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.     

The growth of structure in the intergalactic medium

A 'stochastic adhesion' model is introduced, with the purpose of describing the formation and evolution of mildly non-linear structures, such as sheets and filaments, in the intergalactic medium (IGM), after hydrogen reionization. The model is based on replacing the overall force acting on the baryon fluid – which results from the combination of local gravity, pressure gradients and Hubble drag – by a mock external force, self-consistently calculated from first-order perturbation theory. A small kinematic viscosity term prevents shell-crossing on small scales (which arises because of the approximate treatment of pressure gradients). The emerging scheme is an extension of the well-known adhesion approximation for the dark matter dynamics, from which it differs only by the presence of a small-scale 'random' force, characterizing the IGM. Our algorithm is the ideal tool to obtain the skeleton of the IGM distribution, which is responsible for the structure observed in the low column density Ly α forest in the absorption spectra of distant quasars.     

The relation between Lyman α absorbers and gas-rich galaxies in the local Universe

We use high-resolution hydrodynamical simulations to investigate the spatial correlation between weak  ( N _{H  i} < 10¹⁵ cm⁻²)  Lyα absorbers and gas-rich galaxies in the local Universe. We confirm that Lyα absorbers are preferentially expected near gas-rich galaxies and that the degree of correlation increases with the column density of the absorber. The real-space galaxy auto-correlation is stronger than the cross-correlation (correlation lengths   r _0,gg= 3.1 ± 0.1 Mpc  h ⁻¹  and   r _0,ag= 1.4 ± 0.1 Mpc  h ⁻¹  , respectively), in contrast with the recent results of Ryan-Weber, and the auto-correlation of absorbers is very weak. These results are robust to the presence of strong galactic winds in the hydrodynamical simulations. In redshift space, a further mismatch arises since at small separations the distortion pattern of the simulated galaxy–absorber cross-correlation function is different from the one measured by Ryan-Weber. However, when sampling the intergalactic medium along a limited number of lines-of-sight, as in the real data, uncertainties in the cross-correlation estimates are large enough to account for these discrepancies. Our analysis suggests that the statistical significance of difference between the cross-correlation and auto-correlation signal in current data sets is ∼1σ only.     

The phase diagram of the intergalactic medium and the entropy floor of groups and clusters: are clusters born warm?

We point out that two problems of observational cosmology, namely the facts (i) that ≳60 per cent of the baryonic content of the Universe is not observed at   z ∼ 0  and (ii) that the properties of small clusters do not agree with simple expectations, could be closely related. As shown by recent studies, the shock heating associated with the formation of large-scale structures heats the intergalactic medium (IGM) and leads to a 'warm IGM' component for the gas. In the same spirit, we suggest the intracluster medium (ICM) to be a mixture of galaxy-recycled, metal-enriched gas and intergalactic gas, shock heated by the collapsing much larger scales. This could be obtained through two processes: (1) the late infalling gas from the external warm IGM is efficiently mixed within the halo and brings some additional entropy, or (2) the shocks generated by larger non-linear scales are also present within clusters and can heat the ICM. We show that, if assumption (1) holds, the entropy brought by the warm IGM is sufficient to explain the observed properties of clusters, in particular the entropy floor and the   L _X– T   relation. On the other hand, we note briefly that scenario (2) would require a stronger shock heating because of the larger density of the ICM as compared with filaments. Although the efficiency of these two processes remains to be checked on a quantitative level, they have the advantage of dispensing with the need to invoke any strong preheating from supernovae or quasars (which has otherwise been introduced for the sole purpose of reproducing the behaviour of clusters). Matter ejection by galaxies is included in the present calculations and, consistently with the metal-enrichment requirements, is indeed shown to yield only a quite moderate entropy increase. Our scenario of clusters being 'born warm' can be checked through the predicted redshift evolution of the entropy floor.     

Semi-analytic approach to understanding the distribution of neutral hydrogen in the Universe

Analytic derivations of the correlation function and the column density distribution for neutral hydrogen in the intergalactic medium (IGM) are presented, assuming that the non-linear baryonic mass density distribution in the IGM is lognormal. This ansatz was used earlier by Bi & Davidsen to perform one-dimensional simulations of lines of sight and analyse the properties of absorption systems. We have taken a completely analytic approach, which allows us to explore a wide region of the parameter space for our model. The analytic results have been compared with observations to constrain various cosmological and IGM parameters, whenever possible. Two kinds of correlation functions are defined: (i) along the line of sight (LOS); and (ii) across the transverse direction. We find that the effects on the LOS correlation owing to changes in cosmology and the slope of the equation of state of the IGM, γ , are of the same order, which means that we cannot constrain both the parameters simultaneously. However, it is possible to constrain γ and its evolution using the observed LOS correlation function at different epochs provided that one knows the background cosmology. We suggest that the constraints on the evolution of γ obtained using the LOS correlation can be used as an independent tool to probe the reionization history of the Universe. From the transverse correlation function, we obtain the excess probability, over random, of finding two neutral hydrogen overdense regions separated by an angle θ . We find that this excess probability is always less than 1 per cent for redshifts greater than 2. Our models also reproduce the observed column density distribution for neutral hydrogen, and the shape of the distribution depends on γ . Our calculations suggest that one can rule out γ >1.6 for z ≃2.31 using the column density distribution. However, one cannot rule out higher values of γ at higher redshifts.     

