The thermal history of the intergalactic medium

At redshifts z ≳2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Ly α forest. This photoheated gas follows a tight temperature–density relation which introduces a cut-off in the distribution of widths of the Ly α absorption lines ( b -parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high signal-to-noise ratio quasar spectra and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At a redshift z ∼3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of He  ii .     

The H i opacity of the intergalactic medium at redshifts 1.6 < z < 3.2

We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Lyα forest absorption (DA) over the redshift range  2.2 < z < 3.2  . Our measurement of DA excludes absorption from metal lines or the Lyα lines of Lyman-limit systems and damped Lyα systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between  1.6 < z < 2.2  , we find that the redshift evolution of DA is well described over  1.6 < z < 3.2  as   A (1 + z )^γ  , where   A = 0.0062  and  γ= 2.75  . We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less H  i absorption than expected at   z = 3  , implying that the ultraviolet background is  ∼40  per cent higher than expected. Our data appears to be consistent with an H  i ionization rate of  Γ∼ 1.4 × 10⁻¹² s⁻¹  .     

Tracing the warm–hot intergalactic medium in the local Universe

We present a simple method for tracing the spatial distribution and predicting the physical properties of the Warm–Hot Intergalactic Medium (WHIM), from the map of galaxy light in the Local Universe. Under the assumption that biasing is local and monotonic we map the  ∼2 h ⁻¹ Mpc  smoothed density field of galaxy light into the mass-density field, from which we infer the spatial distribution of the WHIM in the Local Supercluster. Taking into account the scatter in the WHIM density–temperature and density–metallicity relation, extracted from the z = 0 outputs of high-resolution and large-box-size hydrodynamical cosmological simulations, we are able to quantify the probability of detecting WHIM signatures in the form of absorption features in the X-ray spectra, along arbitrary directions in the sky. To illustrate the usefulness of this semi-analytical method we focus on the WHIM properties in the Virgo cluster region.     

Metal enrichment of the intergalactic medium

I demonstrate by means of high-resolution cosmological simulations, which include modelling of a two-phase interstellar medium, that the dominant mechanism for transporting heavy elements from protogalaxies into the intergalactic medium (IGM) is the merger mechanism as discovered by Gnedin & Ostriker. Direct ejection of the interstellar gas by supernovae plays only a minor role in transporting metals into the IGM: for a realistic cosmological scenario only a small fraction of all metals in the IGM is delivered by the supernova-driven winds, while most of the metals in the IGM are transported by the merger mechanism. As a result, the metallicity distribution in the IGM is highly inhomogeneous, in agreement with studies of the QSO metal absorption systems, and the predicted metallicity distribution of Lyman alpha absorbers as a function of their column density is in excellent agreement with the observational data.     

Primordial non-Gaussianities in the intergalactic medium

We present results from the first high-resolution hydrodynamical simulations of non-Gaussian cosmological models. We focus on the statistical properties of the transmitted Lyman-α flux in the high-redshift intergalactic medium. Imprints of non-Gaussianity are present and are larger at high redshifts. Differences larger than 20 per cent at   z > 3  in the flux probability distribution function for high-transmissivity regions (voids) are expected for values of the non-linearity parameter   f _NL=±100  when compared to a standard Λ cold dark matter cosmology with   f _NL= 0  . We also investigate the one-dimensional flux bispectrum: at the largest scales (corresponding to tens of Mpc), we expect deviations in the flux bispectrum up to 20 per cent at   z ∼ 4  (for   f _NL=±100  ), significantly larger than deviations of ∼3 per cent in the flux power spectrum. We briefly discuss possible systematic errors that can contaminate the signal. Although challenging, a detection of non-Gaussianities in the interesting regime of scales and redshifts probed by the Lyman-α forest could be possible with future data sets.     

Searching for the missing baryons in theWarm‐Hot Intergalactic Medium

At low redshift (z < 2), almost half of the baryons in the Universe are not found in bound structures like galaxies and clusters and therefore most likely reside in a Warm‐Hot Intergalactic Medium (WHIM), as predicted by simulations. Attempts to detect WHIM filaments at cosmological distances in absorption towards bright background sources have yielded controversial results that I review here. I argue that a secure detection of absorption features by the WHIM is at the limit of the XMM‐Newton capabilities, but feasible. A proper characterisation of the whole WHIM belongs to the realm of future X‐ray missions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)