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Keywords:

  • galaxies: elliptical and lenticular, cD;
  • galaxies: individual: NGC 2768;
  • galaxies: kinematics and dynamics

ABSTRACT

There are only a few tracers available to probe the kinematics of individual early-type galaxies beyond one effective radius. Here we directly compare a sample of planetary nebulae (PNe), globular clusters (GCs) and galaxy starlight velocities out to approximately four effective radii, in the S0 galaxy NGC 2768. Using a bulge-to-disc decomposition of a K-band image we assign PNe and starlight to either the disc or the bulge. We show that the bulge PNe and bulge starlight follow the same radial density distribution as the red subpopulation of GCs, whereas the disc PNe and disc starlight are distinct components. We find good kinematic agreement between the three tracers to several effective radii (and with stellar data in the inner regions). Further support for the distinct nature of the two galaxy components comes from our kinematic analysis. After separating the tracers into bulge and disc components we find the bulge to be a slowly rotating pressure-supported system, whereas the disc reveals a rapidly rising rotation curve with a declining velocity dispersion profile. The resulting Vrot/σ ratio for the disc resembles that of a spiral galaxy and hints at an origin for NGC 2768 as a transformed late-type galaxy. A two-component kinematic analysis for a sample of S0s will help to elucidate the nature of this class of galaxy.