next previous
Up: Radial velocities of

1. Introduction

Planetary nebulae are among the best velocity tracers of stellar populations and have as such been successfully used to trace the dynamics of extragalactic old stellar populations (e.g. Ciardullo et al. 1993). They are the most luminous objects among the old stellar population, are easy to find, with most of their flux coming out in a few bright emission lines, and yield accurate radial-velocity determinations using these narrow emission lines. In contrast to integrated stellar light, they are identifiable point probes of the gravitational potential field of the host galaxy.

In view of this, it is perhaps surprising that little use has been made of planetary nebulae for the study of the dynamics of the Milky Way. The high obscuration, especially towards the Galactic Centre, makes them difficult to detect, and their distances are unknown to significant factors. Schneider & Terzian (1983) used PN for the outer Milky Way, deriving a rising rotation curve in the outer Galaxy. It seems possible that this result was dominated by Malmquist-like bias caused by uncertainties in the distances, which can cause systematic errors in the derived properties of a sample even if the scale itself has only non-systematic effects. The same effect has caused the scale height of the PN population to be underestimated by a factor of 2 (Zijlstra & Pottasch 1991). In the case of a flat Galactic rotation curve, either under- or overestimation of the distance to an individual object will lead to a positive velocity gradient.

The compilation of data in Schneider et al. (1982) is still the main source for velocities of Galactic planetary nebulae. They combined all measurements in the literature, correcting for systematic offsets between different sources. The data used are of varying accuracy. Recently, Kohoutek & Pauls (1995) have presented well-determined radial velocities for 76 nebulae in the direction of the Galactic Centre. Spyromilio (1995) gives accurate velocities for a few non-bulge PN. In the present paper we present new data on 71 nebulae. The targets were selected on the basis of their Galactic longitude to be within 15 degrees of the Galactic Centre. Preference was given to objects with size and flux consistent with bulge membership, but very faint objects were not observed. Previous velocity determinations are available for many of the observed nebulae, but many of these are rather uncertain. Comparing the velocity as function of longitude and derived distance shows that most of the planetary nebulae discussed belong to the Galactic bulge.


next previous
Up: Radial velocities of

Copyright by the European Southern Observatory (ESO)
web@ed-phys.fr