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.

Modified gravity scenarios where a change of regime appears at acceleration scales a < a0 have been proposed. Since for 1M⊙ systems the acceleration drops below a0 at scales of around 7000 AU, a statistical survey of wide binaries with relative velocities and separations reaching 10 4 AU and beyond should prove useful to the above debate. We apply the proposed test to the best currently available data. Results show a constant upper limit to the relative velocities in wide binaries which is independent of separation for over three orders of magnitude, in analogy with galactic flat rotation curves in the same a < a0 acceleration regime. Our results are suggestive of a breakdown of Kepler's third law beyond a ≈ a0 scales, in accordance with generic predictions of modified gravity theories designed not to require any dark matter at galactic scales and beyond.

The X-ray afterglow plateau emission observed in many Gamma-ray Bursts (GRBs) has been interpreted as either being fueled by fallback onto a newly formed black hole, or by the spin-down luminosity of an ultra-magnetized millisecond neutron star. If the latter model is assumed, GRB Xray afterglow light curves can be analytically reproduced. We fit a sample of GRB X-ray plateaus, interestingly yielding a distribution in the magnetic field versus spin period (B-P) diagram consistent with B ∝ P 7/6 . This is expected from the well-established physics of the spin-up line minimum period for Galactic millisecond pulsars. From the normalisation of the relation we obtain perfectly matches spin-up line predictions for the expected masses (∼ 1M ) and radii (∼ 10 km) of newly born magnetars, and mass accretion rates consistent with GRB expectations of 10 −4 M /s <Ṁ < 10 −1 M /s. Short GRBs with extended emission (SEE) appear towards the high period end of the distribution, while the long GRBs (LGRBs) towards the short period end. This result is consistent with spin-up limit expectations where the total accreted mass determines the position of the neutron star in the B-P diagram. The P-B distribution for LGRBs and SEE are statistically different, further supporting the idea that the fundamental plane relation (Dainotti et al. , 2017, a tri-dimensional correlation between the X-ray Luminosity at the end of the plateau, the end time of the plateau and the 1 − s peak luminosity in the prompt emission, is a powerful discriminant among those populations. Our conclusions are robust against suppositions regarding the GRB collimation angle and magnetar breaking index, which shifts the resulting magnetar properties parallel to the spin-up line, and strongly support a magnetar origin for GRBs presenting X-ray plateaus.

We use data from the Hipparcos catalogue to construct colour–magnitude diagrams for the solar neighbourhood, which are then treated using advanced Bayesian analysis techniques to derive the star formation rate history, SFR(t), of this region over the last 3 Gyr. The method we use allows the recovery of the underlying SFR(t) without the need of assuming any a priori structure or condition on SFR(t), and hence yields a highly objective result. The remarkable accuracy of the data permits the reconstruction of the local SFR(t) with an unprecedented time resolution of ≈50 Myr. An SFR(t) that has an oscillatory component of period ≈0.5 Gyr is found, superimposed on a small level of constant star formation activity. Problems arising from the non‐uniform selection function of the Hipparcos satellite are discussed and treated. Detailed statistical tests are then performed on the results, which confirm the inferred SFR(t) to be compatible with the observed distribution of stars.

Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulae to derive an estimate of the halo spin parameter λ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of λ, which we apply to a large volume‐limited sample of galaxies taken from the Sloan Digital Sky Survey data base. The large numbers involved (11 597) allow the derivation of reliable λ distributions, as signal adds up significantly in spite of the errors in the inferences for particular galaxies. We find that if the observed distribution of λ is modelled with a lognormal function, as often done for this distribution in dark matter haloes that appear in cosmological simulations, we obtain parameters λ0= 0.04 ± 0.005 and σλ= 0.51 ± 0.05, interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of λ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.