Distribution Of Dark Matter Halos Research Articles

Positrons from particle dark-matter annihilation in the Galactic halo: Propagation Green’s functions

We calculate the propagation of positrons from dark-matter particle annihilation in the Galactic halo in different models of the dark matter halo distribution using our three-dimensional code, and present fits to our numerical propagation Green's functions. We show that the Green's functions are not very sensitive to the dark matter distribution for the same local dark matter energy density. We compare our predictions with the computed cosmic ray positron spectra (``background'') for the ``conventional'' cosmic ray nucleon spectrum which matches the local measurements, and a modified spectrum which respects the limits imposed by measurements of diffuse Galactic $\ensuremath{\gamma}$ rays, antiprotons, and positrons. We conclude that significant detection of a dark matter signal requires favorable conditions and precise measurements unless the dark matter is clumpy which would produce a stronger signal. Although our conclusion qualitatively agrees with those of previous authors, it is based on a more realistic model of particle propagation and thus reduces the scope for future speculations. A reliable background evaluation requires new accurate positron measurements and further developments in modeling production and propagation of cosmic ray species in the Galaxy.

Ellipsoidal collapse and an improved model for the number and spatial distribution of dark matter haloes

The Press–Schechter, excursion set approach allows one to make predictions about the shape and evolution of the mass function of bound objects. The approach combines the assumption that objects collapse spherically with the assumption that the initial density fluctuations were Gaussian and small. The predicted mass function is reasonably accurate, although it has fewer high-mass and more low-mass objects than are seen in simulations of hierarchical clustering. We show that the discrepancy between theory and simulation can be reduced substantially if bound structures are assumed to form from an ellipsoidal, rather than a spherical, collapse. In the original, standard, spherical model, a region collapses if the initial density within it exceeds a threshold value, δsc. This value is independent of the initial size of the region, and since the mass of the collapsed object is related to its initial size, this means that δsc is independent of final mass. In the ellipsoidal model, the collapse of a region depends on the surrounding shear field, as well as on its initial overdensity. In Gaussian random fields, the distribution of these quantities depends on the size of the region considered. Since the mass of a region is related to its initial size, there is a relation between the density threshold value required for collapse and the mass of the final object. We provide a fitting function to this δec(m) relation which simplifies the inclusion of ellipsoidal dynamics in the excursion set approach. We discuss the relation between the excursion set predictions and the halo distribution in high-resolution N-body simulations, and use our new formulation of the approach to show that our simple parametrization of the ellipsoidal collapse model represents an improvement on the spherical model on an object-by-object basis. Finally, we show that the associated statistical predictions, the mass function and the large-scale halo-to-mass bias relation, are also more accurate than the standard predictions.

Effects of merging histories on angular momentum distribution of dark matter haloes

The effects of merging histories of proto-objects on the angular momentum distributions of the present-time dark matter haloes are analysed. An analytical approach to the analysis of the angular momentum distributions assumes that the haloes are initially homogeneous ellipsoids and that the growth of the angular momentum of the haloes halts at their maximum expansion time. However, the maximum expansion time cannot be determined uniquely, because in the hierarchical clustering scenario each progenitor, or subunit, of the halo has its own maximum expansion time. Therefore the merging history of the halo may be important in estimating its angular momentum. Using the merger tree model by Rodrigues &38; Thomas, which takes into account the spatial correlations of the density fluctuations, we have investigated the effects of the merging histories on the angular momentum distributions of dark matter haloes. It was found that the merger effects, that is, the effects of the inhomogeneity of the maximum expansion times of the progenitors which finally merge together into a halo, do not strongly affect the final angular momentum distributions, so that the homogeneous ellipsoid approximation happens to be good for the estimation of the angular momentum distribution of dark matter haloes. This is because the effect of the different directions of the angular momenta of the progenitors cancels out the effect of the inhomogeneity of the maximum expansion times of the progenitors. The contribution of the orbital angular momentum to the total angular momentum when two or more pre-existing haloes merge together was also investigated. It is shown that this contribution is more important than that of the angular momentum of diffuse accreting matter to the total angular momentum, especially when the mergers occur many times.

Biasing and the distribution of dark matter haloes

In hierarchical models of gravitational clustering, virialized haloes are biased tracers of the matter distribution. As discussed by Mo & White (1996), this bias is nonlinear and stochastic. They developed a model which allows one to write down analytic expressions for the mean of the bias relation, in the initial Lagrangian, and the evolved, Eulerian spaces. We provide analytic expressions for the higher order moments as well. We also show how the Mo-White approach can be extended to compute the evolution, not just of the haloes, but of the dark matter distribution itself. The model predictions for the mean and scatter in the Eulerian bias relation, as well as for the Eulerian halo-mass and halo-halo correlation functions, are in reasonable agreement with numerical simulations of hierarchical gravitational clustering for haloes of a wide range of masses, whereas the predictions for the corresponding Lagrangian quantities are accurate only for massive haloes.

An excursion set model for the distribution of dark matter and dark matter haloes

A model of the gravitationally evolved dark matter distribution, in the Eulerian space, is developed. It is a simple extension of the excursion set model that is commonly used to estimate the mass function of collapsed dark matter haloes. In addition to describing the evolution of the dark matter itself, the model allows one to describe the evolution of the Eulerian space distribution of the haloes. It can also be used to describe density profiles, on scales larger than the virial radius, of these haloes, and to quantify the way in which matter flows in and out of Eulerian cells. When the initial Lagrangian space distribution is white noise Gaussian, the model suggests that the Inverse Gaussian distribution should provide a reasonably good approximation to the evolved Eulerian density field, in agreement with numerical simulations. Application of this model to clustering from more general Gaussian initial conditions is discussed at the end.