Glimpsing through the high-redshift neutral hydrogen fog

We analyse the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤ z _em≤ 6.42 to obtain tighter constraints on the volume-averaged neutral hydrogen fraction, x _{H  i} , at z ≈ 6. We study separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with a semi-analytical model of the Lyα forest, we conclude that x _{H  i} evolves smoothly from 10^−4.4 at   z = 5.3  to 10^−4.2 at   z = 5.6  , with a robust upper limit x _{H  i} < 0.36 at   z = 6.3  . The frequency and physical sizes of the peaks imply an origin in cosmic underdense regions and/or in H  ii regions around faint quasars or galaxies. In one case (the intervening H  ii region of the faint quasar RD J1148+5253 at   z = 5.70  along the line of sight of SDSS J1148+5251 at   z = 6.42  ) the increase of the peak spectral density is explained by the first-ever detected transverse proximity effect in the H  i Lyα forest; this indicates that at least some peaks result from a locally enhanced radiation field. We then obtain a strong lower limit on the foreground QSO lifetime of t _Q > 11 Myr. The observed widths of the peaks are found to be systematically larger than the simulated ones. Reasons for such discrepancy might reside either in the photoionization equilibrium assumption or in radiative transfer effects.     

Colour corrections for high-redshift objects due to intergalactic attenuation

Corrections to the magnitudes of high-redshift objects due to intergalactic attenuation are computed using current estimates of the properties of the intergalactic medium (IGM). The results of numerical simulations are used to estimate the contributions to resonant scattering from the higher-order Lyman transitions. Differences of 0.5–1 mag from the previous estimate of Madau are found. Intergalactic k _IGM-corrections and colours are provided for high-redshift starburst galaxies and Type I and Type II quasi-stellar objects for several filter systems used in current and planned deep optical and infrared surveys.     

Fluorescent Lyman α emission from gas near a QSO at redshift 4.28

We use integral field spectroscopy with the Gemini North Telescope to detect probable fluorescent Lyman α (Lyα) emission from gas lying close to the luminous QSO PSS 2155+1358 at redshift 4.28. The emission is most likely coming not from primordial gas, but from a multiphase, chemically enriched cloud of gas lying about 50 kpc from the QSO. It appears to be associated with a highly ionized associated absorber seen in the QSO spectrum. With the exception of this gas cloud, the environment of the QSO is remarkably free of neutral hydrogen. We also marginally detect Lyα emission from a foreground subdamped Lyα absorption-line system.     

The entropy and energy of intergalactic gas in galaxy clusters

Studies of the X-ray surface brightness profiles of clusters, coupled with theoretical considerations, suggest that the breaking of self-similarity in the hot gas results from an 'entropy floor', established by some heating process, which affects the structure of the intracluster gas strongly in lower-mass systems. By fitting analytical models for the radial variation in gas density and temperature to X-ray spectral images from the ROSAT PSPC and ASCA GIS, we have derived gas entropy profiles for 20 galaxy clusters and groups. We show that, when these profiles are scaled such that they should lie on top of one another in the case of self-similarity, the lowest-mass systems have higher-scaled entropy profiles than more massive systems. This appears to be due to a baseline entropy of depending on the extent to which shocks have been suppressed in low-mass systems. The extra entropy may be present in all systems, but is detectable only in poor clusters, where it is significant compared with the entropy generated by gravitational collapse. This excess entropy appears to be distributed uniformly with radius outside the central cooling regions.
We determine the energy associated with this entropy floor, by studying the net reduction in binding energy of the gas in low-mass systems, and find that it corresponds to a pre-heating temperature of 0.3 keV. Since the relationship between entropy and energy injection depends upon gas density, we are able to combine the excesses of 70140 keV cm² and 0.3 keV to derive the typical electron density of the gas into which the energy was injected. The resulting value of implies that the heating must have happened prior to cluster collapse but after a redshift z 710. The energy requirement is well matched to the energy from supernova explosions responsible for the metals which now pollute the intracluster gas.