Age dating Luminous Red Galaxies observed with the Southern African Large Telescope

Abstract

There have been a number of attempts to measure the expansion rate of the Universe using agedating of Luminous Red Galaxies (LRGs). Assuming that stars in LRGs form at the same time, age-dating of two populations of LRGs at different redshifts can provide an estimate of the time difference associated with the corresponding redshift interval (dz/dt). This gives a direct estimate of the Hubble parameter H(z) at the average redshift of the two populations. We explore the validity of this method by using two different sets of data. Firstly, we measure a value of the Hubble parameter H(z) at a redshift of z 0.47. Our measurement is based on analysis of high quality spectra of LRGs obtained with the Southern African Large Telescope (SALT) in two narrow redshift ranges of z 0.40 and z 0.55 as part of an initial pilot study. Ages were estimated from fitting single stellar population models to the full observed spectra. The actual measurement presents the best estimate of H(z) via this method at z 0.5 to date. Secondly, we select a homogeneous sample of passively evolving galaxies over 0.20 < z < 0.40 from the Sloan Digital Sky Survey Data Release Seven (SDSS DR7) catalogue by applying a refined criteria, which is based on absolute magnitude. Three additional H(z) measurements were obtained at z 0.28, z 0.30 and z 0.32. We present the H(z) estimates and their cosmological constraints using the two different datasets.