6. FORNAX

We regard the Fornax cluster as a much better proving ground for distance estimators than Virgo. Virgo is really the knot at the center of three filaments: UMa stretching to the north, the Southern Extension curving towards us, and a filament almost directly away from us, which includes not only spirals, but early-type galaxies as wen such as N4365 and then the W cloud with N4261. In contrast, Fornax offers most of its galaxies within 0.02 radian, it has had two good SNIa (198ON and 1992A), it 1s excellent for SBF, D_n - , PNLF, and SN, and it is not too bad for TF and Cepheids. Figure 13 shows this central region lying within a 1 degree radius.

Figure 13. The central 2×2 degrees of the Fornax cluster. The largest elliptical is NGC 1399 and the large spiral is NGC 1365. NGC 1316 is 3 degrees to the southwest. Fornax is about four times more compact than Virgo.

Table 2 predicts a distance to Fornax of 17.6 ± 0.5 Mpc. If the true distance is as great as 20 Mpc then SBF would be shown to be a bad distance estimator at the 5 - , level. Let us see what SNIa have to say about Fornax. The two SNIa have the following parameters:

1980N in NGC1316: V_max = 12.44, B_max = 12.49, m₁₅ = 1.29

1992A in NGC1380: V_max = 12.55, B_max = 12.60, m₁₅ = 1.33

Sandage and Tammann (1993) fit distant SNIa to provide a tie to the Hubble flow:

5 log v₂₂₀ = m_B(max) + 3.150 ± 0.08

5 log v₂₂₀ = m_V(max) + 3.256 ± 0.06

from which we can deduce that the distance to Fornax is v₂₂₀ = 1398 km s^-1. The results from Hamuy et al. (1995) are very similar, and Faber et al. (1989) measure a distance of 1411 km s^-1 using D_n - . So it seems quite secure that the distance to Fornax is about 1400 km s^-1. A recent calibration of the zero point of SNIa (as standard candles) by Sandage et al. (1996)

(6.6)

which implies that the distance modulus of Fornax is 32.0 ± 0.2. Combining this with the distance of 1400 km s^-1 above, we get H₀ = 55, consistent with the results of Sandage et al. (1996). However, this implies a distance to the Fornax cluster of 25 ± 2.5 Mpc, which differs from the SBF distance by 3. The SBF result, 17.6 Mpc, divided into 1400 km s^-1, implies H₀ = 80 km s^-1 Mpc^-1.

Rather than arguing about which Hubble constant is correct, we can now actually compare distances directly. At this conference the first Cepheid distance to a Fornax galaxy (NGC 1365) is reported, and the result is 18.2 ± 0.8 Mpc. This is consistent with the SBF distance and inconsistent at the 2.5, level with the SNIa calibration. Since we are dealing with an empirical science, it is essential not to simply average in this new result, but instead to pause and take stock in how this result agrees with the predictions from previous data and methods. The new Cepheid result is a strong endorsement of SBF and casts doubt on the validity of SNIa as a reliable distance estimator. We do not know yet what is wrong, but we must regard any Hubble constant derived from SNIa with skepticism.