Planetary Nebulae as Standard Candles. IV. A Test in the Leo I Group

We present the results of a planetary nebula survey of galaxies in the Leo 1 group performed with the Kitt Peak and Cerro Tololo 4 m telescopes and on-band off-band λ5007 interference filters. In all, we detected 249 PN candidates populating the top 1.5 mag of the planetary nebula luminosity function (PNLF), including 93 in the E0 galaxy NGC 3379, 54 in the E6 galaxy NGC 3377, and 102 in the S0 spiral NGC 3384. In all three galaxies, the luminosity specific planetary nebula number densities are roughly the same, and the derived stellar death rates are in remarkable agreement with the predictions of stellar evolution theory. Using statistically complete and homogeneous samples of planetaries, we show that the shape of the [O III] λ5007 planetary nebula luminosity function is the same in each galaxy and indistinguishable from that observed in M31 and M81. By comparing the PNLFs with an empirical law, we find the most likely distances to NGC 3377, 3379, and 3384 are 10.3, 9.8, and 10.1 Mpc, respectively, with formal 1 σ errors of ~ 10%. This invariance in galaxies with differing Hubble types and metallicities demonstrates that the PNLF is an excellent standard candle for early-type galaxies.