A SYSTEMATIC EFFECT IN THE USE OF PLANETARY-NEBULAE AS STANDARD CANDLES

The distances determined by Jacoby et al. (1990a) for 6 early type galaxies members of the Virgo cluster of galaxies, using the method of the planetary nebulae luminosity function (PNLF) are correlated with the magnitude of the parent galaxy. A similar trend is found within the Leo I group of galaxies and is confirmed by a comparison between the PNLF distances and distance estimates from the surface brightness fluctuation method. This effect works in the sense that more luminous galaxies are assigned smaller distances. Another strange systematic feature is the increase of the specific PN density from luminous to faint galaxies, clearly seen in Virgo and Leo I data. We show that these systematic trends can be due to an exponential part in the bright end of the PNLF. If this is the case, the short value derived for the Virgo distance by the PNLF method should be regarded uncertain. We analyze shortly the use of an exponential LF as a distance indicator and note that if it is applied to the planetary nebulae, one should select the calibrators to match at least the Hubble type of the measured galaxies. As a preliminary example the relative distance mu(Vir) - mu(Leo I) is derived by the exponential LF method to be 1.15 +/- 0.04 (with a fixed value for the slope of the LF) instead of the original value of 0.88. We emphasize the need of a detailed study of the PNLF in galaxies within Virgo and closer with a large luminosity and type range.