2005MNRAS.362.1295F

Dark energy is the invisible fuel that seems to drive the current acceleration of the Universe. Its presence, which is inferred from an impressive convergence of high-quality observational results along with some apparently successful theoretical predictions, is also supported by the current estimates of the age of the Universe from dating of local and high-z objects. In this paper, we test the viability of several dark energy scenarios in the light of the age estimates of the high-redshift (z= 3.91) quasar APM 08279+5255. Using a chemodynamical model for the evolution of spheroids, we first reevaluate its current estimated age, as given by Hasinger, Schartel & Komossa. An age of 2.1 Gyr is set by the condition that Fe/O abundance ratio (normalized to solar values) of the model reaches 3.3, which is the best-fitting value obtained in the above reference. In the detailed chemodynamical modelling, the iron enrichment defines three relevant time-scales: (i) ∼0.3 Gyr for the central region of the galaxy housing the quasar to reach a solar iron abundance; (ii) ∼1 Gyr for the Fe/O abundance ratio to reach the solar value and (iii) ∼2 Gyr for a highly suprasolar Fe/O abundance ratio (Fe/O = 2.5, suggested by the quasar APM 08279+5255). Therefore, a high value of the Fe/O abundance ratio for a quasar is a strong evidence that the quasar is old, which represents a severe constraint for cosmological scenarios. It is shown that for the currently accepted value of the matter density parameter, most of the existing dark energy scenarios cannot accommodate this old high-redshift object unless the Hubble parameter is as low as H₀= 58km/s/Mpc, as recently advocated by Sandage and collaborators. Even considering less-stringent age limits, only cosmological models that predict a considerably old Universe at high-z can be compatible with the existence of this object. This is the case of the conventional ΛCDM scenario and some specific classes of brane world cosmologies.