Planetary nebulae as standard candles. VIII. Evidence for a change in the luminosity function cutoff at low metallicity

We search for a change in the [O III] λ5007 planetary nebula luminosity function (PNLF) with metallicity by combining the planetary nebula (PN) observations of four metal-poor galaxies in the Local Group: M32, NGC 185, NGC 205, and the Small Magellanic Cloud. We show that the most likely magnitude cutoff for the PNs in our sample is 0.28^+0.11_-0.17 mag fainter than the value of M* observed in the bulge of M31, but consistency with the M31 measurement cannot be ruled out beyond the 70% confidence level. The data suggest that PN-based distances may depend slightly on the underlying stellar population, with a factor of 10 in metal abundance translating into an apparent shift in the value of M* by 0.25 mag (D(PNLF) ∝ Z^-0.05). However, because this variation is small, and population differences among luminous early-type cluster galaxies are usually slight, the effect is not important for cosmological distance determinations. The observed shift in the luminosity function is in good agreement with the theoretically derived metallicity dependence of Dopita, Jacoby, & Vassiliadis.