Population II standard candle calibration of the Fundamental Plane of groups and clusters of galaxies and the Hubble Constant

Hubble’s constant (H0) is calculated using the Fundamental Plane (FP) relation between half-light radius, surface brightness, and velocity dispersion for groups and clusters of galaxies. The Tip of the Red Giant Branch (TRGB), a standard candle based on the constant luminosity of low-mass stars at helium core flash, is used to calibrate the cluster FP. TRGB stars yield the distances to the nearby clusters Virgo and Leo. Half-light radii are derived by assuming spherical model density profiles that are consistent with the cluster FP. Altogether this is a new distance ladder based on Baade’s Population II exclusively; it is independent of Cepheid variable stars and all secondary standard candles calibrated by them. I obtain H0 = 56± 4 km s −1 Mpc (stat. err.), which is 20% lower than the Hubble Space Telescope Key Project result based on Cepheids. The primary TRGB and Cepheid calibrations are in excellent agreement, and the Key Project distances to Virgo and Leo are consistent with the TRGB distances and density models. The latter comparison also yields a new constraint on the Cepheid metallicity slope of γ < 0 (92% confidence).