Based on the comparisons between PNLF and Cepheid distances, between PNLF and SBF (Ciardullo, Jacoby, & Tonry (1993)) distances, and between PNLF spiral and PNLF elliptical distances within a cluster, there is repeated evidence that PNLF distances are accurate to ~ 8%. Similar claims are being made for SN Ia. Yet, PNLF distances suggest values for H0 near 75 km/s/Mpc while SN Ia distances are between 55 and 67 km/s/Mpc (Sandage et al. (1996); Riess et al. (1996); Hamuy et al. (1996)) and both methods are based on Cepheids for calibration. While an H0 of 67 is within the combined error budget of the methods, an H0 of 55 stresses the PNLF method (and others) severely.
The PNLF and SN Ia distance comparison shown in Table 2 for galaxies in common in the Virgo and Fornax clusters summarizes the issue. For Virgo, these include a single SN Ia in each of NGC 4374, NGC 4382, and NGC 4486, although none was observed very well. Although SN1991bg in NGC 4374 was observed very well, we reject it for being anomalously underluminous (Phillips (1993); Leibundgut et al. (1993)). For Fornax, 2 SN Ia occurred in NGC 1316, and both were reasonably well observed (Hamuy et al. (1991)). Distances are given as the average of the 3 galaxies in Virgo.
|Cluster||PNLF Distance||SN Ia Distance||Discrepancy|
|Virgo||15.1 ± 0.9||17.4 ± 2.6||0.8|
|Fornax||17.7 ± 0.5||22.9 ± 1.5||3.3|
The SN Ia zero-point is set using the well observed SN Ia SN1981B and SN1990N. No others (SN1895B, SN1937C, SN1960F, SN1972E) were adequately observed for their peak magnitudes and decline rates to be determined at the same level of reliability as these 2 primary calibrators. From Sandage et al. (1996), we have MmaxB = -19.3 ± 0.08 for SN1981B and SN1990N. The apparent peak magnitudes for the Virgo and Fornax SN Ia are taken from Leibundgut et al. (1991).
Evidently, the discrepancy in Virgo is not significant while the discrepancy in Fornax is significant. Thus, either the PNLF distance to Fornax is incorrect (although 2 other galaxies yield the same distance and the Cepheid distance is nearly identical), or the 2 well observed SN Ia in Fornax are underluminous by 0.5 mag. There is evidence that the Fornax SN Ia are, in fact, somewhat fast declining ones, but this can only explain about 0.15 mag of the discrepancy. If the decline rate is considered, using the Hamuy et al. (1996) slope of 0.78, then the discrepancy is reduced to a 2.2 event, which still is marginally significant. Because the discrepancies between the Fornax SN Ia distance and Fornax distances from the HST Cepheids, SBF, and PNLF are all very similar, the SN Ia Fornax inconsistency cannot be solved by appealing solely to errors in the PNLF method. For now, this issue remains open.