37. The nearby flow field

We now step back somewhat and investigate the Hubble flow between us and Fornax, derived from galaxies and groups of galaxies inside 20 Mpc, each having Cepheid-based distances and expansion velocities individually corrected for a Virgocentric flow model (Kraan-Korteweg 1986). Figure 34 captures those results in graphical form. At 3 Mpc the M81-NGC 2403 Group (for which both galaxies of this pair have Cepheid distance determinations) gives H₀ = 75 km/sec/Mpc after averaging their two velocities. Working further out to M101, the NGC 1023 Group and the Leo Group, the calculated values of H₀ range from 62 to 99 km/sec/Mpc. An average of these independent determinations including Virgo and Fornax, gives H₀ = 70 (± 3)_r km/sec/Mpc. This determination, as before, uses a Virgocentric flow model with a 1/ R_Virgo infall velocity fall-off, scaled to a Local Group infall velocity of +200 km/sec, which was determined ab initio by minimizing the velocity residuals for the galaxies with Cepheid-based distances.

Figure 34. The velocity-distance relation for local galaxies having Cepheid-based distances. Circled dots mark the velocities and distances of the parent groups or clusters. The one-sided ``error'' bars with galaxy names attached mark the velocities associated with the individual galaxies having direct Cepheid distances. The broken line represents a fit to the data giving H₀ = 70 ± 3 km/sec/Mpc. The 95% confidence interval on the observed scatter is ± 16 km/sec/Mpc, and is shown by the thin diverging broken lines; the solid lines indicate one-sigma limits.

The foregoing determination of H₀ is again predicated on the assumption that the inflow-corrected velocities of both Fornax and Virgo are not further perturbed by other mass concentrations or large-scale flows, and that the 25,000 Mpc³ volume of space delineated by them is at rest with respect to the distant galaxy frame. To avoid these local uncertainties we now step out from Fornax to the distant flow field. There we explore three applications: (i) Use of the Tully-Fisher relation calibrated by Cepheids locally, and now including NGC 1365 and about two dozen additional galaxies in the Fornax cluster. Ultimately these calibrators are tied into the distant flow field at 10,000 km/sec defined by the the Tully-Fisher sample of galaxies in clusters (Aaronson et al. 1980). (ii) Using the distance to Fornax to tie into averages over previously published differential moduli for independently selected distant-field clusters, (iii) Recalibrating the Type Ia supernova luminosities at maximum light, and applying that calibration to events as distant as 30,000 km/sec.