The Distance to the Hercules Supercluster. II. Application of Tertiary Indicators and an Estimate of the Hubble Constant

Abstract

Using data provided in a previous paper, we calculate distances on the zero point of de Vaucouleurs's distance scale to 77 individual galaxies in CGCG field 108, where the core of the Hercules supercluster is found. Three largely independent techniques are applied: (1) the luminosity index of spirals in conjunction with total magnitudes, isophotal diameters, and effective apertures; (2) the 21 cm line width of spirals in conjunction with total magnitudes and isophotal diameters; and (3) the (U - B) colors and mean effective surface brightnesses of ellipticals and lenticulars in conjunction with total magnitudes. The methods yield results which are fairly consistent, although the published 21 cm line-width data either show evidence of large errors or are not well represented by revised versions of the Tully-Fisher relation. However, in the mean the three methods agree, giving 35.1 ± 0.2 (external mean error, inclusive of zero point) for the distance modulus, indicating that the Hercules supercluster is 105 ± 11 Mpc distant. Using published radial velocities, the Hubble constant is found to be H₀ = 105 ± 6 (internal mean error; ± 11 external mean error) km s⁻¹Mpc⁻¹, consistent with previous estiinates based on de Vaucouleurs's tertiary distance scale. An analysis of the velocity-distance relation toward CGCG field 108 seems to confirm the existence of two groups of foreground galaxies. While the number of objects in these groups is too small to establish firmly the form of the relation within the scatter of the distance moduli, the mean distances of the groups are approximately consistent with the same value of the Hubble constant derived from the Hercules supercluster. The data suggest that the velocity-distance relation toward CGCG field 108 is linear on a scale of 100 Mpc.