Steps toward the Hubble constant. IV - Distances to 39 galaxies in the general field leading to a calibration of the galaxy luminosity classes and a first hint of the value of H sub 0

Distances to 39 late-type (Sc-Sd-Sm-Ir) field galaxies have been obtained from the sizes of their H ii regions as measured on red-sensitive plates taken with the Mount Wilson and the Palomar reflectors. The distances follow from the calibration in Paper I, and are combined with apparent magnitudes to give absolute magnitudes. The sample is increased to 48 galaxies by adding members of the Local and NGC 2403 groups, whose distances are known from Cepheids. The distribution of absolute magnitudes among the van den Bergh luminosity classes (Lc) is determined, and a new absolute calibration of the mean total galaxian luminosity with luminosity class is obtained. The derived Mpg0, = f(L ) relation is tested for shape by comparing it with the relative calibration for Virgo cluster spirals of different L , and with van den Bergh's initial calibration that used the ratio of redshifts to obtain relative distances of field galaxies in Humason, Mayall, and Sandage. The adopted absolute magnitudes, combined with the apparent magnitudes of Virgo cluster spirals, gives the distance to the Virgo cluster as (rn - M)0 = 31.45 1 0.09, or D = 19.5 1 0.8 Mpc. Two methods (the Hubble diagram for first-ranked E galaxies in clusters, and the color-magnitude relation for E galaxies in the Coma cluster compared with Virgo) show that the Virgo cluster has no detectable peculiar velocity. The systemic velocity is v0 = 1111 1 75 (rms) km 1 This, combined with D, gives a first hint at the value for the Hubble constant as H0 = 57 1 6 km s ' Mpc `. The value agrees well with that obtained in Paper VI which follows, where many field spirals are used at large redshifts beyond any possible local velocity anisotropy. The value of H0 obtained here via the Virgo cluster depends not on the data for the Virgo cluster alone, but on the total statistical sample of 84 brightest E galaxies that make up the Hubble-diagram formulation for clusters. Subject headings: cosmology - galaxies - redshifts