X iv : a st ro - p h / 01 09 07 5 v 1 5 S ep 2 00 1 Detection , photometry and slitless radial velocities of 535 planetary nebulae in the flattened elliptical galaxy NGC 4697

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 (FORS) at the Cassegrain focus of the first 8-meter 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 – 2 – studies. The PNLF also provides an estimate of the specific PN formation rate: (6 ± 2) × 10 −12 PNs per year per solar luminosity. 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 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.