The Galaxy as Fundamental Calibrator of the Extragalactic Distance Scale - Part Two - Comparisons of Metric and Photometric Scale Lengths and Three Further Tests of the Long and Short Distance Scales

Abstract

Metric and photometric parameters of the Galaxy are used to perform three further tests of the extragalactic distance scales: <P />1. The zero points, α, β of the correlations between luminosity index, Λ<SUB>c</SUB>, and the absolute magnitude, -M<SUB>T</SUB><SUP>0</SUP> = α -3(Λ<SUB>c</SUB> - 1), and linear isophotal diameter, log D<SUB>0</SUB> = β - 0.6(Λ<SUB>c</SUB> - 1), of spirals are derived by means of the corresponding quantities for the Galaxy. The galactic-based zero points, α = 19.15±0.25, β=4.18±0.05 are in excellent agreement with the zero points (19.15±0.16 and 4.18±0.045) of the Λ<SUB>c</SUB> scale previously derived from 16 nearby calibrating galaxies on the short distance scale. <P />2. The implications of the long and short distance scales for the absolute magnitude and for several metric and photometric diameters of the Galaxy are tested by comparison with four galaxies previously identified as closely similar to our Galaxy by a multiparameter taxonomic procedure. If these galaxies are at the distances implied by the short scale, their metric and photometric parameters agree within 0.1-0.2 mag with the corresponding parameters of the Galaxy, while on the long scale they depart from it by 1.3-1.4 mag. <P />3. This conclusion is independently confirmed by comparison of the consequences of the distances attributed to the Virgo E cluster on the short and long scales (11.8 vs. 22.5 Mpc) for the absolute magnitudes of the globular clusters which, according to Hanes, are observed at a mean apparent magnitude υ&gt; = 23.3 ± 0.4. The corresponding mean absolute magnitude of the clusters, υ&gt; = -7.22 on the short scale, and -8.46 on the long scale, imply υ(RR)&gt; = +0.78 on the short scale, in near perfect agreement with the mean of modern determinations (+0.8±0.15), but υ(RR)&gt; =-0.46 on the long scale, which is 1.26 mag too bright. The same discrepancy arises for the zero point of the period-luminosity relation of long-period variables which determines R<SUB>sun</SUB> in agreement with RR Lyrae variables. <P />The five different, and substantially independent, types of metric, photometric, and kinematic tests of the extragalactic distance scale presented in Papers I and II bypass many of the uncertainties of the traditional step-by-step approach. They all agree with the short distance scale within 0.1-0.2 mag; they all reject the long scale at extremely high probability levels. Hence, the values of the Hubble constant in the range 90 ≤ H<SUB>0</SUB> ≤ 100 km s<SUP>-1</SUP>, previously derived from a variety of indicators calibrated in the short scale, are confirmed. <P />The validity of the Galaxy as a standard is discussed in Appendix A. The time scale argument often used as a prop to the long scale is shown in Appendix B to be inconclusive.