21 centimeter line width distances of cluster galaxies and the value of H0

Abstract

Locally calibrated blue and infrared Tully-Fisher (TF) relations are applied to an essentially (i.e., 82%) complete sample of 81 Sab-Sm galaxies which are bona fide members of the Virgo Cluster. Because of the proximate completeness of the sample a nearly unbiased Virgo modulus of (m - M)^0^ = 31.60 + 0.15 can be derived. The result is in perfect agreement with independent recent determinations which gives additional justification to the adopted local calibrators. The distance modulus from the blue TF relation carries higher weight because infrared H magnitudes are available for only 25 Virgo members. The remaining H magnitudes are derived from a well defined (B-H)/log {DELTA}v_21_ relation; nevertheless, the 25 members suffice to show that the bTF and iTF relations give almost identical distance moduli from any selected Virgo subsample. The intrinsic scatter about the two TF relations is σ_M_ = 0.7 mag. This is considerably larger than the observed scatter in the UMa Cluster and in 10 more distant clusters for which B and H magnitudes and 21 cm line widths are available; their reduced scatter discloses that the hitherto investigated samples merely contain the brightest spirals at any given line width. Distance determinations of these clusters can therefore only be achieved by fitting the upper envelopes of their TF relations onto the blue and infrared upper envelopes of the Virgo Cluster. The resulting distances define a linear expansion law with a small scatter. The linearity of the expansion can be extended for the range from v ~ 200 km s^-1^ out to v ~ 20,000 km s^-1^ using external data. The present cluster data require a Hubble constant of H_0_ = 56.6 +/- 0.9 km s^-1^ Mpc^-1^ which implies through first-ranked cluster galaxies 51 < H_0_(global) < 56 at very large distances; the external error of these values is probably less than 20%. A search for Local Group motion toward the microwave background dipole remains inconclusive.