5.2. Results from SBF Surveys

The ramifications of existing SBF data for peculiar velocity surveys and the Hubble constant are preliminary, but they are encouraging in terms of what they portend for the knowledge this method will bring in the near future. In the very early days of peculiar velocity work, Tonry & Davis (1981) and Aaronson et al. (1982) estimated values of ~ 250 km s^-1 for the infall of the Local Group into the Virgo cluster. Model fits to the SBF data for Local Supercluster galaxies confirm this value, and show that it is remarkably insensitive to the assumed density profile around Virgo (Tonry 1995). Another early scientific result of SBF studies has been validation of the large peculiar motions of elliptical galaxies in the Hydra-Centaurus region originally detected using the D_n- technique (Dressler 1994). More generally, intercomparison of the SBF and TF / D_n- velocity fields in the coming years will provide an important consistency check. Preliminary tests of this sort have shown good agreement to within the quoted errors (Tonry 1995; Tonry et al. 1997).

The zero point of the SBF method (i.e., the value of for a given (V - I) color) was poorly known until recently, but has now been determined from a comparison of SBF and Cepheid distances. Taking the distances in Mpc to the Local Group, the M81, CenA, NGC 1023, NGC 3379, NGC 7331 groups, and the Virgo cluster from published Cepheid data, Tonry et al. (1997) obtained the zero point given in equation 10. By working with groups, Tonry et al. were able to include 10 galaxies with Cepheid distances and a total of 44 SBF galaxies in the calibration. However, this comparison suffers from the nagging possibility that the SBF objects, which are preferentially ellipticals and S0s, may not lie at precisely the same distances as the Cepheid galaxies, which are late-type spirals, in the same group. Indeed, there are currently only five galaxies with both Cepheid and SBF distances. One of these, NGC 5253, gives a discordant result. If the remaining four are used, Tonry et al. (1997) find an SBF zero point -1.82 ± 0.06, in reasonable agreement with the preferred value of -1.74± 0.05 found from the group comparison.

Thus calibrated, the SBF technique can be used as a temporary bridge between Cepheid distances, still too few in number to be reliable calibrators, and the secondary DIs that probe the far-field of the Hubble flow. Tonry et al. (1997) used SBF distances for groups and individual galaxies to provide absolute calibrations for TF, D_n-, and Type Ia supernovae (Section 6). In so doing, they obtained distances in Mpc for relatively distant galaxies, and thus estimates of the Hubble constant. The mean value was found to be H₀ = 81 ± 6 km s^-1 Mpc^-1 from SBF-calibrated secondary DIs. Such a large value of H₀, if it holds up, may prove problematic for Big Bang cosmology, as discussed in Section 2. However, it should be kept in mind that the absolute calibration of SBF is tied to the Cepheid distance scale, and that the latter might change in coming years as the HST Key Project (Section 2) continues.