A potter sits down at the wheel, and the first movements of the hands set the fate of the future vessel. If the lump of clay stays flat and wide, it will spin for a long time, gaining mass and heat. If the potter squeezes it into a dense ball, the wheel slows down and the piece hardens quickly. Galaxies are much the same: some live for billions of years as blue disks, giving birth to stars, while others freeze into red spheroids where stars have long since stopped forming. What makes a galaxy change its shape and stop making stars? Astrophysicists have been wrestling with this question for several decades.
For a long time, galaxy mergers were the prime suspect. When two stellar cities collide, their disks are destroyed and stirred together, and the gas in the center flares up, feeding the supermassive black hole. The black hole releases energy, the gas is swept away, and the galaxy goes quiet. But there is a snag: mergers increase a galaxy's size, whereas quiescent galaxies at the same mass are, on the contrary, smaller than star-forming ones. So there must be another mechanism linking shape and quiescence. The authors of the new work traced the evolution of more than two thousand galaxies in the EAGLE simulation to find that mechanism.
It turned out that up to a certain point, future quiescent and future star-forming galaxies look identical. Their shape and rotation are indistinguishable. But roughly 8-10 billion years ago their paths diverge, and this divergence coincides with the onset of the shutdown of star formation. The difference in size and black hole mass appears even earlier. The authors then identified four channels through which a galaxy's shape governs its quiescence.
The first channel is mergers. They simultaneously increase size, suppress rotation, make the galaxy more elongated, and grow the black hole, which then quenches star formation. The second channel is black hole growth itself. If you take galaxies with the same merger history, the compact ones, with a small disk, grow their black hole faster and go quiet more often. The link between the disk fraction and black hole mass is negative and strong: a correlation coefficient of about minus 0.5. The third channel is size. At equal stellar and black hole masses, future quiescent galaxies are 20-60 percent more compact, which means the feedback from the black hole is more efficient in them. The fourth channel is secular disk instability. A compact, flat disk spontaneously warps, turning into an elongated system while barely gaining any stellar mass. This channel accounts for about half of the elongated galaxies at a mass of around 40 billion solar masses.
The work shows that a galaxy's morphology is not a passive consequence of its history but an active player. It governs black hole growth and the shutdown of star formation. This matters for semi-analytic models: to reproduce the observed size difference between quiescent and star-forming galaxies, one must explicitly account for how black hole growth depends on galaxy structure. The next step is to test these predictions against observations, for instance data on compact massive galaxies in the early Universe and on elongated galaxies in the local Universe.
In the EAGLE simulation, galaxies with a mass of about 10^10.6 solar masses that never experienced major mergers can turn from a flat disk into a prolate "cigar" through internal instability, while barely gaining any stellar mass.