Quiescent luminous red galaxies as cosmic chronometers: on the significance of mass and environmental dependence

¹ College of Science, China Three Gorges University, 443002 Yichang, PR China
² Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Science, 100012 Beijing, PR China
e-mail: luyj@nao.cas.cn
³ National Astronomical Observatories, Chinese Academy of Science, 100012 Beijing, PR China
⁴ Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁵ Center of High Energy Physics, Peking University, 100871 Beijing, PR China
⁶ Department of Astronomy and Astrophysics, Peking University, 100871 Beijing, PR China

Received: 11 February 2015
Accepted: 23 September 2015

Abstract

Context. Massive luminous red galaxies (LRGs) are believed to be evolving passively and can be used as cosmic chronometers to estimate the Hubble constant (the differential age method). However, different LRGs may be located in different environments. The environmental effects, if any, on the mean ages of LRGs, and the ages of the oldest LRGs at different redshift, may limit the use of the LRGs as cosmic chronometers.

Aims. We aim to investigate the environmental and mass dependence of the formation of “quiescent” LRGs, selected from the Sloan Digital Sky Survey (SDSS) data release 8, and to pave the way for using LRGs as cosmic chronometers.

Methods. Using the population synthesis software STARLIGHT, we derive the stellar populations in each LRG through the full spectrum fitting and obtain the mean age distribution and the mean star formation history (SFH) of those LRGs.

Results. We find that there is no apparent dependence of the mean age and the SFH of quiescent LRGs on their environment, while the ages of those quiescent LRGs depend weakly on their mass. We compare the SFHs of the SDSS LRGs with those obtained from a semi-analytical galaxy formation model and find that they are roughly consistent with each other if we consider the errors in the STARLIGHT-derived ages. We find that a small fraction of later star formation in LRGs leads to a systematical overestimation (~28%) of the Hubble constant by the differential age method, and the systematical errors in the STARLIGHT-derived ages may lead to an underestimation (~ 16%) of the Hubble constant. However, these errors can be corrected by a detailed study of the mean SFH of those LRGs and by calibrating the STARLIGHT-derived ages with those obtained independently by other methods.

Conclusions. The environmental effects do not play a significant role in the age estimates of quiescent LRGs; and the quiescent LRGs as a population can be used securely as cosmic chronometers, and the Hubble constant can be measured with high precision by using the differential age method.