New Kinematics for the Central Spheroid in Polar Disk Galaxy NGC4650A

Type Conference Paper
Names E. Iodice, M. Arnaboldi, R. Saglia, L. Sparke, O. Gerhard
Conference Name IAU Symposium
Volume 235
Pages 207
Date 2007
Library Catalog NASA ADS
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mass models do not reproduce the observed trend with radius and previous conclusions on the halo flattening are no longer valid.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.
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