Age-dating luminous red galaxies observed with the Southern African Large Telescope

Abstract

We measure a value for the cosmic expansion of $H(z) = 89 \\pm 23$(stat) $\\pm$\n44(syst) km s$^{-1}$ Mpc$^{-1}$ at a redshift of $z \\simeq 0.47$ based on the\ndifferential age technique. This technique, also known as cosmic chronometers,\nuses the age difference between two redshifts for a passively evolving\npopulation of galaxies to calculate the expansion rate of the Universe. Our\nmeasurement is based on analysis of high quality spectra of Luminous Red\nGalaxies (LRGs) obtained with the Southern African Large Telescope (SALT) in\ntwo narrow redshift ranges of $z \\simeq 0.40$ and $z \\simeq 0.55$ as part of an\ninitial pilot study. Ages were estimated by fitting single stellar population\nmodels to the observed spectra. This measurement presents one of the best\nestimates of $H(z)$ via this method at $z\\sim0.5$ to date.\n