H 0 from Type Ia Supernovae

The Hubble diagrams in B, V , and I of a complete sample of 35 SNe Ia with (B − V) < 0.06 and 1200 < v ∼ < 30 000 km s −1 have a scatter of only 0. m 1, after small corrections are applied for differences in decline rate ∆m15 and color (B −V). The tightness of the Hubble diagrams proves blue SNe Ia to be the best " standard candles " known. Their absolute magnitudes MB,V,I are calibrated by eight SNe Ia with Cepheid distances from HST. Combining this calibration with the appropriate Hubble diagrams yields a large-scale value of H0 = 58.5 ± 6.3 at the 90-percent confidence level. The Hubble diagram of SNe Ia has so small scatter that it seems feasable to determine Λ " locally " , i.e. within z ∼ < 0.12, once 100-200 SNe Ia with good photometry will be available. Such a local determination would minimize evolutionary effects and K-term corrections. Clusters of galaxies have provided useful Hubble diagrams through brightest cluster members, TF distances, and Dn − σ and fundamental plane distances, but with significantly more scatter (σ = 0. m 2−0. m 3) than SNe Ia. The zeropoint calibration of these Hubble diagrams is an additional problem, which is aggravated by the high weight of any adopted distance of the Virgo cluster and by selection effects of clusters with only few well studied members. If a Virgo cluster modulus of (m − M) = 31.60 ± 0.20 is adopted for calibration, — a value which is well secured by Cepheids, SNe Ia, and the TF method, and which also agrees with less definitive distances from the globular cluster luminosity function, novae, and the Dn −σ method, — one finds H0 = 55±5. The reasons are explained why some authors have found higher values from clusters. The determination of H0 from field galaxies is beset by selection effects of magnitude-limited samples (Malmquist bias). Authors who have properly allowed for bias have consistently obtained H0 ≈ 55±5 within v ∼ < 5000 km s −1 based on the TF and other methods. The calibration rests only on Cepheids, independent of any adopted Virgo cluster modulus. Giving highest weight to SNe Ia it is concluded that H0 = 58 ± 6.