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 , < \mbox { 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 .