The age-redshift relation for Luminous Red Galaxies in the Sloan Digital Sky Survey

TL;DR

This study analyzes Luminous Red Galaxies from SDSS across redshifts 0 to 0.4, revealing passive evolution with little change in metallicity or alpha-element ratios, and a clear age-redshift relation consistent with cosmological models.

Contribution

It provides a detailed calibration of SDSS spectra to the Lick system and derives galaxy ages, metallicities, and abundance ratios, demonstrating passive evolution over the studied redshift range.

Findings

Little evidence for evolution in metallicity or alpha-elements.

Age decreases with redshift, consistent with cosmological lookback times.

More massive LRGs are younger than less massive ones.

Abstract

We present a detailed analysis of 17,852 quiescent, Luminous Red Galaxies (LRGs) selected from Sloan Digital Sky Survey (SDSS) Data Release Seven (DR7) spanning a redshift range of 0.0 < z < 0.4. These galaxies are co-added into four equal bins of velocity dispersion and luminosity to produce high signal-to-noise spectra (>100A^{-1}), thus facilitating accurate measurements of the standard Lick absorption-line indices. In particular, we have carefully corrected and calibrated these indices onto the commonly used Lick/IDS system, thus allowing us to compare these data with other measurements in the literature, and derive realistic ages, metallicities ([Z/H]) and alpha-element abundance ratios ([alpha/Fe]) for these galaxies using Simple Stellar Population (SSP) models. We use these data to study the relationship of these galaxy parameters with redshift, and find little evidence for evolution in metallicity or alpha-elements (especially for our intermediate mass samples). This demonstrates that our subsamples are consistent with pure passive evolving (i.e. no chemical evolution) and represent a homogeneous population over this redshift range. We also present the age-redshift relation for these LRGs and clearly see a decrease in their age with redshift (5 Gyrs over the redshift range studied here) which is fully consistent with the cosmological lookback times in a concordance Lambda CDM universe. We also see that our most massive sample of LRGs is the youngest compared to the lower mass galaxies. We provide these data now to help future cosmological and galaxy evolution studies of LRGs, and provide in the appendices of this paper the required methodology and information to calibrate SDSS spectra onto the Lick/IDS system.