Probing dark energy at galactic and cluster scales

Abstract

We investigate dark matter halo properties as a function of a time-varying dark energyequation of state. The dynamics of the collapse of the halo is governed by theform of the quintessence potential, the time evolution of its equation of state, theinitial conditions of the field and its homogeneous nature in the highly non-linearregime. These have a direct impact on the turnaround, virialization and collapsetimes, altering in consequence the non-linear density contrast and virial radius. Wecompute halo concentrations using the Eke, Navarro and Steinmetz algorithm,examining two extreme scenarios: first, we assume that the quintessence field doesnot exhibit fluctuations on cluster scales and below—homogeneous fluid; second,we assume that the field inside the overdensity collapses along with the darkmatter—inhomogeneous fluid. The Eke, Navarro and Steinmetz prescription reveals, ingeneral, higher halo concentrations in inhomogeneous dark energy models than intheir homogeneous equivalents. Halo concentrations appear to be controlled byboth changes in formation epochs of the halo cores and differing virializationoverdensities. We derive physical halo properties in all models and discuss theirobservational implications. We examine two possible methods for comparing observationswith theoretical predictions. The first method works on galaxy cluster scales andconsists of fitting the observed x-ray cluster gas density distributions to thosepredicted for an Navarro–Frenk–White profile. The second method works on galaxyscales and involves the observational measurement of the so-called central densityparameter.