The expansion field: the value of H 0

Abstract

Any calibration of the present value of the Hubble constant (H 0) requires recession velocities and distances of galaxies. While the conversion of observed velocities into true recession velocities has only a small effect on the result, the derivation of unbiased distances which rest on a solid zero point and cover a useful range of about 4–30 Mpc is crucial. A list of 279 such galaxy distances within v < 2,000 km s−1 is given which are derived from the tip of the red-giant branch (TRGB), from Cepheids, and/or from supernovae of type Ia (SNe Ia). Their random errors are not more than 0.15 mag as shown by intercomparison. They trace a linear expansion field within narrow margins, supported also by external evidence, from v = 250 to at least 2,000 km s−1. Additional 62 distant SNe Ia confirm the linearity to at least 20,000 km s−1. The dispersion about the Hubble line is dominated by random peculiar velocities, amounting locally to <100 km s−1 but increasing outwards. Due to the linearity of the expansion field the Hubble constant H 0 can be found at any distance >4.5 Mpc. RR Lyr star-calibrated TRGB distances of 78 galaxies above this limit give H 0 = 63.0 ± 1.6 at an effective distance of 6 Mpc. They compensate the effect of peculiar motions by their large number. Support for this result comes from 28 independently calibrated Cepheids that give H 0 = 63.4 ± 1.7 at 15 Mpc. This agrees also with the large-scale value of H 0 = 61.2 ± 0.5 from the distant, Cepheid-calibrated SNe Ia. A mean value of H 0 = 62.3 ± 1.3 is adopted. Because the value depends on two independent zero points of the distance scale its systematic error is estimated to be 6%. Other determinations of H 0 are discussed. They either conform with the quoted value (e.g. line width data of spirals or the D n −σ method of E galaxies) or are judged to be inconclusive. Typical errors of H 0 come from the use of a universal, yet unjustified P–L relation of Cepheids, the neglect of selection bias in magnitude-limited samples, or they are inherent to the adopted models.