NGC 2217, a case of apparent counterrotation

Abstract

We present spectroscopic and photometric observations of the almost face-on SB0 NGC 2217, a galaxy showing the recently discovered phenomenon of counter rotation. In the inner regions of the galaxy, the emission lines indicate line-of-sight velocities opposite in direction from those of the stars. The gas in these regions circulates around the bar major axis, in a disklike structure with faint spiral arms, as visible from Hα images. By contrast, in the outer regions gas and stars show similar velocities, with orbits that are probably coplanar. On the basis of a simple model of a warped and twisted gas disk, we deduce that the counterrotation observed in the inner parts is probably caused by a projection effect. Surface photometry in the V band indicates a complex luminosity distribution, with the superposition of different components. A synthetic image of the bar has been obtained by subtracting from the original frame a model for the spheroidal component. We estimate the integrated bar luminosity to be between 5% and 10% of the total galaxy luminosity. From the particular configuration and kinematics of our gas model we try to estimate a range of values for the tumbling velocity of the bar. A comparison with models of bars from the literature shows a partial disagreement with our estimated value. This discrepancy could indicate, as for the other galaxies presenting counterrotation, that the gas configuration is not in equilibrium, but results from a recent acquisition from outside.