Exploring Cluster Elliptical Galaxies as Cosmological Standard Rods

Abstract

We explore the possibility of calibrating massive cluster elliptical galaxies as cosmological standard rods using the fundamental plane relation combined with a correction for luminosity evolution. Although cluster ellipticals certainly formed in a complex way, their passive evolution out to redshifts of about 1 indicates that basically all major merging and accretion events took place at higher redshifts. Therefore, a calibration of their luminosity evolution can be attempted. We propose to use the Mg-σ relation for that purpose because it is independent of distance and cosmology. We discuss a variety of possible caveats, ranging from dynamical evolution to uncertainties in stellar population models and evolution corrections to the presence of age spread. Sources of major random and systematic errors are analyzed as well.

We apply the described procedure to nine elliptical galaxies in two clusters at z = 0.375 and derive constraints on the cosmological model. For the best-fitting Λ-free cosmological model we obtain q₀ ~ 0.1, with 90% confidence limits being 0 < q₀ < 0.7 (the lower limit being due to the presence of matter in the universe). If the inflationary scenario applies (i.e., the universe has flat geometry), then, for the best-fitting model, matter and Λ contribute about equally to the critical cosmic density (i.e., Ω_m ~ Ω_Λ ~ 0.5). With 90% confidence, Ω_Λ should be smaller than 0.9.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Associated Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Based on observations carried out at the European Southern Observatory, La Silla, Chile.