Steps toward the Hubble constant. VII. Distances to NGC 2403, M101, and the Virgo cluster using 21 centimeter line widths compared with optical methods: the global value of H0.

The discovery by Tully and Fisher that the absolute B magnitudes of galaxies and their 21 cm line widths (corrected to edge-on orientation) are very tightly correlated has opened the way to test the previously determined distances to the M81-NGC 2403 and the Ml01 groups. The recalibration of the Tully-Fisher relation gives MpgO, = - 6.88Av21 - 2.36 based on a Hyades modulus of 3.03 mag. Application to the nearby groups gives 21 cm radio moduli of (m - M)o = 27.64 + 0.09 for M81-NGC 2403, and 29.08 + 0.16 for MiOl, based on adopted distances to M31, M33, and the Hyades alone. Comparison with the previous optical values of (m - M)0 = 27.55 + 0.13 and 29.30 + 0.3 provides support for the present local distance scale. Application of the 21 cm method to 19 spirals within the central 60 (radius) of the Virgo cluster gives (m - M)0 = 31.4 + 0.2 for the distance modulus to the cluster, again based on a Hyades modulus of 3.03, although difficulties remain that cast doubt on the adopted precepts for the internal absorption correction. Data from six optical methods give (m - M)0 = 31.50 + 0.08 for the Virgo cluster. The agreement with the 21 cm method again provides a satisfactory test of the current distance scale. Adopting a revised Hyades modulus of (m - M)0 = 3.23 gives a weighted mean Virgo modulus of (m - M)0 = 31.70 + 0.08 (D = 21.9 + 0.9 Mpc), which we adopt. This modulus provides the absolute magnitude calibration of first-ranked cluster E galaxies, and hence the calibration of the ridgeAine of the velocity-magnitude relation for great clusters. The resulting global value of the Hubble constant is H0 = 50.3 + 4.3 a km s ' Mpc-1, which gives H0-1 = (19.4 1 1.6) x 10 years. Comparison with the age of globular clusters, increased by the formation time of our Galaxy since the creation event, shows that the Friedmann models (A = 0) with q0 > 21 seem inadmissible. Therefore, the conclusion would be that the Universe has happened only once. Subject headings: cosmology - galaxies: clusters of - galaxies: redshifts - radio sources: 21 cm radiaticn