We have studied the structural and dynamical parameters of two samples of elliptical-like-objects (ELOs) formed in a set of self-consistent hydrodynamical simulations. ELO stellar masses, projected half- stellar mass radii, and stellar central l.o.s. velocity dispersions have been found to define a dynamical Fundamental Plane (FP), whose physical origin we report on.

From a sample of more than 100 remnants from major and minor hydrodynamic binary galaxy merger simulations (Cox 2004; Cox et al. 2005), we find that stellar remnants are mostly oblate while dark matter halos are mostly prolate or triaxial. Shapes are determined by iteratively diagonalizing a moment-of-inertia tensor. The preferred axes of the two shapes are almost always nearly perpendicular. This can be understood by considering the influence of angular momentum and dissipation during the merger. If binary major mergers of spiral galaxies are responsible for the formation of elliptical galaxies or some subpopulation of elliptical galaxies, then the galaxies can be be expected to be oblate and the dark matter halos prolate with the two preferred axes perpendicular to each other.

We present high resolution two dimensional velocity fields from integral field spectroscopy along with derived rotation curves for nine low surface brightness galaxies. This is a positive step forward in terms of both data quality and number of objects studied. We fit NFW and pseudo-isothermal halo models to the observations. We find that the pseudo-isothermal halo better represents the data in most cases than the NFW halo, as the resulting concentrations are lower than would be expected for ΛCDM.

The nonperturbative renormalization group flow of Quantum Einstein Gravity (QEG) is reviewed. It is argued that there could be strong renormalization effects at large distances, in particular a scale dependent Newton constant, which mimic the presence of dark matter at galactic and cosmological scales.

The standard model of cosmology considers the existence of two components of unknown nature, “dark matter” and “dark energy”, which determine the cosmological evolution. Their nature remains unknown, and other models can also be considered. In particular, it may be possible to reinterpret the recent cosmological observations so that the Universe does not contain two fluids of unknown natures, but only one fluid with particular properties. After a brief review of constraints on this unifying “dark fluid”, we will discuss a specific model of dark fluid based on a complex scalar fluid.

One can solve the Jeans equation analytically for equilibrated dark matter structures, once given two pieces of input from numerical simulations. These inputs are 1) a connection between phase-space density and radius, and 2) a connection between velocity anisotropy and density slope, the α – β relation. The first (phase-space density vs. radius) has been analysed through several different simulations, however the second (α – β relation) has not been quantified yet. We perform a large set of numerical experiments in order to quantify the slope and zero-point of the α – β relation. This allows us to conclude that equilibrated dark matter structures indeed have zero central velocity anisotropy, central density slope of α0 ≈ –0.8, and outer anisotropy of approximately β∞ ≈ 0.5.

Cosmological simulations of structure formation predict that galaxies are dramatically modified by galaxy harassment during the assembly of galaxy clusters, losing a substantial fraction of their stellar mass which today must be in the form of intracluster stars. Simulations predict non-uniform spatial and radial velocity distributions for these stars. Intracluster planetary nebulae are the only abundant component of the intracluster light whose kinematics can be measured at this time. Comparing these velocity distributions with simulations will provide a unique opportunity to investigate the hierarchical cluster formation process as it takes place in the nearby universe.

We present HI data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce non-circular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. We performed a harmonic decomposition of the velocity field in order to search for alleged non-circular motions needed to “hide” a cusp: in DDO 47 non-circular motions are globally at a level of 2–3 km s-1, far from being sufficient to reconcile the observed rotation curve with the ΛCDM predictions. We conclude that the dark matter halo around DDO 47 is truly cored and that a cusp cannot be hidden by non-circular motions. More details are shown in Gentile et al. (2005).

We use the mass-to-light gradients in early-type galaxies to infer the global dark matter fraction, fd = Md/M*, for these systems. We discuss implications about the total star formation efficiency in dark-matter halos and show that the the trend of fd with mass produces virial mass-to-light ratios which are consistent semi-analitical models. Preliminary kurtosis analysis of the quasi-constant M/L galaxies in Romanowsky et al. seems at odd with Dekel et al. simulations.

The mass distribution of galaxy clusters can be determined from the study of the projected phase-space distribution of cluster galaxies. The main advantage of this method as compared to others, is that it allows determination of cluster mass profiles out to very large radii. Here I review recent analyses and results on this topic. In particular, I briefly describe the Jeans and Caustic methods, and the problems one has to face in applying these methods to galaxy systems. Then, I summarize the most recent and important results on the mass distributions of galaxy groups, clusters, and superclusters. Additional covered topics are the relative distributions of the dark and baryonic components, and the orbits of galaxies in clusters.

