Andreu Raig

On the Origin of the Inner Structure of Halos

We calculate by means of the Press-Schechter formalism the density profile developed by dark matter halos during accretion, i.e., the continuous aggregation of small clumps. We find that the shape of the predicted profile is similar to that shown by halos in high-resolution cosmological simulations. Furthermore, the mass-concentration relation is correctly reproduced at any redshift in all the hierarchical cosmologies analyzed except for very large halo masses. The role of major mergers, which can cause the rearrangement of the halo structure through violent relaxation, is also investigated. We show that, as a result of the boundary conditions imposed by the matter continuously infalling into the halo during the violent relaxation process, the shape of the density profile emerging from major mergers is essentially identical to the shape the halo would have developed through pure accretion. This result explains why, according to high-resolution cosmological simulations, relaxed halos of a given mass have the same density profile regardless of whether they have had a recent merger and why both spherical infall and hierarchical assembly lead to very similar density profiles. Finally, we demonstrate that the density profile of relaxed halos is not affected by the capture of clumps of intermediate mass either.

Scaling Evolution of Universal Dark Matter Halo Density Profiles

Dark matter halos show a universal density profile with a scaling such that less massive systems are typically denser. This mass-density relation is well described by a proportionality between the characteristic density of halos and the mean cosmic density at halo formation time. It has recently been shown that this proportionality could be the result of the following simple evolutionary picture. Halos form in major mergers with essentially the same, cosmogony-dependent, dimensionless profile and then grow inside out as a consequence of accretion. Here we verify the consistency of this picture and show that it predicts the correct zero point of the mass-density relation.

The Effects of the Peak-Peak Correlation on the Peak Model of Hierarchical Clustering

In two previous papers a semianalytical model was presented for the hierarchical clustering of halos via gravitational instability from peaks in a random Gaussian field of density fluctuations. This model is better founded than the extended Press-Schechter model, which is known to agree with numerical simulations, and it makes similar predictions. The specific merger rate, however, shows a significant departure at intermediate captured masses. The origin of this was suspected to be the rather crude approximation used for the density of nested peaks. Here, we seek to verify this suspicion by implementing a more accurate expression for the latter quantity, which accounts for the correlation among peaks. We confirm that the inclusion of the peak-peak correlation improves the specific merger rate, while the good behavior of the remaining quantities is preserved.

Testing the modified Press-Schechter model against N-body simulations

A modified version of the extended Press–Schechter model for the growth of dark-matter haloes was introduced in two previous papers, with the aim of explaining the mass–density relation shown by haloes in high-resolution cosmological simulations. In this model, major mergers are well separated from accretion, thereby allowing a natural definition of halo formation and destruction. This makes it possible to derive analytic expressions for halo formation and destruction rates, the mass accretion rate and the probability distribution functions of halo formation times and progenitor masses. The stochastic merger histories of haloes can be readily derived and easily incorporated into semi-analytical models of galaxy formation, thus avoiding the usual problems encountered in the construction of Monte Carlo merger trees from the original extended Press–Schechter formalism. Here we show that the predictions of the modified Press–Schechter model are in good agreement with the results of N-body simulations for several scale-free cosmologies.

The Structure of Dark-Matter Halos: Origin and Evolution

With the aid of the Modified Press-Schechter model, we calculate the halo inner structure in hierarchical cosmologies by considering that they grow through an alternate sequence of major mergers and long periods of gentle accretion. The density profile so predicted closely resembles that shown in N-body simulations at any epoch considered.

An Analytical Model of Galaxy Formation

We present an analytical model of galaxy formation and evolution. After giving a rapid overview of the strategy employed and the main ingredients included in the model, we focus on describing the main differences between this model and other similar ones available.

MODELLING THE GROWTH OF DARK-MATTER HALOS

A new model for the cosmological growth of dark matter halos is presented. We compare theoretical predictions with results of N-body simulations.