Kinematics of planetary nebulae in the outskirts of the elliptical galaxy NGC 4697

TL;DR

This study measures the velocities of planetary nebulae in NGC 4697 using Subaru telescope data, extending kinematic analysis to larger radii to investigate dark matter distribution and galaxy dynamics.

Contribution

It introduces a new method for radial velocity measurement of extragalactic planetary nebulae and extends the kinematic data of NGC 4697 to larger galactic radii.

Findings

Outer velocity dispersion is slightly lower than previous estimates.

No evidence of rotation detected at 5 effective radii.

Dark matter halo presence is inconspicuous, with uncertain mass.

Abstract

We describe the implementation of slitless radial velocity measurements of extragalactic planetary nebulae (PNs) with the 8.2 m Subaru telescope and its Cassegrain imaging spectrograph, FOCAS. As a first application, we have extended a previous search for PNs in NGC 4697 to larger angular distances from its center. A total of 218 PNs were detected, and their radial velocities were measured. We have added 56 new PN detections to the existing sample of 535, observed previously with the ESO VLT + FORS imaging spectrograph; 36 of these new 56 PNs are located at angular distances larger than 230 arcsec from the center of NGC 4697. We compare the new FOCAS velocities with the earlier FORS velocities, for 158 of the 162 reobserved sources, finding good agreement. We now have kinematic information extending out to 5 effective radii from the center. The outer line-of-sight velocity dispersion is a bit lower than estimated earlier. This result is compatible with the existence of a dark matter halo plus some degree of radial anisotropy, but the dark matter halo is rather inconspicuous, and it is still unclear how massive it can be. A more detailed global dynamical study of the whole set of PN velocities will be required to decide if they permit to narrow down the range of possible dark matter distributions in NGC 4697. The new radial velocities reveal no evidence of rotation at 5 effective radii.