Chemically Decoupled Nuclei in the Spiral Galaxies NGC 4216 and NGC 4501

Abstract

By using bidimensional spectral data obtained at the 6m telescope for the Virgo spirals NGC 4216 and NGC 4501, we have found chemically distinct metal-rich nuclei in these galaxies. Under the assumption of equal ages for the nuclear and bulge stellar populations, the metallicity difference between the nuclei and their environments in the galaxies is estimated as a factor of 2. But we have also found an age difference between the nucleus and the bulge in NGC 4216: age-metallicity disentangling on the diagrams (H-beta, Mgb), (H-beta, [MgFe]), and (H-beta, $<Fe>$) results in an age estimate for the nucleus of 8 - 12 billion years, the bulge being older by a factor of 1.5 - 2; and the self-consistent metallicity difference estimate is then a factor of 3. The solar magnesium-to-iron ratios in the galactic nuclei show evidence for a long duration of the secondary nuclear star formation bursts which produced the chemically distinct stellar subsystems. Detailed morphological and kinematical analyses made for the stellar and gaseous structures in the centers of NGC 4216 and 4501 have revealed the presence of circumnuclear stellar-gaseous disks with radius of some hundreds parsecs which demonstrate fast axisymmetric rotation and lie exactly in the planes of the main galactic disks.