31. Implications of a cepheid distance to the Fornax cluster

Thirty-seven long-period Cepheid variables have been discovered in the Fornax cluster spiral galaxy, using the Hubble Space Telescope. The resulting V and I period-luminosity relations give a true modulus of µ₀ = 31.28 ± 0.07 mag, corresponding to a distance of 18.0 ± 0.6 Mpc. A Cepheid distance to the Fornax cluster offers several means of estimating the Hubble constant. First, associating this distance with the Fornax cluster as a whole gives a local Hubble constant of H₀= 73 (± 7)_random [± 18]_systematic km/sec/Mpc. Second, the Fornax cluster provides a means of calibrating a wide variety of secondary distance indicators. Recalibrating the Tully-Fisher relation using NGC 1365 and 6 nearby spiral galaxies, applied to 15 clusters out to 100 Mpc gives H₀ = 76 (± 2)_r [± 8]_s km/sec/Mpc. A broad-based set of differential moduli established from Fornax out to Abell 2147, nearly a factor of ten in distance further, gives H₀ = 72 (± 1)_r [± 7]_s km/sec/Mpc. With the addition of two Type Ia supernova calibrators in Fornax and correcting the supernova peak luminosities for decline rate, gives H₀ = 68 (± 5)_r [± 8]_s km/sec/Mpc, out to a distance in excess of 500 Mpc. Seven Cepheid-based distances to groups of galaxies out to and including the Virgo and Fornax clusters yield H₀ = 70 (± 3)_r [± 16]_s km/sec/Mpc. These major distance determination methods agree to within their statistical errors. The resulting value of the Hubble constant, encompassing all those determinations which are based directly on Cepheids or tied to secondary distance indicators, is found to be H₀ = 72 (± 3)_r [± 12]_s km/sec/Mpc, out to cosmologically significant distances.

Hubble (1929) announced his discovery of the expansion of the Universe nearly 70 years ago. Despite decades of effort, and continued improvements in the actual measurement of extragalactic distances, convergence on a consistent value for the absolute expansion rate, as parameterized by the Hubble constant, H₀, was not forthcoming. However, progress in the last few years has been rapid and dramatic (see, for instance, Freedman, Madore & Kennicutt 1997; Mould, Sakai, Hughes & Han 1997; Tammann & Federspiel 1997). This accelerated pace has occurred primarily as a result of the improved resolution of the Hubble Space Telescope (and its consequent ability to discover classical Cepheid variables at distances a factor of ten further than can routinely be achieved from the ground), giving accurate zero points to a number of recently refined methods which can measure precise relative distances beyond the realm of the Cepheids. These combined efforts are providing a more accurate distance scale for local galaxies, and are indicating a convergence among various secondary distance indicators in establishing an absolute calibration of the far-field Hubble flow.

Soon after the December 1993 HST servicing mission it was clear that the measurement of Cepheids in the Virgo cluster (part of the original design specifications for the telescope) was feasible (Freedman et al. 1994a). And although the subsequent discovery of Cepheids in the Virgo galaxy M100 (Freedman et al. 1994b) and subsequent refinements (Ferrarese et al. 1996) were important steps in resolving outstanding differences in the extragalactic distance scale (Mould et al. 1995), the Virgo cluster is complex both in its geometric and its kinematic structure, and there still remain large uncertainties in both the velocity and distance to this cluster. Virgo clearly was, and still is, not an ideal test site for an unambiguous determination of the cosmological expansion rate or the calibration of secondary distance indicators. In this paper we discuss the implications of a Cepheid distance to the next major clustering of galaxies, the Fornax cluster, which is a much less complicated system than Virgo.