Polar ring galaxies, where matter is in equilibrium in perpendicular orbits around spiral galaxies, are ideal objects to probe the 3D shapes of dark matter halos. The conditions to constrain the halos are that the perpendicular system does not strongly perturb the host galaxy, or that it is possible to derive back its initial shape, knowing the formation scenario of the polar ring. The formation mechanisms are reviewed: mergers, tidal accretion, or gas accretion from cosmic filaments. The Tully-Fisher diagram for polar rings reveals that the velocity in the polar plane is higher than in the host plane, which can only be explained if the dark matter is oblate and flattened along the polar plane. Only a few individual systems have been studied in details, and 3D shapes of their haloes determined by several methods. The high frequency of warps could be explained by spontaneous bending instability, if the disks are sufficiently self-gravitating, which can put constraints on the dark matter flattening.


Recent X-ray observations have been showing that the old vision of clusters as peacefully relaxed structures is in most cases absolutely wrong. Coupled with optical and radio observations, these X-ray data show evidence for violent mechanisms. Results based on XMM-Newton gas temperature maps for four clusters will be presented here.


We present the preliminary results from a combined X-ray/ lensing investigation of the galaxy cluster RBS 864 (z = 0.29). Morphological and spectral studies of X-ray data show a relaxed appearance of this cluster, thus allowing us to derive a good mass estimate. From optical data we found the presence of a very high number of arc candidates and performed a strong lensing mass analysis. We found a large discrepancy in the comparison of the X-ray and lensing masses.


The hierarchical nature of halo formation in the Cold Dark Matter (CDM) model suggests that there should be mass dependent trends in the radial velocity and velocity anisotropy profiles of dark matter haloes, with more massive systems exhibiting the effects of more pronounced infall. We present results from a sample of high resolution cosmological N-body simulations of individual dwarf, galaxy and cluster mass haloes, and demonstrate that such trends with mass are indeed observed.


The bulge contribution to the inner density profile of a galaxy is well traced by the bulge's NIR surface brightness profile. We map the surface brightness profiles of bulges using HST/NICMOS imaging. Most bulges show nuclear components of 20–200 pc scales, and frequently also unresolved nuclear star clusters (<20 pc). These nuclear components are the densest parts of bulges, and gave bulges the “reputation” of having high stellar concentration. Actually, the Sérsic r1/n bulges typically have low concentrations, 1 < n < 2.5. We cannot confirm the bi-modal distribution of bulge profile slopes reported by others, and suggest that the alleged bi-modality is related to selectively excluding/including nuclear components in low/high-n bulges.

We use the weak gravitational lensing effect to study the mass distribution of a sample of 50 southern Abell clusters (0.05 < z < 0.3) having LX > 5 × 1044 erg s-1 observed with ESO-VLT under uniform sky conditions and subarsecond (0.6'') image quality. Their dynamical equibrium is assesed through comparison of the clusters mass estimates made by weak-lensing, velocity-dispersions and X-ray techniques. So far, for 24 clusters (Cypriano et al. 2004), we find: a) the center of their mass and light distributions are coincident for 77% of the sample; b) the elongations of the fitted mass profiles and of the light of the cD galaxies generally match with each other; c) although most of the clusters are found to be in dynamical equilibrium, those with TX ≥ 8 keV (or σv ≥ 1120 km s-1) are the discordant ones. The preliminary bright arc statistics for our whole sample (LZLS) suggests the presence of a cut-off at z ~ 0.07 which is qualitatively consistent with predictions done in a ΛCDM cosmology (Meneghetti et al. 2003).


I briefly highlight the salient properties of modified-inertia formulations of MOND, contrasting them with those of modified-gravity formulations, which describe practically all theories propounded to date. Future data (e.g. the establishment of the Pioneer anomaly as a new physics phenomenon) may prefer one of these broad classes of theories over the other. I also outline some possible starting ideas for modified inertia.


We explore the possibility that galaxy clusters or halos could be suited environments to host fermions like massive neutrinos in their degenerate state. Equilibrium density profiles for galaxy clusters and halos are calculated where the contribution of fermion particles to an isotropic collisionless component is taken into account. Fermions are supposed to cluster in CDM protoclusters while their phase space density is sufficient for not neglecting the Pauli's exclusion principle.


Using a wavelet algorithm, we have mapped the temperature structure of the three relaxed clusters of galaxies Abell 478, Abell 1795 and Abell 2029. The findings of significant non-radial thermal structures outside the core region of two of these clusters question the validity limits of the elliptical symmetry hypothesis required for deriving cluster mass profiles from gas brightness and temperature profiles measurements.