Paola Belloni

Exploring Cluster Elliptical Galaxies as Cosmological Standard Rods

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-? 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 ?-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 ? contribute about equally to the critical cosmic density (i.e., ?m ? ?? ? 0.5). With 90% confidence, ?? should be smaller than 0.9.