Dynamics of galactic disks

5 problems

• When and how did galactic bars first form?

  Open

  The James Webb Space Telescope has found bars in galaxies only two to three billion years after the Big Bang. It is not known whether bars form on their own in cooled-down disks or are triggered by collisions with other galaxies.

• Do observed bar speeds agree with dark-matter friction predictions?

  Contested

  A rotating bar of stars in a galaxy center should be braked by the surrounding dark matter. Many observed bars still turn fast, which may conflict with the standard dark-matter model.

• Relative roles of disk heating and radial migration

  Open

  Older stars in the disk move more randomly and many no longer orbit where they were born. How much of this comes from spiral arms, the bar, gas clouds or passing satellites is not settled.

• Origin of the Gaia phase spiral in the Milky Way disk

  Contested

  Gaia showed that nearby stars, plotted by height above the disk against vertical speed, trace a snail-shell spiral. It may be the ringing from a passing dwarf galaxy or from the Milky Way's own bar and arms.

• Are spiral arms long-lived density waves or recurrent transients?

  Contested

  Spiral arms could be a fixed pattern that stars pass through, like a traffic jam, or short-lived patterns that keep reforming. Simulations mostly give short-lived arms, while classic theory assumes lasting ones.