An old city gains new suburbs. In the center, narrow streets where life has nearly stopped, while on the outskirts construction is booming. That is what a spiral galaxy looks like when its core stars have long aged and new stars are still being born in the outer disk. For a long time, astronomers thought galaxies evolve in one direction: spirals, with their twisted arms and active star formation, gradually lose gas, merge with each other, and turn into ellipticals, smooth 'retirees' with no hint of spiral structure. But new research shows that this path is not always final.

Astronomers used data from the MaNGA survey, which takes spectra of thousands of galaxies with high spatial resolution. In each spiral, they looked for regions where star formation had long ceased. The criterion was strict: if within some radius more than 95% of points have an age indicator D4000 above the threshold, that region was called a 'quenched central core.' They found 247 such cores, enough to compare their properties with fully quenched early-type galaxies.

The comparison yielded a striking result. The cores of spirals are practically indistinguishable from lenticular galaxies, disk systems without spiral arms, in the size-mass relation, in the relation between stellar velocity dispersion and mass, and in the profiles of chemical composition and rotation. The slope of the velocity dispersion-mass relation for cores is 0.314, while for lenticulars it is 0.310. For elliptical galaxies, this slope is noticeably smaller: 0.233. The fraction of slowly rotating cores decreases with mass in the same way as for lenticulars, while for ellipticals it increases. All this points to a common origin: the quenched cores of spirals are former lenticular galaxies.

How is this possible? A galaxy that has already 'died,' having lost its cold gas and stopped forming stars, can get a second life. Fresh gas flows in from intergalactic space or from neighboring dwarf galaxies. It settles onto the old core, forming a new disk. Star formation ignites anew, and spiral arms wind up. The old core remains in the center as a bulge, a compact spherical concentration. Thus, a 'retired' galaxy turns into a 'parent' galaxy.

This discovery settles a long-standing question about the fate of 'red nuggets,' compact massive galaxies that abound in the early universe. It was thought they must have merged into giant elliptical galaxies. But now it is clear that many of them did not disappear; instead, they grew new disks and became the bulges of modern spirals. Galaxies, like old cities, do not only decay but also rebuild themselves.