A critical review of selected techniques for measuring extragalactic distances

We review seven of the most reliable indicators used for deriving distances to galaxies as far way as 100 Mpc: globular-cluster luminosity functions (GCLF), novae, Type Ia supernovae (SN Ia), H I-line-width relations, planetary-nebula luminosity functions (PNLF), surface-brightness fluctuations (SBF), and fundamental-plane relationships for elliptical galaxies (Dn - sigma). In addition, we examine the use of Cepheid variables since these serve to set zero points for most of the methods. We pay particular attention to the uncertainties inherent in these methods, both internal and external. We then test these uncertainties by comparing distances derived with each technique to distances derived from surface-brightness fluctuations. We find that there are small systematic offsets between the PNLF, GCLF, and SBF methods, with the PNLF and GCLF distances being on average 6% and 13% larger than those of the SBF method. The dispersion between the PNLF and SBF distances is 8%; the GCLF-SBF dispersion is 16%, the SN Ia-SBF dispersion is 28%, Dn-sigma-SBF dispersion is 26%, and the Tully-Fisher-SBF dispersion is 32%. The latter value drops to 14%, however, when one considers only well-mixed groups, suggesting that the spiral galaxies measured with Tully-Fisher are not always spatially coincident with the groups' elliptical galaxies. In the mean, all the methods agree extremely well. We also present a summary of distances to the Virgo Cluster. Weighted and unweighted averages of the seven methods yield Virgo distances of 16 ± 1.7 and 17.6 ± 2.2 Mpc, respectively. The overlap among all the indicators is well within the expected accuracies of the methods. Using the weighted or unweighted Virgo distances to bootstrap to the Coma Cluster, we find the Hubble constant to be either 80 ± 11 or 73 ± 11 km s-1 Mpc-1 respectively.