Quiescent luminous red galaxies (LRGs) as cosmic chronometers: on the
  significance of mass and environmental dependence
  Massive luminous red galaxies (LRGs) are believed to be evolving passively
and can be used as cosmic chronometers to estimate the Hubble constant.
However, different LRGs may be located in different environments. The
environmental effects may limit the use of the LRGs as cosmic chronometers. We
aim to investigate the environmental and mass dependence of the formation of
'quiescent' LRGs selected from the Sloan Digital Sky Survey Date Release 8 and
to pave the way for using LRGs as cosmic chronometers. 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. 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 to those obtained independently by other
methods. The environmental effects do not play significant role in the age
estimates of quiescent LRGs, and the quiescent LRGs as a population can be used
securely as cosmic chronometers.
