Giant H II regions as distance indicators - II. Application to H II galaxies and the value of the Hubble constant

The (integrated) Hβ luminosities of giant H II regions and of Hn galaxies (giant H II regions in dwarf galaxies) can be predicted with an accuracy comparable to the observational errors from the velocity widths of their emissionline profiles and the nebular oxygen abundances. The zero point of the correlation between these parameters is calibrated using the homogeneous sample of giant H II regions in nearby galaxies with well determined distances discussed in the first paper of this series. The calibration is applied to distant H II galaxies to obtain a value for Hubble's constant H₀. After correcting the fluxes for Malmquist bias and the radial velocities for the motion of the Local Group we find $$H_0=89\pm10\enspace\text{km s}^{-1}\enspace\text{Mpc}^{-1}$$. From four H II galaxies in the Virgo cluster, a true distance modulus of $$(m-M)_{\text {vir}}=30.9\pm0.15$$ is obtained for that cluster. The effects of the large-scale inhomogeneities in the Hubble flow shown to exist by Dressier et al. are emphasized and illustrated using the distance to Virgo to calibrate the kinematical distances of six rich clusters out to redshifts of ∼9000 km s⁻¹. We conclude that values of H₀ obtained using ‘nearby’ (cz>5000 km s⁻¹) galaxies may be systematically biased by streaming motions. A detailed discussion of possible systematic errors is presented.