Steps toward the Hubble constant. III. The distance and stellar content of the M101 group of galaxies

The distance to M101 and its five late-type companions (NGC 5204, 5474, 5477, 5585, and Ho IV) is obtained by six methods from new data on the stellar content, after the reality of the group is tested using criteria of adjacency, resolvability, redshifts, and dynamical stability. The brightest stars in M101 are isolated and measured relative to a new photometric sequence that covers the range 12.6 < B < 21.7. Analysis of the color-magnitude diagram shows the brightest resolved blue star in M101 to have B = 18.97. Correction of older data suggests B(3) = 20.6 for NGC 5474, and B(3) = 20.9 in NGC 5585. Despite a serious study, no red supergiant variables with B - V> 2.0 were found in M101 brighter than a limit of V = 21. Further, after a thorough search, no Cepheids were found brighter than a limit of B = 22.5. Eight irregular blue variables were isolated from the total plate material that extends from 1909 to the present. From these data, and from the largest H ii regions in the six group members, the distance is calculated to be D = 7.2 i 1 Mpc, or (m - M)0 = 29.3 + 0.3 which is more than 10 times that adopted by Hubble in 1936. This result forms the basis for extending the calibrations of H ii regions and of stars in Papers I and II of this series to the Sc I galaxies so that the distance scale can be carried outward into the general field (Papers IV and VI). The velocity-distance ratio for the M101 group alone, obtained from the adopted distance and from v0 = 402 i 4.2 km (and allowing 30 km s - 1mean random motion for the group [Paper V] in the error calculation), is vlr = 55.5 + 8.7km 1 Mpc, which agrees well with the global value of the Hubble constant itself (Paper VI), showing the lack of measurable perturbation of the Hubble expansion flow at M101 (cf. Paper V). Subject headings: Cepheids and W Virginis stars - galaxies, clusters of - galaxies, individual - luminous stars