Decoupled nuclei and nuclear polar rings in regular spiral galaxies NGC 7217

Abstract

The regular isolated Sab galaxy NGC 7217 has been studied with the Multi-Pupil Fiber Spectrograph of the 6m telescope of the Special Astrophysical Observatory RAS (Nizhnij Arkhyz, Russia) in two spectral ranges, the blue one including the strong absorption lines Mg I and Fe I and the red one including the emission lines Hα and [N II]lambda 6583. We confirm the existence of a circumnuclear gaseous polar disk with a radius of 3'' which we reported earlier. The same area, with a radius of 3\arcsec -4\arcsec, elongated orthogonally to the line of nodes, is distinguished by high values of the Lick index < Fe> and shows a Mg/Fe ratio lower than solar. This implies that there were at least two discrete star formation bursts in the circumnuclear region with a temporal separation of a few Gyrs. We relate this pair of bursts to the complex structure of the global brightness profile of the galaxy, which may be decomposed into three exponential segments with different scalelengths.