To a limited degree, the 3 indirect distance comparisons shown in Figure 1 illustrate that the PNLF distances to ellipticals are on the same system as the spiral distances. In addition, tests relying purely on PNLF distances have been described for the NGC 1023 group (Ciardullo, Jacoby, & Harris (1991)), the Coma I group (Jacoby, Ciardullo, and Harris 1996), and the Leo group (Feldmeier, Ciardullo, & Jacoby (1997)). Figure 2 summarizes the 4 PNLF and 2 Cepheid distances to the Leo group.
Figure 2. A comparison between PNLF and Cepheid distances in the Leo Group. NGC 3368 (M96) has been measured using both the PNLF (Feldmeier et al. (1997)) and with Cepheids (Tanvir et al. (1995)). In addition, group member NGC 3351 has been measured with Cepheids (Graham et al. (19967)).
Tests to different galaxies cannot be conclusive, since the two galaxies are not guaranteed to be at exactly the same distance. Nevertheless, the deviations in distance (spiral minus elliptical) for the 3 purely PNLF tests are as follows. NGC 1023 group: 0.00 ± 0.21 mag (NGC 1023); Coma I group: +0.08 ± 0.15 mag (NGC 4278) and -0.42 ± 0.12 mag (NGC 4494); Leo group: -0.12 ± 0.24 mag (NGC 3377), -0.11 ± 0.24 mag (NGC 3379), and -0.18 ± 0.24 (NGC 3384). At face value, assuming that in each of these 3 cases the spiral and associated ellipticals are at exactly the same distance, the average offset is -0.16 mag for the 3 cases (0.00 for NGC 1023, -0.34 for Coma I, and -0.13 for Leo) in the sense that our elliptical distances are too large.
This conclusion is too simplistic because we have not addressed the assumption that the spiral galaxy is always at the same distance as the comparison galaxy. For the Coma I group, for example, the GCLF (Fleming et al. (1995)) and the SBF (Simard & Pritchet) methods concur that NGC 4494 is beyond NGC 4565, and curiously, the deviance for this galaxy is the largest we see. If removed from the sample, the spiral-elliptical offset is reduced to -0.02 mag, a level that is too small to consider reliable within the assumptions. Until this problem can be addressed more thoroughly, we apply no correction to our elliptical galaxy distances.