The expansion field: the value of H0
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- Tammann, G.A., Sandage, A. & Reindl, B. Astron Astrophys Rev (2008) 15: 289. doi:10.1007/s00159-008-0012-y
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Any calibration of the present value of the Hubble constant (H0) 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 H0 can be found at any distance >4.5 Mpc. RR Lyr star-calibrated TRGB distances of 78 galaxies above this limit give H0 = 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 H0 = 63.4 ± 1.7 at 15 Mpc. This agrees also with the large-scale value of H0 = 61.2 ± 0.5 from the distant, Cepheid-calibrated SNe Ia. A mean value of H0 = 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 H0 are discussed. They either conform with the quoted value (e.g. line width data of spirals or the Dn−σ method of E galaxies) or are judged to be inconclusive. Typical errors of H0 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.