We use a semianalytical approach, and the standard σ 8 = 1 cold dark matter (SCDM) cosmological model to study the gravitational collapse and virialization, the structure, and the global and statistical properties of isolated dark matter (DM) galactic halos which emerge from primordial Gaussian fluctuations. Firstly, from the statistical properties of the primordial density fluctuation field the possible mass aggregation histories (MAHs) are generated. Secondly, these histories are used as the initial conditions of the gravitational collapse. To calculate the structure of the virialized systems we have generalized the secondary infall model to allow arbitrary MAHs and internal thermal motions. The average halo density profiles we obtained agree with the profile derived as a fitting formula to results of N-body cosmological simulations by Navarro et al. (1996Navarro et al. ( , 1997. The comparison of the density profiles with the observational data is discussed, and some possible solutions to the disagreement found in the inner regions are proposed.The results of our approach, after considering the gravitational dragging of the baryon matter that forms a central disk in centrifugal equilibrium, show that the empirical Tully-Fisher (TF) relation and its scatter can be explained through the initial cosmological conditions, at least for the isolated systems. The σ 8 = 1 SCDM model produces galaxies with high velocities when compared to observations, but when the SCDM power spectrum is normalized to σ 8 = 0.57 an excellent agreement with the

We investigate the in‐spiralling time‐scales of globular clusters (GCs) in dwarf spheroidal (dSph) and dwarf elliptical (dE) galaxies, due to dynamical friction (DF). We address the problem of these time‐scales having been variously estimated in the literature as much shorter than a Hubble time. Using self‐consistent two‐component (dark matter and stars) models, we explore mechanisms which may yield extended DF time‐scales in such systems in order to explain why dwarf galaxies often show GC systems. As a general rule, dark matter and stars both give a comparable contribution to the dynamical drag. By exploring various possibilities for their gravitational make‐up, it is shown that these studies help to constrain the parameters of the dark matter haloes in these galaxies, as well as to test alternatives to dark matter. Under the assumption of a dark halo having a central density core with a typical King core radius somewhat larger than the observed stellar core radius, DF time‐scales are naturally extended upwards of a Hubble time. Cuspy dark haloes yield time‐scales ≲4.5 Gyr, for any dark halo parameters in accordance with observations of stellar line‐of‐sight velocity dispersion in dSph galaxies. We confirm, after a detailed formulation of the DF problem under the alternative hypothesis of modified Newtonian dynamics (MOND) and in the lack of any dark matter, that due to the enhanced dynamical drag of the stars, the DF time‐scales in MOND would be extremely short. Taking the well‐measured structural parameters of the Fornax dSph and its GC system as a case study, we conclude that requiring DF time‐scales comparable to the Hubble time strongly favours dark haloes with a central core.

A B S T R A C TWithin the framework of the cold dark matter (CDM) cosmogony, a central cusp in the density profiles of virialized dark haloes is predicted. This prediction disagrees with the soft inner halo mass distribution inferred from observations of dwarf and low surface brightness galaxies, and some clusters of galaxies. By analysing data for some of these objects, we find that the halo central density is nearly independent of the mass from galactic to galaxy cluster scales, with an average value of around 0.02 M ( pc 23 . We show that soft cores can be produced in the CDM haloes by introducing a lower cut-off in the power spectra of fluctuations and assuming high orbital thermal energies during halo formation. However, the scale invariance of the halo central density is not reproduced in these cases. The introduction of self-interaction in the CDM particles offers the most attractive alternative to the core problem. We propose gravothermal expansion as a possible mechanism to produce soft cores in the CDM haloes with self-interacting particles. A global thermodynamical equilibrium can explain the central density scale invariance. We find a minimum cross-section capable of establishing isothermal cores in agreement with the observed shallow cores. If s is the crosssection, m x the mass of the dark matter particle and v the halo velocity dispersion, then sam x < 4 Â 10 225 100 km s 21 v 21 cm 2 GeV 21 .

A class of long gamma-ray bursts (GRBs) presenting light curves with an extended plateau phase in their X-ray afterglows obeys a correlation between the rest-frame end-time of the plateau, T a , and its corresponding X-ray luminosity, L a , (Dainotti et al). In this work we perform an analysis of a total sample of 176 Swift GRBs with known redshifts, exhibiting afterglow plateaus. By adding a third parameter that is the peak luminosity in the prompt emission, L peak , we discover the existence of a new three-parameter correlation. The scatter of data about this plane becomes smaller when a class-specific GRB sample is defined. This sample of 122 GRBs is selected from the total sample by excluding GRBs with associated supernovae (SNe), X-ray flashes and short GRBs with extended emission. With this sample the three-parameter correlation identifies a GRB "fundamental plane." Moreover, we further limit our analysis to GRBs with light curves with good data coverage and almost flat plateaus, 40 GRBs forming our "gold sample." The intrinsic scatter, s =  0.27 0.04 int , for the three-parameter correlation for this last sub-class is more than two times smaller than the value for the -L T a a one, making this the tightest three-parameter correlation that involves the afterglow plateau phase. Finally, we also show that a slightly less tight correlation is present between L peak and a proxy for the total energy emitted during the plateau phase, L T a a , confirming the existence of an energy scaling between the prompt and afterglow phases.