Allan Sandage and the Distance Scale

TL;DR

This paper reviews Allan Sandage's extensive work on the cosmic distance scale, highlighting his contributions to calibrating the Hubble constant through various indicators like Cepheids, SNe Ia, and TRGB, leading to a refined value of H0.

Contribution

It provides a comprehensive historical and methodological overview of Sandage's work, emphasizing the development and application of different distance indicators to determine H0.

Findings

H0 calibrated at 62.3 using Cepheids with HST

H0 refined to 62.9 using TRGB distances

Combined estimate of H0 is 64.1 km/s/Mpc

Abstract

Allan Sandage returned to the distance scale and the calibration of the Hubble constant again and again during his active life, experimenting with different distance indicators. In 1952 his proof of the high luminosity of Cepheids confirmed Baade's revision of the distance scale (H0 ~ 250 km/s/Mpc). During the next 25 years, he lowered the value to 75 and 55. Upon the arrival of the Hubble Space Telescope, he observed Cepheids to calibrate the mean luminosity of nearby Type Ia supernovae (SNe Ia) which, used as standard candles, led to the cosmic value of H0 = 62.3 +/- 1.3 +/- 5.0. Eventually he turned to the tip of the red-giant branch (TRGB) as a very powerful distance indicator. A compilation of 176 TRGB distances yielded a mean, very local value of H0 = 62.9 +/- 1.6 and shed light on the streaming velocities in the Local Supercluster. Moreover, TRGB distances are now available for six SNe Ia; if their mean luminosity is applied to distant SNe Ia, one obtains H0 = 64.6 +/- 1.6 +/- 2.0. The weighted mean of the two independent large-scale calibrations yields H0 = 64.1 km/s/Mpc within 3.6%.