The age-redshift relation for luminous red galaxies obtained from full spectrum fitting and its cosmological implications

Abstract

The relative age of galaxies at different redshifts can be used to infer the Hubble parameter and put constraints on cosmological models. We select luminous red galaxies (LRGs) from the Sloan Digital Sky Survey Data Release7 and then cross match it with the MPA/JHU catalog of galaxies to obtain a large sample of quiescent LRGs at redshift z ∼ 0.03-0.39. The total number of quiescent LRGs, 23,883, is divided into four sub-samples according to their velocity dispersions and each sub-sample is further divided into 12 redshift bins. The spectra of the LRGs in each redshift and velocity bin are co-added in order to obtain a combined spectrum with a relatively high signal-to-noise ratio. Adopting the GalexEV/SteLib model, we estimate the mean ages of the LRGs from these combined spectra by using the full-spectrum fitting method. We check the reliability of the estimated age by using Monte Carlo simulations and find that the estimates are robust and reliable. Assuming that the LRGs in each sub-sample and each redshift bin were on average formed at the same time, the Hubble parameter at the present time H 0 is estimated from the age-redshift relation obtained for each sub-sample, which is compatible with the H 0 value measured by other methods. We demonstrate that a systematic bias (up to ∼20%) may be introduced to the H 0 estimation because of recent star formation in the LRGs due to the later major mergers at z ≲ 0.4, but this bias may be negligible for those sub-samples with large velocity dispersions. Using the age-redshift relations obtained from the sub-sample with the largest velocity dispersion or the two sub-samples with high velocity dispersions, we find H 0 = 65+7-3 km s-1 Mpc-1 or H 0 = 74 +5-4 km s-1 Mpc-1 by assuming a spatially flat ΛCDM cosmology. With upcoming surveys, such as the Baryon Oscillation Spectroscopic Survey, even larger samples of quiescent massive LRGs may be obtained, and thus the Hubble parameter can be measured with high accuracy usimg the age-redshift relation. © 2012. The American Astronomical Society. All rights reserved.