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M.B. Laychak, R. Ciardullo (Penn State), J.J. Feldmeier (CWRU), G.H. Jacoby (WIYN)
We report the results of [O~III] \lambda 5007 surveys for planetary nebulae in the galaxies NGC~3351 and NGC~4258. Using on-band/off-band images taken in [O~III] \lambda 5007, as well as images taken in H\alpha, we identify 20 and 58 PN candidates in these galaxies, respectively. >From these data, an adopted M31 distance of 770~kpc, and the empirical planetary nebula luminosity function (PNLF) we derive a distance to NGC~3351 of 10.1+0.5-0.7~Mpc, and a distance to NGC~4258 of 7.9+0.2-0.3~Mpc. The latter distance is particulary interesting, since it is in excellent agreement with the galaxy's Cepheid distance of 7.7 ±0.3~Mpc, but ~1.6~\sigma larger than the geometrical distance of 7.2 ±0.3~Mpc to the system's nuclear maser.
We use our new data to compare the PNLF, Cepheid, and surface brightness fluctuation (SBF) distance scales. Using 13 galaxies with both Cepheid and PNLF distances, we show that the scatter between the techniques appears consistent with the internal errors of the methods. Similarly, we use the distances to 28 galaxies with both PNLF and SBF measurements to show that the residuals generated by these distance indicators are perfectly consistent with that expected from random scatter. However, we also find that a systematic scale error exists in the distance ladder, such that SBF distances (which use Cepheids to set their zero point) are ~15% larger than PNLF distances (which also use Cepheids as their zero-point calibration). We discuss the possibility that this offset is due to systematic differences between the stellar populations probed by the various techniques.