An old city sprouts new suburbs: life in the center has stalled, while construction bustles on the outskirts. That is how many spiral galaxies are built. At their middle sits an old core where stars stopped forming long ago, and around it lies a young disk with spiral arms. Previously, astronomers thought galaxies evolve only one way: from spirals into smooth 'retirees' without arms. New research shows the opposite can happen.

The team used data from the MaNGA survey, which takes spectra of thousands of galaxies. In each spiral, they located a region where star formation has ceased, a 'quenched core.' Then they compared these cores with lenticular galaxies, which have a disk but no spiral arms. It turned out that the cores are nearly indistinguishable from lenticular galaxies in size, mass, stellar motions, and chemical composition. The slope of the velocity dispersion–mass relation for cores is 0.314, while for lenticulars it is 0.310. Thus, the quenched cores of spirals are former lenticular galaxies.

How does this work? A galaxy that has already 'died,' having lost its gas and stopped forming stars, can get a second life. Fresh gas flows in from intergalactic space, settles onto the old core, and swirls into a new disk. Star formation ignites anew. The old core remains in the center as a bulge. In this way, a 'retired' galaxy turns into a 'parent' galaxy. This explains the fate of 'red nuggets,' the compact massive galaxies from the early universe: they did not vanish but instead grew new disks and became the bulges of modern spirals.