|Annu. Rev. Astron. Astrophys. 1999. 37: 445-486
Copyright © 1999 by Annual Reviews. All rights reserved
6.2. The Steps Series from NGC 2403 to the Global Expansion Field
The Steps series has been reviewed elsewhere (Sandage 1998a). Only a few aspects need be summarized here in Table 2, but with a few added comments.
|I||Data on angular sizes of the first three largest HII regions are given in galaxies in the Local Group. Distance-degenerate effects are discussed.||Sandage & Tammann (1974a)|
|II||Identification and photometry of the brightest resolved stars are given in 11 nearby galaxies with known Cepheid distances. Later data of the same kind for M81 and M101 are in Sandage (1983a, 1984).||Sandage & Tammann (1974b)|
|III||The stellar content of M101 is given based on a long series of 200-inch photometrically calibrated photographs. The distance modulus of m - M = 29.3 is 11 times the distance of m - M = 24.0 given by Hubble (Holmberg 1950). Kelson et al (1996), with m - M = 29.34, confirms the Steps value. Ho = 56 ± 9 km s-1 Mpc-1.||Sandage & Tammann (1974c)|
|IV||Calibration of the absolute magnitude of the van den Bergh luminosity classes. Modulus of the Virgo cluster derived as m - M = 31.45 ± 0.09. Ho = 57 ± 6.||Sandage & Tammann (1974d)|
|V, VI||The quietness of the local velocity field is derived from the photometric distances of nearby galaxies compared with kinematic distances. Redshift data in Steps VI for Sc I galaxies to v = 20,000 km s-1 are combined with Zwicky et al (1961-1968) magnitudes to give a Hubble diagram requiring Ho = 57 ± 3. Observational selection bias, using a form of a Spaenhauer diagram, was discussed for the first time.||Sandage & Tammann (1975a, 1975b)|
|VII||Virgo cluster distance modulus of m - M = 31.70 ± 0.08 derived using Tully-Fisher method. Ho = 50 ± 4.||Sandage & Tammann (1976)|
|VIII||Type Ia supernovae calibrated via brightest stars in IC 4182 gives Ho = 50 ± 7.||Sandage & Tammann (1982)|
|IX||A new method is introduced to tie the Virgo Cluster redshift to the remote expansion field devoid of all local velocity anomalies. The method was later made more precise by Jerjen & Tammann (1993), Federspiel et al. (1998). Virgo modulus derived as m - M = 31.70 ± 0.09. Ho = 52 ± 2.||Sandage & Tammann (1990)|
|X||Globular clusters are used to give a Virgo Cluster modulus revising an earlier value by Harris et al. (1991) who used an incorrect calibration of MV (RR). Using the Oosterhoff-Preston-Arp period-metallicity relation for RR Lyrae stars (Arp 1955, Kinmann 1959) as calibrated from pulsation equations (Sandage 1990b, c, 1993b, c) gives m - M (Virgo) = 31.64 ± 0.25. Ho = 57 ± 5.||Sandage & Tammann (1995)|
The five distance indicators listed by Hubble were Cepheids, brightest stars, novae, average galaxy luminosities, and brightest cluster galaxies. These were all eventually used in the programs to determine the distances to M31, the M81 and M101 groups, and ultimately to galaxies farther away in the remote expansion field.
Cepheids and the brightest cluster galaxies, together with the Palomar and Mount Wilson programs to calibrate them, were described earlier. The Steps program added the new distance indicators of (a) the angular size of HII regions and the linear calibration of the first three largest, and (b) supernovae of type Ia. In addition, novae, brightest stars, and galaxy luminosity functions that calibrated the van den Bergh luminosity classes in late type spirals were used, thereby completing Hubble's (1951) list. A few details of the Steps series are given in Table 2.
The definitive solution to the distance scale problem via the traditional ladder approach has had to await the results of the Hubble Space Telescope (HST) for Cepheid distances of galaxies that have produced supernovae of type Ia. The reviews by Teerikorpi (1997), Branch (1998), and the calibrations of SNe Ia at maximum light via Cepheids (Sandage, Tammann, & Saha 1998, Saha et al 1999) continue to favor the long distance scale with Ho = 55 ± 5, although other solutions still occur in the literature (Madore et al 1998).