FM 4: Planetary Nebulae as Probes of Galactic Structure and Evolution

The uninformed reader may be surprised to learn that we have discovered more PNe in other galaxies than we have found in our own. What are the reasons for this observational effort? Historically, the first motivation is related to their potential for distance determination. The PN luminosity function was empirically shown to have a surprisingly constant bright end, which allows it to be used as a standard candle. Since those bright PNe are easily discoverable with narrow-band filter techniques, it was possible to obtain reliable distances to galaxies up to 20 Mpc away. Twenty years ago, the PNLF distances were among the first to point to a Hubble constant near 70 km/s/Mpc, subsequently confirmed with other techniques.

Paradoxically, we still lack a theoretical understanding of the constancy of the PNLF bright end for a variety of stellar populations. In particular, it is not clear why the PNLFs of elliptical galaxies are not significantly fainter that those of populations with modern, on-going star formation. We would like to put together a group of experts and discuss possible solutions to this problem, which could involve new ideas related to what percentage of PNe are produced by binary systems, or what could be the intrinsic width of the initial-final mass relation for the evolution of single intermediate-mass and low-mass stars.

Planetary nebulae can be utilized as probes of the old stellar population in our galaxy and beyond, their chemical abundances are relatively easy to calculate, their spectra are fairly easily gathered with modern spectroscopy, and they represent a definite stage of stellar evolution, thus can be associated to a specific progenitor mass range. The population of Galactic and Magellanic Cloud PNe has been studied in details by spectroscopic means to determine the metal distribution in our galaxy and in the Magellanic neighbors. Moving now to harder observational goals, we have started measuring chemical abundances in extragalactic PNe. What limits can we impose on galaxy formation by merging from the observed metallicity gradients in galaxies with a variety of morphologies? To what extent stellar migration affect the scenarios that we detect using PNe compared to HII regions? This kind of project will eventually need the light-collecting power of 30-m telescopes, but we think it is timely to start discussing possible strategies for target selection and preliminary observations.

It was clear from the very beginning that extragalactic PNe would be excellent kinematic probes of the stellar populations they represent. As soon as new, more efficient slitless methods for radial velocity measurement were implemented, this potential began to be exploited. We now know of a few flattened elliptical galaxies that show a Keplerian decline of line-of-sight velocity dispersion with projected distance from the galaxy center, opening a way to learn about dark matter distribution or, alternatively, about the presence of radial anisotropy in the velocity dispersion. We would like to discuss empirical ways to decide which of the two interpretations is correct. We also know of elliptical galaxies where PNe indicate a clear presence of dark matter, in agreement with other techniques (e.g. X-ray emitting gas, or globular clusters). This indicates that PNe are indeed reliable kinematic probes. PNe also provide important kinematic information in spiral galaxies, in particular those which appear edge-on, because the angular separation from the galactic plane gives directly the physical distance of the PN from the plane. We would like to discuss what we can learn about angular momentum distribution from this kind of information. Can we impose limits on theoretical modeling of spiral galaxy formation?

The IAU PN Working Group proposes to hold a PN Focus Meeting in connection with the GA in Honolulu to discuss the new topics that have emerged in recent years. There is a lively debate of PN as probes of galactic evolution and populations. While typically the WG organizes a IAU Symposium on Planetary Nebulae every 5 years, the last full meeting devoted to Extragalactic PNe was held almost 10 years ago at ESO (Garching, Germany), with Chair Letizia Stanghellini and Jeremy Walsh (ESO).The time is certainly mature for a short Extragalactic PN focus meeting.