Planetary Nebulae Beyond the Milky Way – Historical Overview

Abstract

Up to 10% of the total luminosity of a planetary nebula,∼500 L , can be emitted in the dominant cooling line, [O iii] λ5007. This, coupled with the narrowness of the line (∼15-25 km s−1), makes it extremely easy to detect PNe in external galaxies using a narrow-band filter tuned to the galaxy redshift. The availability of multiple independent distance indicators for our closest neighbouring galaxies, the Magellanic Clouds and M 31, means that the luminosities of the PNe in these galaxies (in particular the large numbers of PNe in the LMC) can be used to calibrate PN luminosity functions, which have been used over the past 15 years as probes of the Hubble constant. Given the fact that we still do not have accurate distances for most planetary nebulae in the Milky Way, this has been an astonishing development. Unlike H ii regions, which cannot be used to probe elliptical galaxies, planetary nebulae can be used to probe the dynamics and metallicity of any type of galaxy. Today, via accurate radial velocity measurements, extragalactic PNe are being used as dynamical mass probes of galaxies, and even of the stellar mass content of the intracluster regions of galaxy clusters, with dedicated ‘planetary nebula spectrographs’ being built to further such studies. The angular resolution of the Hubble Space Telescope has turned out to be ideally suited to the study of PNe in the LMC and SMC, and the exploitation of the accurately known distances to these two galaxies has allowed reliable luminosities and masses to be derived for the central stars and nebulae, thereby putting PN research on a much more quantitative footing. In this overview of past and recent studies of extragalactic planetary nebulae (PNe), I have divided the research into two main areas: (a) surveys for planetary nebulae in other galaxies, PN luminosity functions and intracluster PNe; (b) detailed studies of planetary nebulae in the Magellanic Clouds, particularly studies carried out with the HST.