Mon. Not. R. Astron. Soc., 441, 2212-2229 (2014/July-1)
Regrowth of stellar discs in mature galaxies: the two-component nature of NGC 7217 revisited with VIRUS-W.
FABRICIUS M.H., COCCATO L., BENDER R., DRORY N., GOSSL C., LANDRIAU M., SAGLIA R.P., THOMAS J. and WILLIAMS M.J.
Abstract (from CDS):
Previous studies have reported the existence of two counter-rotating stellar discs in the early-type spiral galaxy NGC 7217. We have obtained high-resolution optical spectroscopic data (R ~ 9000) with the new fibre-based Integral Field Unit instrument VIRUS-W at the 2.7-m telescope of the McDonald Observatory in Texas. Our analysis confirms the existence of two components. However, we find them to be corotating. The first component is the more luminous (~77 percent of the total light), has the higher velocity dispersion (~170 km/s) and rotates relatively slowly (projected vmax = 50 km/s). The lower luminosity second component (~23 percent of the total light) has a low velocity dispersion (~20 km/s) and rotates quickly (projected vmax = 150 km/s). The difference in the kinematics of the two stellar components allows us to perform a kinematic decomposition and to measure the strengths of their Mg and Fe Lick indices separately. The rotational velocities and dispersions of the less luminous and faster component are very similar to those of the interstellar gas as measured from the [Oiii] emission. Morphological evidence of active star formation in this component further suggests that NGC 7217 may be in the process of (re)growing a disc inside a more massive and higher dispersion stellar halo. The kinematically cold and regular structure of the gas disc in combination with the central almost dust-free morphology allows us to compare the dynamical mass inside of the central 500pc with predictions from a stellar population analysis. We find agreement between the two if a Kroupa stellar initial mass function is assumed.