Quasars as high-redshift cosmological probes

speaker: Giada Bargiacchi


Quasars (QSOs) have the potential to be the next rung of the cosmic distance ladder beyond type Iasupernovae (SNe) as they can be turned into standardizable candles through their non-linear X-ray to ultraviolet (UV) luminosity relation. Indeed, we have verified that this relation is intrinsic to QSO physics and not artifact of selectonbiases or redshift evolution by applying the statistical Efron & Petrosian method. Extending the cosmological analyses with high-redshift data is the key to distinguishing between different cosmological models that are instead degenerate at low redshifts, and allowing a better constraint on a possible dark energy (DE) evolution. We use the most updated QSO sample combined with SNe and, when possible, baryon acoustic oscillations (BAOs) data to test different kinds of cosmological models. We find that a joint analysis of QSO+SNe with BAO is only possible in the context of a flat Universe. We also find that the matter component, ΩM,0 , is fully consistent with ΩM,0 = 0.3 in all the data sets assuming a flat ΛCDM model. Yet, all the other analysed models show a statistically significant deviation at 2- 3 σ from this prediction by making use of the SNe+QSO+BAO sample. In the models where the DE density evolves with time, the SNe+QSO+BAO data always prefer ΩM,0 > 0.3, w0 < −1 and wa greater, but statistically consistent, than wa = 0. This DE phantom behaviour is mainly driven by the contribution of SNe+QSOs, while BAO are closer to the prediction of the flat ΛCDM model