Kinematic evidence for different Planetary Nebulae Populations in the elliptical galaxy NGC 4697

Abstract

We have analysed the magnitudes, kinematics and positions of a complete sample of 320 PNs in the elliptical galaxy NGC 4697. We show (i) that the PNs in NGC 4697 do not constitute a single population that is a fair tracer of the distribution of all stars. The radial velocity distributions, mean velocities, and dispersions of bright and faint subsamples differ with high statistical confidence. (ii) Using the combined data for PNs brighter than 26.2, we have identified a subpopulation of PNs which is azimuthally unmixed and kinematically peculiar, and which thus neither traces the distribution of all stars nor can it be in dynamical equilibrium with the galaxy potential. (iii) The planetary nebula luminosity functions (PNLF) of two kinematic subsamples in NGC 4697 differ with 99.7% confidence, ruling out a universal PNLF. We estimate that the inferred secondary PN population introduces an uncertainty in the bright cutoff magnitude of ~ 0.15 mag for this galaxy. -- We argue that this secondary PN distribution may be associated with a younger, >~ 1 Gyr old stellar population, perhaps formed in tidal structures that have now fallen back onto the galaxy, as has previously been suggested for the X-ray point sources in this galaxy, or coming from a more recent merger/accretion with a red galaxy. The use of PNs for extragalactic distance determinations is not necessarily compromised, but their use as dynamical tracers of dark halos will require deep observations and careful analysis of large PN samples.