Formation of Structure in the Universe

1.3.2.1 Relative Distance Methods

One piece of good news is that the several methods of measuring the relative distances to galaxies now mostly seem to be consistent with each other (Jacoby et al. 1992; Fukugita, Hogan, & Peebles 1993). These methods use either (a) ``standard candles'' or (b) empirical relations between two measurable properties of a galaxy, one distance-independent and the other distance-dependent. (a) The old favorite standard candle is Type Ia supernovae; a new one is the apparent maximum luminosity of planetary nebulae (Jacoby et al. 1992). Sandage et al. (1996) and others (van den Bergh 1995, Branch et al. 1996, cf. Schaefer 1996) get low values of h approx 0.55 from HST Cepheid distances to SN Ia host galaxies, including the seven SNe Ia with what Sandage et al. characterize as well-observed maxima that lie in six galaxies for which HST Cepheid distances are now available. But taking account of an empirical relationship between the SN Ia light curve shape and maximum luminosity leads to higher h = 0.65 ± 0.06 (Riess, Press, & Kirshner 1996) or h = 0.63 ± 0.03 (Hamuy et al. 1996), although Tammann & Sandage (1995) disagree that the increase in h can be so large.

(b) The old favorite empirical relation used as a relative distance indicator is the Tully-Fisher relation between the rotation velocity and luminosity of spiral galaxies (and the related Faber-Jackson or D_n - sigma relation). A newer one is based on the decrease in the fluctuations in elliptical galaxy surface brightness on a given angular scale as comparable galaxies are seen at greater distances (Tonry 1991); a new SBF survey gives h = 0.81 ± 0.06 (Tonry et al. 1997).

The ``mid-term'' value of the Hubble constant from the HST key project is h = 0.73 ± 0.10 (Freedman 1997). This is based on the standard distance to the LMC of 50 kpc (corresponding to a distance modulus of 18.50). But the preliminary results from the Hipparcos astrometric satellite suggest that the Cepheid distance scale must be recalibrated, and that the quoted distance to the LMC is too low by about 10% (Feast & Catchpole 1997, Feast & Whitelock 1997). An increase in the LMC distance of about 7% is also obtained using the preliminary Hipparcos recalibration of the zero point and metallicity dependence of the RR Lyrae distance scale (Gratton et al. 1997, Ried 1997; cf. Alcock et al. 1996b), thus removing a long-standing discrepancy. The implication is that the Hubble parameter determined by Cepheid calibrators must be decreased, by perhaps 10%. This applies to the HST key project, and it also applies to the SN Ia results for h, which are based on Cepheid distances; thus, forexample, the Hamuy et al. (1996) value would decrease to about h = 0.57, with a corresponding t₀ = 11.4 Gyr for Omega = 1.