THE EXTRAGALACTIC DISTANCE SCALE .8. A COMPARISON OF DISTANCE SCALES

Some 300 recent determinations of the distances to 12 nearby galaxies, one group, and three clusters by various authors using a variety of independent new methods are compared with the long and short extra-galactic distance scales. All comparisons show that there is close agreement (within 0.3 mag in the distance modulus DM) between the short-scale moduli and the new estimates by others at all distances from the Large Magellanic Cloud (DELTA = 0.05 Mpc) to the Coma Cluster (DELTA = 83 Mpc). The mean systematic difference DELTADM(short - others) is only -0.04 +/- 0.02 mag. There is little evidence for a progressive Malmquist bias in the short scale compared with all others over the whole distance modulus interval (18 < DM < 35). If y = DM(short scale) and x = DM(others), an impartial line solution for the 16 objects gives for the median moduli y - 27.75 = (0.999 +/- 0.007)(x - 27.79), with a standard deviation of only 0.09 mag. The long scale differs systematically from all the others by about +0.25 mag within the Local Group (DM < 26) and by + 1.01 mag outside (26 < DM < 35). Accidental errors are also much larger in the long-scale moduli of eight individual galaxies in the interval 26 < DM < 31 (sigma congruent-to 0.4 mag) than in the two other scales (sigma congruent-to 0.1 mag). The mean value of the Hubble expansion ratio for 11 objects in the distance interval of 1 to 16 Mpc is [H] = 88.8 +/-5.7 km s-1 Mpc-1. The Coma Cluster alone gives 86.0. A linear solution for all 12 objects gives [H] = 87.3 +/- 1.1 km s-1 Mpc-1, with a dispersion of 84 km s-1. There is no appreciable systematic departure from linearity. The major arguments still supporting the long scale and detracting from the short scale are briefly reviewed. Apart from this persistent, unresolved dichotomy, the main remaining uncertainties impacting both distance scales at the 0.1-0.2 mag level (zero points, Galactic and internal extinction corrections, local anisotropy of the velocity field) are discussed.