Circumnuclear star-forming rings in galaxies as a late stage of decoupled nucleus formation

摘要

During our study of chemically decoupled galactic galactic nuclei, we have found that they are often surrounded by compact circumnuclear stellar disks. In NGC 1023 and 7331, where intermediate-age stellar populations dominate in the chemically decoupled nuclei, we have also detected an age difference between the nuclei and circumnuclear disks, the nuclei being older by 2-3 Gyr. Since the magnesium-to-iron ratios are also lower in the disks, we conclude that a secondary star formation burst may start simultaneously over an extended central area, finish soon in the very center and continue for several billion years in the circumnuclear rings. We have caught one galaxy at the moment of this late stage of decoupled nucleus formation, namely, during intense star formation in the circumnuclear ring: it is the giant elliptical galaxy NGC 759.