relatively high moisture content, other types have a very low moisture content. The expanding then shrinking of wood is caused by both the wood’s moisture content and the humidity level in the air. As humidity increases, the moisture content of wood will also increase and the wood expands. As humidity decreases, moisture content will also decrease and the wood will shrink. This is known as the Equilibrium Moisture Content, or EMC.>>