| Abstract |
NGC4650A is the prototype for Polar Ring Galaxies (PRGs). Its luminous
components, inner spheroid and polar structure, have been studied with
optical and near-infrared photometry, optical spectroscopy and in the
radio. The central spheroid is an exponential thick disk, characterized
by a very bright nucleus; the polar structure has been shown to be a
disk, rather than a ring. The question about the shape of the dark halo
of NGC4650A is still open: the biggest uncertainties in the mass models
proposed until 1996 for the dark halo shape were maily related to the
difficulty in measuring velocity dispersion profiles along the major
axis of the central spheroid. In order to constraint the dark halo
shape, we have obtained high resolution spectra (with FORS2@UT4 on the
ESO VLT) of the Calcium triplet absorption lines on the photometric axes
of the stellar spheroid in NGC4650A. The new CaT spectra are better
tracers of the kinematics for the NGC4650A spheroid than was available
before. Along the major axis, the observed rotation and velocity
dispersion measurements show the presence of a kinematically decoupled
nucleus, and a flat velocity dispersion profile. The minor axis
kinematics is determined for the first time: along this direction some
rotation is measured, and the velocity dispersion is nearly constant and
slightly increases at larger distances from the center. The new high
resolution kinematic data suggest that the stellar component in NGC4650A
resembles a nearly-exponential oblate spheroid supported by rotation.
The main implications of these results on the previous mass models for
NGC4650A concern the law adopted to fit the velocity dispersion profile:
the measured flat velocity dispersion profile along the spheroid major
axis shows that both the linear decreasing fit and the exponential
empirical law adopted in the previous mass models do not reproduce the
observed trend with radius and previous conclusions on the halo
flattening are no longer valid. Moreover, the new kinematic data set
constraints on current models for the formation scenarios of PRG,
supporting a slow accretion rather then a secondary strong dissipative
event. |