A study of extragalactic planetary nebulae populations based on spectroscopy. I. Data compilation and first findings

We compile published spectroscopic data and [O III] magnitudes of almost 500 extragalactic planetary nebulae (PNe) in 13 galaxies of various masses and morphological types. This is the first paper of a series that aims to analyze the PN populations and their progenitors in these galaxies. Although the samples are not complete or homogeneous we obtain some first findings through the comparison of a few intensity line ratios and nebular parameters. We find that the ionized masses and the luminosities in H$β$, L$_{Hβ}$, of around 30 objects previously identified as PNe indicate that they are most likely compact HII regions. We find an anticorrelation between the electron densities and the ionized masses in M31, M33, and NGC 300 which suggests that most of the PNe observed in these galaxies are probably ionization bounded. This trend is absent in LMC and SMC suggesting that many of their PNe are density bounded. The He II/H$β$ values found in many PNe in LMC and some in M33 and SMC are higher than in the other galaxies. Photoionization models predict that these high values can only be reached in density bounded PNe. We also find that the brightest PNe in the sample are not necessarily the youngest since there is no correlation between electron densities and the H$β$ luminosities. The strong correlation found between L$_{Hβ}$-L$_{[O III]}$ implies that the so far not understood cut off of the planetary luminosity function (PNLF) based on [O III] magnitudes can be investigated using L$_{Hβ}$, a parameter much easier to study.