Context. Among the methods employed to measure the mass of galaxy clusters, the techniques based on lensing and X-ray analyses are perhaps the most widely used; however, the comparison between these mass estimates is often difficult and, in several clusters, the results apparently inconsistent. Aims. We aim at investigating potential biases in lensing and X-ray methods to measure the cluster mass profiles. Methods. We performed realistic simulations of lensing and X-ray observations that were subsequently analyzed using observational techniques. The resulting mass estimates were compared with the input models. Three clusters obtained from state-of-the-art hydrodynamical simulations, each of which projected along three independent lines-of-sight, were used for this analysis. Results. We find that strong lensing models can be trusted over a limited region around the cluster core. Extrapolating the strong lensing mass models to outside the Einstein ring can lead to significant biases in the mass estimates, if the BCG is not modeled properly, for example. Weak-lensing mass measurements can be strongly affected by substructures, depending on the method implemented to convert the shear into a mass estimate. Using nonparametric methods which combine weak and strong lensing data, the projected masses within R 200 can be constrained with a precision of ∼10%. Deprojection of lensing masses increases the scatter around the true masses by more than a factor of two because of cluster triaxiality. X-ray mass measurements have much smaller scatter (about a factor of two less than the lensing masses), but they are generally biased toward low values between 5 and 10%. This bias is entirely ascribable to bulk motions in the gas of our simulated clusters. Using the lensing and the X-ray masses as proxies for the true and the hydrostatic equilibrium masses of the simulated clusters and by averaging over the cluster sample, we are able to measure the lack of hydrostatic equilibrium in the systems we have investigated. Conclusions. Although the comparison between lensing and X-ray masses may be difficult in individual systems due to triaxiality and substructures, using a large number of clusters with both lensing and X-ray observations may lead to important information about their gas physics and allow use of lensing masses to calibrate the X-ray scaling relations.

We study the evolution of ionization fronts around the first protogalaxies by using high‐resolution numerical cosmological (Λ+ cold dark matter, CDM, model) simulations and Monte Carlo radiative transfer methods. We present the numerical scheme in detail and show the results of test runs from which we conclude that the scheme is both fast and accurate. As an example of interesting cosmological application, we study the reionization produced by a stellar source of total mass M=2×108 M⊙ turning on at z≈12, located at a node of the cosmic web. The study includes a spectral energy distribution of a zero‐metallicity stellar population, and two initial mass functions (IMFs; Salpeter/Larson). The expansion of the ionization front (I‐front) is followed as it breaks out from the galaxy and is channelled by the filaments into the voids, assuming (in a 2D representation) a characteristic butterfly shape. The ionization evolution is very well tracked by our scheme, as realized by the correct treatment of the channelling and shadowing effects resulting from overdensities. We confirm previous claims that both the shape of the IMF and the ionizing power metallicity dependence are important to correctly determine the reionization of the Universe.

We investigate a new implementation of the smoothed particle hydrodynamics technique designed to improve the realism with which galaxy formation can be simulated. In situations where cooling leads to the coexistence of phases of very different density and temperature, our method substantially reduces artificial overcooling near phase boundaries, prevents the exclusion of hot gas from the vicinity of cold ‘clouds’ and allows relative motion of the two phases at each point. We demonstrate the numerical stability of our scheme in the presence of extremely steep density and temperature gradients, as well as in strong accretion shocks and cooling flows. In addition, we present new implementations of star formation and feedback which simulate the effect of energy injection into multiphase gas more successfully than previous schemes. Our feedback recipes deposit thermal energy separately in cold dense gas and hot diffuse gas, and can explicitly re‐inject cold gas into the hot phase. They make it possible to dampen star formation effectively, to reheat cold gas, and to drive outflows into the galaxy halo and beyond. We show feedback effects to be strongest in small‐mass objects where much of the gas can be expelled. After idealized tests, we carry out a first low‐resolution study of galaxy formation in a Λ‐cold dark matter universe. Feedback results in substantial and mass‐dependent reductions in the total baryonic mass gathered on to the final object as well as in significant modulation of the star formation history.

Motivated by the relative lack of neutral hydrogen around Lyman‐break galaxies deduced from recent observations, we investigate the properties of the Lyα forest around high‐redshift galaxies. The study is based on improved numerical SPH simulations implementing, in addition to standard processes, a new scheme for multiphase and outflow physics description. Although on large scales our simulations reproduce a number of statistical properties of the intergalactic medium (because of the small filling factor of shock‐heated gas), they underpredict the Lyα optical depth decrease inside 1 Mpc h−1 of the galaxies by a factor of ≈15. We interpret this result as arising from the combined effect of infall occurring along the filaments, which prevents efficient halo gas clearing by the outflow, and the insufficient increase of (collisional) hydrogen ionization produced by the temperature increase inside the hot, outflow‐carved bubble. Unless either feedback is not properly modelled in cosmological simulations or an observational selection bias is present, we speculate that local photoionization could be a viable explanation to solve the puzzle.

We study the gravitational lensing magnification produced by the intervening cosmological matter distribution, as deduced from three different hierarchical models (SCDM, LCDM, CHDM) on very high redshift sources, particularly supernovae in protogalactic (Pop III) objects. By means of ray-shooting numerical simulations we find that caustics are more intense and concentrated in SCDM models. The magnification probability function presents a moderate degree of evolution up to z ≈ 5 (CHDM) and z ≈ 7 (SCDM/LCDM). All models predict that statistically large magnifications, µ > ∼ 20 are achievable, with a probability of the order of a fraction of percent, the SCDM model being the most efficient magnifier. All cosmologies predict that above z ≈ 4 there is a 10% chance to get magnifications larger than 3. We have explored the observational perspectives for Pop III SNe detection with NGST when gravitational magnification is taken into account. We find that NGST should be able to detect and confirm spectroscopically Type II SNe up to a redshift of z ≈ 4 in the J band (for T SN = 25000 K); this limit could be increased up to z ≈ 9 in the K band, allowing for a relatively moderate magnification. Possibly promising strategies to discriminate among cosmological models using their GL magnification predictions and very high-z SNe are sketched. Finally, we outline and discuss the limitations of our study.