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 i ndependent new methods are compared with the long and short extra-gala ctic distance scales. All comparisons show that there is close agreeme nt (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 Larg e 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 Malmquis t bias in the short scale compared with all others over the whole dist ance modulus interval (18 < DM < 35). If y = DM(short scale) and x = D M(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 standa rd deviation of only 0.09 mag. The long scale differs systematically f rom 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 t he 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 Hubbl e expansion ratio for 11 objects in the distance interval of 1 to 16 M pc 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 syste matic departure from linearity. The major arguments still supporting t he 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 ani sotropy of the velocity field) are discussed.