The Age-Redshift Relation For Luminous Red Galaxies Obtained From the Full Spectrum Fitting and Its Cosmological Implications

TL;DR

This study uses full-spectrum fitting of luminous red galaxies from SDSS to estimate their ages and derive the Hubble parameter, providing a method to constrain cosmological models with future larger samples.

Contribution

It introduces a robust age estimation method from combined spectra of LRGs and applies it to derive the Hubble parameter, considering systematic biases and future survey prospects.

Findings

Estimated H_0 values consistent with other methods.

Systematic bias up to 20% due to recent star formation.

Larger samples from future surveys can improve H_0 measurement accuracy.

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 23,883 quiescent luminous red galaxies (LRGs) from the SDSS DR7 and divide them 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 relatively high $S/N$. Adopting the GalexEV/SteLib model, we estimate the mean ages of the LRGs from these combined spectra by 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 $\sim 20%$) may be introduced to the $H_0$ estimation because of recent star formation in the LRGs due to the later major mergers at $z\la 0.4$, 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 $H_0= 65^{+7}_{-3}\kmsmpc$ or $H_0= 74^{+5}_{-4}\kmsmpc$ by assuming a spatially flat $\Lambda$CDM cosmology. With upcoming surveys, such as the Baryon Oscillation Spectroscopic Survey (BOSS), even larger samples of quiescent massive LRGs may be obtained, and thus the Hubble parameter can be measured with high accuracy through the age--redshift relation.