An independent estimate of H(z) at z = 0.5 from the stellar ages of brightest cluster galaxies

TL;DR

This study measures the Hubble parameter at redshift 0.5 using the ages of brightest cluster galaxies, providing an independent estimate that helps address tensions between different cosmological measurements.

Contribution

It introduces a new, homogeneous sample of brightest cluster galaxies for cosmic chronometer analysis, reducing systematic errors in H(z) measurements without assuming a specific cosmology.

Findings

Measured H(z=0.5) = 72.1 ± 33.9 (stat) ± 7.3 (syst) km/s/Mpc

Reduced systematic errors to 10% by selecting oldest, most massive galaxies

Projected Hubble constant H0 = 54.6 ± 25.7 (stat) ± 5.5 (syst) km/s/Mpc, consistent with other measurements

Abstract

Several cosmological observations (e.g., Cosmic Microwave Background (CMB), Supernovae Type Ia, and local distance ladder measurements such as Cepheids) have been used to measure the global expansion rate of the Universe, i.e., the Hubble constant, $H_{0}$. However, these precision measurements have revealed tensions between different probes that are proving difficult to solve. Independent, robust techniques must be exploited to validate results or mitigate systematic effects. We use the Cosmic Chronometer (CC) method, which leverages the differential age evolution of passive galaxies, to measure $H(z)$, without any assumption of the underlying cosmology. Unlike previous CC studies, we used only brightest cluster galaxies (BCGs), the oldest and most massive galaxies in the Universe, to construct a pure and homogeneous sample. In this work we used a sample of 53 BCGs in massive, Sunyaev-Zel'dovich selected galaxy clusters (0.3 $< z <$ 0.7) with Southern African Large Telescope (SALT) spectroscopic observations. We used optical spectra to measure D4000$_{\rm n}$ of the BCGs to obtain a new direct measurement of $H(z) = 72.1 \pm 33.9(\rm stat) \pm 7.3$(syst) km s$^{-1}$ Mpc$^{-1}$ at $z=0.5$. By using BCGs, we significantly reduced the systematic errors to 10% by minimising the stellar mass and metallicity dependence of the method. The dominant uncertainty, and limitation for our study, is statistical, and we need larger, homogeneous samples of the oldest, most massive galaxies. By using the $Planck$+BAO prior of $\Omega_{m}$ and $\Omega_{\Lambda}$, the projected Hubble constant is $H_{0}$ = $54.6 \pm 25.7(\rm stat) \pm 5.5$(syst) km s$^{-1}$ Mpc$^{-1}$, consistent with both CMB and Cepheid measurements.