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)     

Probing subparsec structure in the Lyman α forest with gravitational microlensing

We present the results of microlens ray-tracing simulations showing the effect of absorbing material between a source quasar and a lensing galaxy in a gravitational lens system. We find that, in addition to brightness fluctuations due to microlensing, the strength of the absorption line relative to the continuum varies with time, with the properties of the variations depending on the structure of the absorbing material. We conclude that such variations will be measurable via ultraviolet spectroscopy of image A of the gravitationally lensed quasar Q2237+0305 if the Lyman α clouds between the quasar and the lensing galaxy possess structure on scales smaller than ∼0.1 pc. The time-scale for the variations is on the order of years to decades, although very short-term variability can occur. While the Lyman α lines may not be accessible at all wavelengths, this approach is applicable to any absorption system, including metal lines.     

The redshift distribution of absorption-line systems in QSO spectra

A statistical analysis of the space–time distribution of absorption-line systems (ALSs) observed in QSO spectra within the cosmological redshift interval   z = 0.0–3.7  is carried out on the base of our catalogue of absorption systems ( Ryabinkov et al. 2003 ). We confirm our previous conclusion that the z -distribution of absorbing matter contains non-uniform component displaying a pattern of statistically significant alternating maxima (peaks) and minima (dips). Using the wavelet transformation, we determine the positions of the maxima and minima and estimate their statistical significance. The positions of the maxima and minima of the z -distributions obtained for different celestial hemispheres turn out to be weakly sensitive to orientations of the hemispheres. The data reveal a regularity (quasi-periodicity) of the sequence of the peaks and dips with respect to some rescaling functions of z . The same periodicity was found for the one-dimensional correlation function calculated for the sample of the ALSs under investigation. We assume the existence of a regular structure in the distribution of absorption matter, which is not only spatial but also temporal in nature with characteristic time varying within the interval 150–650 Myr for the cosmological model applied.     

Mixing metals in the early Universe

We investigate the evolution of the metallicity of the intergalactic medium (IGM) with particular emphasis on its spatial distribution. We propose that metal enrichment occurs as a two-step process. First, supernova (SN) explosions eject metals into relatively small regions confined to the surroundings of star-forming galaxies. From a comprehensive treatment of blowout we show that SN by themselves fail by more than one order of magnitude to distribute the products of stellar nucleosynthesis over volumes large enough to pollute the whole IGM to the metallicity levels observed. Thus, an additional (but as yet unknown) physical mechanism must be invoked to mix the metals on scales comparable to the mean distance between the galaxies that are most efficient pollutants. From this simple hypothesis we derive a number of testable predictions for the evolution of the IGM metallicity. Specifically, we find that: (i) the fraction of metals ejected over the star-formation history of the Universe is about 50 per cent at     that is, approximately half of the metals today are found in the IGM; (ii) if the ejected metals were homogeneously mixed with the baryons in the Universe, the average IGM metallicity would be     at     However, due to spatial inhomogeneities, the mean of the distribution of metallicities in the diffusive zones has a wide (more than 2 orders of magnitude) spread around this value; (iii) if metals become more uniformly distributed at     as assumed, at     the metallicity of the IGM is narrowly confined within the range     Finally, we point out that our results can account for the observed metal content of the intracluster medium.     

The association between gas and galaxies – I. CFHT spectroscopy and pair analysis

We investigate the relative distribution of the gaseous contents of the Universe (as traced by a sample of Lyα absorbers), and the luminous baryonic matter (as traced by a redshift survey of galaxies in the same volume searched for Lyα absorbers), along 16 lines of sight (LOS) between redshifts 0 and 1. Our galaxy redshift survey was made with the multi-object spectrograph on the Canada–France–Hawaii Telescope and, when combined with galaxies from the literature in the same LOS, gives us a galaxy sample of 636 objects. By combining this with an absorption-line sample of 406 absorbing systems drawn from published works, we are able to study the relationship between gas and galaxies over the latter half of the age of the Universe. A correlation between absorbers and galaxies is detected out to separation of 1.5 Mpc. This correlation is weaker than the galaxy–galaxy correlation. There is also some evidence that the absorbing systems seen in C  iv are more closely related to galaxies, although this correlation could be with column density rather than metallicity. The above results are all consistent with the absorbing gas and the galaxies coexisting in dark matter filaments and knots as predicted by current models where the column density of the absorbing gas is correlated with the underlying matter density.     

The proximity effect in a close group of QSOs

We present an analysis of the proximity effect in a sample of 10 2-Å-resolution QSO spectra of the Ly α forest at     . Rather than investigating variations in the number density of individual absorption lines, we employ a novel technique that is based on the statistics of the transmitted flux itself. We confirm the existence of the proximity effect at the > 99 per cent confidence level. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of     . This value assumes that QSO redshifts measured from high-ionization lines differ from the true systemic redshifts by     . We find evidence at a level of 2.6 σ that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. Allowing for known QSO variability, the significance of the correlation reduces to 2.1 σ .
The QSOs form a close group on the sky and the sample is thus well suited for an investigation of the foreground proximity effect, where the Ly α forest of a background QSO is influenced by the UV radiation from a nearby foreground QSO. From the complete sample we find no evidence for the existence of this effect, implying either that     or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example. Our sample includes the fortunate constellation of a foreground QSO surrounded by four nearby background QSOs. These four spectra all show underdense absorption within ±3000 km s⁻¹ of the redshift of the foreground QSO.     

Cosmic dynamics in the era of Extremely Large Telescopes

The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universe's expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterizing the cosmological redshift drift. We consider the Lyα forest in the redshift range  2 < z < 5  and other absorption lines in the spectra of high-redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise-limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Lyα forest as a function of signal-to-noise ratio and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ∼20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.