The Virgo Cluster Distance from 21 Centimeter Line Widths

The distance of the Virgo cluster is derived in the B band from the 21cm-line width-absolute magnitude relation. The latter is calibrated using 18spirals with Cepheid distances mainly from HST. The calibration is applied to acomplete sample of non-peculiar spirals with i>45 deg and lying within theoptical (n=49) or X-ray (n=35) contour of the cluster, resulting in a meancluster distance of (m-M)_0=31.58+/-0.24 mag (external error) or 20.7+/-2.4Mpc. The mean distance of subcluster A is 0.46+/-0.18 mag smaller than that ofsubcluster B, but the individual distances of the members of the twosubstructures show considerable overlap. The distance modulus is corrected by-0.07 mag for the fact that cluster members have lower H I-surface fluxes andare redder in (B-I) at a given line width than the (field) calibrators.Different sources of the B magnitudes and line widths have little effect on theresulting distance. Different precepts for the internal-absorption correctionchange the result by no more than +/-0.17 mag. The individual distances of thecluster members do not show any dependence on recession velocity, inclination,Hubble type or line width. The dependence on apparent magnitude reflects theconsiderable depth effect of the cluster. The adopted distance is in goodagreement with independent distance determinations of the cluster. Combiningthe cluster distance with the corrected cluster velocity of 1142+/-61 km/sgives H_0=55+/-7 km/s/Mpc (external error). If the Virgo cluster distance isinserted into the tight Hubble diagram of clusters out to 11000 km/s usingrelative distances to the Virgo cluster one obtains a global value ofH_0=57+/-7 km/s/Mpc.Comment: 37 pages, LaTeX using AAS aaspp4 style, 14 eps figures, 7 Tables; to be published in the Astrophysical Journal (Main Journal