Numerous numerical studies indicate that dark matter halos show an almost universal radial density profile. The origin of the profile is still under debate. We investigate this topic and pay particular attention to the velocity dispersion profile. To this end, we have performed high-resolution simulations with two independent codes, ART and GADGET. The radial velocity dispersion can be approximated as a function of the potential by 2 r ¼ aðÈ=È out Þ ðÈ out À ÈÞ, where È out is the outer potential of the halo. For the parameters a and , we find that a ¼ 0:29 AE 0:04 and ¼ 0:41 AE 0:03. We find that the power-law asymptote, 2 / È , is valid out to much larger distances from the halo center than any power-law asymptote for the density profile / rÀn . The asymptotic slope nðr ! 0Þ of the density profile is related to the exponent via n ¼ 2=ð1 þ Þ. Thus, the value obtained for from the available simulation data can be used to obtain an estimate of the density profile below currently resolved scales. We predict a continuously decreasing n toward the halo center with the asymptotic value n P 0:58 at r ¼ 0.

If the gas in filaments and halos shares the same velocity field as the luminous matter, it will generate measurable temperature anisotropies due to the kinematic Sunyaev-Zel'dovich effect. We compute the distribution function of the KSZ signal produced by a typical filament and show it is highly non-Gaussian. The combined contribution of the thermal and kinematic SZ effects of a filament of size Mpc and electron density m Ϫ3 could 3

We compute the power spectrum of the Cosmic Microwave Background temperature anisotropies generated by the Intergalactic Medium. To estimate the electron pressure along the line of sight and its contribution to the Sunyaev-Zeldovich component of the CMB anisotropies, we assume the non-linear baryonic density contrast is well described by a log-normal distribution. For model parameters in agreement with observations and for an experiment operating in the Rayleigh-Jeans regime, the largest IGM contribution corresponds to scales l ≈ 2000. The amplitude is rather uncertain and could be as large as 100 − 200µK 2 , comparable to the contribution of galaxy clusters. The actual value is strongly dependent on the gas polytropic index γ, the amplitude of the matter power spectrum σ 8 , namely C IGM l ∼ (γ 2 σ 8 ) 12 . At all redshifts, the largest contribution comes from scales very close to the baryon Jeans length. This scale is not resolved in numerical simulations that follow the evolution of gas on cosmological scales. The anisotropy generated by the Intergalactic Medium could make compatible the excess of power measured by Cosmic Background Imager (CBI) on scales of l ≥ 2000 with σ 8 = 0.9. Taking the CBI result as an upper limit, the polytropic index can be constraint to γ < 1.5 at 2σ level at redshifts z ∼ 0.1 − 0.4. With its large frequency coverage, the PLANCK satellite will be able to measure the secondary anisotropies coming from hot gas. Cluster and Intergalactic Medium contributions could be separated by cross correlating galaxy/cluster catalogs with CMB maps. This measurement will determine the state of the gas at low and intermediate redshifts.

We present HST STIS observations of four quasar pairs with redshifts 0.84 < z em < 1.56 and angular separation 2-3 arcmin corresponding to ∼1-1.5 h −1 50 Mpc transverse proper distance at z ∼ 0.9. We study the distribution of velocity differences between nearest neighbor H Lyman-α absorption lines detected in the spectra of adjacent QSOs in order to search for the possible correlation caused by the extent or the clustering properties of the structures traced by the absorption lines over such a scale. The significance of the correlation signal is determined by comparison with Monte-Carlo simulations of spectra with randomly distributed absorption lines. We find an excess of lines with a velocity separation smaller than ∆V = 500 km s −1 significant at the 99.97% level. This clearly shows that the Lyman-α forest is correlated on scales larger than 1 h −1 50 Mpc at z ∼ 1. However, out of the 20 detected coincidences within this velocity bin, 12 have ∆V > 200 km s −1. This probably reflects the fact that the scale probed by our observations is not related to the real size of individual absorbers but rather to large scale correlation. Statistics are too small to conclude about any difference between pairs separated by either 2 or 3 arcmin. A damped Lyman-α system is detected at z abs = 1.2412 toward LBQS 0019−0145A with log N(H) ∼ 20.5. From the absence of Zn absorption, we derive a metallicity relative to solar [Zn/H] < −1.75.

We have simulated the formation of large-scale structure arising from COBEnormalized spectra computed by convolving a primordial double-inflation perturbation spectrum with the CDM transfer function. Due to the broken scale invariance ('BSI') characterizing the primordial perturbation spectrum, this model has less small-scale power than the (COBE-normalized) standard CDM model. The particle-mesh code (with 512 3 cells and 256 3 particles) includes a model for thermodynamic evolution of baryons in addition to the usual gravitational dynamics of dark matter. It provides an estimate of the local gas temperature. In particular, our galaxy-finding procedure seeks peaks in the distribution of gas that has cooled. It exploits the fact that "cold" particles trace visible matter better than average and thus provides a natural biasing mechanism. The basic picture of large-scale structure formation in the BSI model is the familiar hierarchical clustering scenario. We obtain particle in cell statistics, the galaxy correlation function, the cluster abundance and the cluster-cluster correlation function and statistics for large and small scale velocity fields. We also report here on a semi-quantitative study of the distribution of gas in different temperature ranges. Based on confrontation with observations and comparison with standard CDM, we conclude that the BSI scenario could represent a promising modification of the CDM picture capable of describing many details of large-scale structure formation.