Young stellar nuclei in lenticular galaxies: NGC 5574 and NGC 7457

The stellar population properties in the centers of NGC 5574 and NGC 7457 ( two lenticular galaxies previously shown to have radially homogeneous blue optical-band colors) are studied by means of integral-field spectroscopy. The compact nuclei of the galaxies appear to be chemically distinct: in NGC 5574 the nucleus is distinguished by a higher than solar iron-to-magnesium ratio, and in NGC 7457 a drop of mean stellar metallicity by a factor of 2 exists between the nucleus and the bulge. Both galaxies demonstrate a rather young mean luminosity-weighted age of the stellar populations in the bulges, not older than 5-7 Gyr; however, the chemically distinct nuclei are still younger, 2-2.5 Gyr old. The kinematics of the stars in the center of NGC 5574 are probably affected by the influence of its global bar. The compact core of NGC 7457, with a radius of about 1."5, shows a visible counterrotation; a combined analysis of the photometric and kinematical two-dimensional maps allows us to conclude that the core rotates axisymmetrically, but its rotation plane is inclined to the main symmetry plane of the galaxy. The stellar velocity dispersion in the centers of both galaxies is anomalously low: 60-80 km s(-1).