Detection, Photometry, and Slitless Radial Velocities of 535 Planetary Nebulae in the Flattened Elliptical Galaxy NGC 4697

Abstract

We have detected 535 planetary nebulae (PNs) in the flattened elliptical galaxy NGC 4697 using the classic on-band, off-band filter technique with the Focal Reducer and Spectrograph at the Cassegrain focus of the first 8 m telescope unit of the ESO Very Large Telescope. From our photometry, we have built the [O III] λ5007 planetary nebula luminosity function (PNLF) of NGC 4697. It indicates a distance of 10.5 ± 1 Mpc to this galaxy, in good agreement with the distance obtained from surface brightness fluctuations and substantially smaller than a previous estimate of 24 Mpc used in earlier dynamical studies. The PNLF also provides an estimate of the specific PN formation rate: (6 ± 2) × 10-12 PNs yr-1 L-1⊙. Combining the information from on-band images with PN positions on dispersed, slitless grism images, we have obtained radial velocities for 531 of the 535 PNs. We describe the slitless velocity method and the calibration procedures that we have followed. The radial velocities have errors of about 40 km s-1 and provide kinematic information up to a distance of almost three effective radii from the nucleus. Some rotation is detected in the outer regions, but the rotation curve of this galaxy appears to drop beyond one effective radius. Assuming an isotropic velocity distribution, the velocity dispersion profile is consistent with no dark matter within three effective radii of the nucleus (however, some dark matter can be present if the velocity distribution is anisotropic). We obtain a blue mass-to-light ratio of 11. Earlier M/L ratios for NGC 4697 were too small because of the too large distance used for their derivation.