Dynamics and stellar populations in early-type galaxies

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Abstract
Optical multiband CCD photometry and multiposition CCD long-slit spectroscopy are presented for the five early-type galaxies NGC 439, NGC 2434, NGC 3706, NGC 6407 and NGC 7192. Light and morphological profiles, rotation velocity and velocity dispersion profiles, as well as Mg2 and 〈Fe〉 line-strength radial profiles are reported. In some cases the measurements extend beyond two effective radii. Peculiar core kinematics are detected in NGC 2434 and NGC 7192, in which the innermost nuclear regions counter-rotate with respect to the main galactic bulk. Corresponding with the kinematically decoupled core, an enhancement in the Mg2 index is observed. A much weaker signature of enhancement with respect to the galactic bulk is found in the corresponding 〈Fe〉 profiles. Should this phenomenon turn out to be common, it might be at least partly responsible for the relatively shallow slope in the 〈Fe〉–Mg2 plane shown by galactic nuclei compared to that traced within galaxies. Under the assumption of axisymmetry, the Jeans equations of stellar equilibrium have been solved to model the dynamical state of each galaxy. In some objects there is evidence for the presence of a significantly massive and extended dark halo (in some cases, even more flattened than the luminous galaxy). The dependence of the local metallicity (as traced by the Mg2 index) on physical parameters that possibly influence the local chemical enrichment has been investigated. In particular, a local dependence of the metallicity on the escape velocity from the potential well has been found (supporting previous results); however, the spread in the local relationship hints at other physical parameters, in addition to the local potential, affecting the metallicity distribution within galaxies. The Mg2 index is found to correlate with the local stellar density ρ (as described by the R surface brightness) rescaled by the total galactic mass M, i.e. with ρM2. The slope of such a correlation within individual galaxies seems consistent with the average relationship derived by Bender, Burstein & Faber for galactic nuclei. A significant spread, reduced when the dark component is added to the scaling mass, is, however, present in the relationship. Part of such a scatter might arise from a residual dependence on the galactic size, possibly incompletely accounted for with the M2 dependence.

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