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 8meter 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

Modelling Kinematics and Dark Matter: The Halos of Elliptical Galaxies

“…The results so far are mixed. PNe velocity dispersions in several intermediate luminosity ellipticals were found to decline with radius, and to require little if any dark matter at 2 − 5R e [2,15]. Two of these (NGC 3379, 4494) were also part of the sample of [4,31], but were consistent with constant M/L also in that study.…”

“…This would seem to be not a likely explanation for several galaxies at the same time, unless selecting round galaxies introduces a significant bias in this sense. It is comforting that the near-edge-on, similar luminosity NGC 4697 also has little evidence for dark matter [2]. See Romanowsky's paper for further discussion [19].…”

“…A test of this is to compare the rotation velocities v and velocity dispersions σ obtained from PN radial velocities with those from ALS, taking into account the greater smoothing of the PN velocity field due to spatial averaging. Mendez used his sample of 535 PN velocities in NGC 4697 for a simple comparison [2]. His sample, restricted to a stripe around the major axis, indeed followed the rotation seen in the ALS data; however, the number of PNe per mean velocity point is ∼ 10 and so the errors are large, ∼ 50 km s −1 .…”

“…From this emission line, PNe can be identified and their radial velocities measured spectroscopically at distances of up to 20-30 Mpc. With 8m class telescopes [2] or the special-purpose Planetary Nebula Spectrograph (PN.S) [3], several hundred PNe have been found in giant elliptical galaxies with distance up to the Virgo cluster. The PN number density approximately follows the surface density of stars; thus a few tens of PNe can be used to measure the stellar kinematics at several effective radii R e , where the surface brightness is too faint for absorption line spectroscopy (ALS).…”

“…Observational tests of this question have been carried out in a number of galaxies, including M31, Cen A, the Leo group galaxies NGC 3377, 3379 and 3384, several Virgo ellipticals, the Fornax galaxies NGC 1399 and NGC 1404, and NGC 4697 [2,7,9,11]. Generally, the number of PNe as a function of isophote follows the luminosity distribution for radii outside ∼ 1 − 2 ′ , in some cases very well.…”