The Kinematics of Planetary Nebulae in the Magellanic Clouds

Radial velocities have been determined for 25 planetary nebulae in the LMC and II in the SMC. Many of the observations were made at 49 Å/mm at H γ . The planetary nebulae are the first old subsystem in the Magellanic Clouds to be studied kinematically in any detail. In addition Table I contains three objects not now considered to be planetaries including Henize SMC N55 which is an interesting [Fe II ] emission line object. In the LMC the overall rotational pattern of the planetaries agrees with that found previously for very young objects. This shows that the old and young objects belong to one (complex) system rather than being essentially independent as has sometimes been suggested; and that the displacement of the centre of rotation 1 kpc north of the optical bar is a fundamental property of LMC dynamics rather than a peculiarity restricted to the youngest objects. The planetary nebulae show a poor detailed correlation with the velocities and densities of the interstellar H I , consistent with their being old objects. A velocity dispersion of 22 ± 3 (s.e.) km s −1 is found, twice that of young objects. This is taken to indicate a collapse of the LMC to a plane at some time in the past (as in the Galaxy). Some indication is found that the velocity dispersion may be a function of magnitude. In the SMC there is also poor detailed correlation between the planetary velocities and the velocities and densities of the interstellar gas. However, the marked division of the planetaries into two compact velocity groups indicates the existence of two distinct groups of objects in the same line of sight, as is also indicated by the double peaks of the 21 cm profiles. The planetaries, being old, cannot belong to the expanding shells proposed by Hindman to account for the 21 cm data. It is suggested, rather, that objects of a wide range of ages follow non-circular orbits in a barred system, the bar being seen end on with gas and stars flowing out at both ends. As in the Galaxy the Magellanic Cloud planetaries cover a wide range of excitation classes. The frequency distribution of these classes is different in the SMC from the LMC whilst the region of the Galaxy so far surveyed is probably closer to the LMC in this respect. In the LMC high excitation objects seem to be more frequent amongst the fainter planetaries.