Take a child's spinning top and set it on the table. It spins steadily, with all its parts moving in the same direction. Now toss a handful of beads onto the floor and blow on them: each bead will fly its own way. That's roughly how astronomers distinguish two types of large galaxies: 'spinning tops' (fast rotators) and 'beads' (slow ones with chaotic stellar motion).
But how do you tell which type a distant galaxy belongs to? Usually, you look at the rotation speed of stars in its center. If it exceeds a certain threshold, it's a 'spinning top'; if not, it's 'beads'. The authors of the new study tested these thresholds using supercomputer simulations from IllustrisTNG, where virtual galaxies are born and evolve according to the laws of physics. They selected thousands of such galaxies and compared them with real data from the MaNGA and ATLAS$^{3D}$ telescopes.
The simulations also showed that virtual galaxies are on average less 'top-like' than real ones. They have too many systems with intermediate rotation and overly massive stellar 'clouds' around the center. This is an important signal for those who refine computer models of the universe.
And one final touch: the authors figured out how to estimate, from observations, the fraction of stars in a distant galaxy's halo — the sparse shell around it. Previously, this required complex measurements; now kinematics, that very 'spinning' and 'shivering' of stars, suffices. True, the method is still rough, but it's a step toward seeing galaxies in more depth.
The IllustrisTNG simulations reproduce a cubic volume of space hundreds of millions of light-years across, and within it, millions of galaxies are born — from dwarfs to giants.