The baryon density at z = 0.9-1.9. Tracing the warm-hot intergalactic medium with broad Lyman α absorption

Abstract

Aims:We present an analysis of the Lyman α forests of five quasar spectra in the near UV. Properties of the intergalactic medium (IGM) at an intermediate redshift interval (0.9≤ z≤1.9) are studied. The amount of baryons in the diffuse photoionised IGM and the warm-hot intergalactic medium (WHIM) are traced to get constraints on the redshift evolution of the different phases of the intergalactic gas.
Methods: The baryon density of the diffuse IGM is determined with photoionisation calculations under the assumption of local hydrostatic equilibrium. We assume that the gas is ionised by a metagalactic background radiation with a Haardt & Madau (2001, Clusters of Galaxies and the High Redshift Universe Observed in X-rays) spectrum. The WHIM is traced with broad Lyman α (BLA) absorption. The properties of a number of BLA detections are studied. Under the assumption of collisional ionisation equilibrium a lower limit to the baryon density could be estimated.
Results: It is found that the diffuse photoionised IGM contains at least ~25% of the total baryonic matter at redshifts 1≲ z≲2. For the WHIM a lower limit of ~2.4% could be determined. Furthermore, the data indicates that the intergalactic gas is in a state of evolution at z∼1.5. We confirm that a considerable part of the WHIM is created between z=1 and z=2.