Effects of merging histories on angular momentum distribution of dark matter haloes

ABSTRA C T The effects of merging histories of proto-objects on the angular momentum distributions of the present-time dark matter haloes are analysed. An analytical approach to the analysis of the angular momentum distributions assumes that the haloes are initially homogeneous ellipsoids and that the growth of the angular momentum of the haloes halts at their maximum expansion time. However, the maximum expansion time cannot be determined uniquely, because in the hierarchical clustering scenario each progenitor, or subunit, of the halo has its own maximum expansion time. Therefore the merging history of the halo may be important in estimating its angular momentum. Using the merger tree model by Rodrigues & Thomas, which takes into account the spatial correlations of the density fluctuations, we have investigated the effects of the merging histories on the angular momentum distributions of dark matter haloes. It was found that the merger effects, that is, the effects of the inhomogeneity of the maximum expansion times of the progenitors which finally merge together into a halo, do not strongly affect the final angular momentum distributions, so that the homogeneous ellipsoid approximation happens to be good for the estimation of the angular momentum distribution of dark matter haloes. This is because the effect of the different directions of the angular momenta of the progenitors cancels out the effect of the inhomogeneity of the maximum expansion times of the progenitors. The contribution of the orbital angular momentum to the total angular momentum when two or more pre-existing haloes merge together was also investigated. It is shown that this contribution is more important than that of the angular momentum of diffuse accreting matter to the total angular momentum, especially when the mergers occur many times.

On the Spatial Distribution of Dark Matter Halos

We study the spatial distribution of dark matter halos in the universe in terms of their number density contrast, related to the underlying dark matter fluctuation via a nonlocal and nonlinear bias random field. The description of the matter dynamics is simplified by adopting the truncated Zeldovich approximation to obtain both analytical results and simulated maps. The halo number density field in our maps and its probability distribution reproduce with excellent accuracy those of halos in a high-resolution N-body simulation with the same initial conditions. Our nonlinear and nonlocal bias prescription matches the N-body halo distribution better than any Eulerian linear and local bias.

BATSE Observations of the Large-Scale Isotropy of Gamma-Ray Bursts

We use dipole and quadrupole statistics to test the large-scale isotropy of the first 1005 gamma-ray bursts observed by the Burst and Transient Source Experiment (BATSE). In addition to the entire sample of 1005 gamma-ray bursts, many subsets are examined. We use a variety of dipole and quadrupole statistics to search for Galactic and other predicted anisotropies and for anisotropies in a coordinate-system independent manner. We find the gamma-ray burst locations to be consistent with isotropy, e.g., for the total sample the observed Galactic dipole moment (cos theta) differs from the value predicted for isotropy by 0.9 sigma and the observed Galactic quadrupole moment (sin(exp 2) b - 1/3) by 0.3 sigma. We estimate for various models the anisotropies that could have been detected. If one-half of the locations were within 86 deg of the Galactic center, or within 28 deg of the Galactic plane, the ensuing dipole or quadrupole moment would have typically been detected at the 99% confidence level. We compare the observations with the dipole and quadrupole moments of various Galactic models. Several Galactic gamma-ray bursts models have moments within 2 sigma of the observations; most of the Galactic models proposed to date are no longer in acceptable agreement with the data. Although a spherical dark matter halo distribution could be consistent with the data, the required core radius is larger than the core radius of the dark matter halo used to explain the Galaxy's rotation curve. Gamma-ray bursts are much more isotropic than any observed Galactic population, strongly favoring but not requiring an origin at cosmological distances.

Faint galaxy counts in a hierarchical universe

We interpret galaxy counts in the B and K bands, and the redshift distribution of faint galaxies, using a model for galaxy formation in a hierarchical universe. We first study how variation of each of the free parameters in our model affects its observational predictions. These parameters control processes such as star formation, supernova feedback and merging, and determine the masses and luminosities of the galaxies that populate the evolving distribution of dark matter haloes. We find that more efficient merging results in a galaxy luminosity function with a lower amplitude at the faint end at all epochs. This causes the slope of the faint counts to decrease, and the redshift distribution to shift to higher values of z

Modeling galaxy formation in evolving dark matter halos

A simple semianalytic model is used to study the radiative cooling of gas in an evolving distribution of cold dark matter halos. Two related models are used to describe the evolving distribution of collisionless dark matter: the Press-Schechter formalism and a recently developed Monte Carlo model. The Monte Carlo method follows the merging of halos and hence extends the fully analytic treatment in several ways. First, plausible estimates of halo lifetimes are made. Second, the local depletion of the primordial gas is followed in each halo as progressively more gas cools and forms stars. Finally, limits are set on the merging of protogalaxies. It is shown that, in the absence of any form of energy input, an unacceptably large fraction of the baryons will cool and condense in low-mass halos at high redshift. In order to counteract efficient cooling at early times, it is postulated that energy input by supernovae associated with the first stars formed in these halos is sufficient to unbind the gas in halos with circular velocities less than a critical value V(crit). Estimates of the present-day luminosity function calculated using this model suggest V(crit) is about 200 km/s or more, and hence that self-regulating or self-limitingmore » star formation operates even in bright L(asterisk) galaxies. 33 refs.« less