1,721,590 research outputs found
Strong Mg II Systems in Quasar and Gamma-Ray Burst Spectra
Full text linkThe incidence of strong Mg II systems in gamma-ray burst (GRB) spectra is a few times higher than in quasar (QSO) spectra. We investigate several possible explanations for this effect, including dust obscuration bias, clustering of the absorbers, different beam sizes of the sources, multiband magnification bias of GRBs, and association of the absorbers with the GRB event or the circumburst environment. We find that (1) the incidence rate of Mg II systems in QSO spectra could be underestimated by a factor of 1.3-2 due to dust obscuration; (2) the equivalent width distribution of the Mg II absorbers along GRBs is consistent with that observed along QSOs, thus suggesting that the absorbers are more extended than the beam sizes of the sources; (3) on average, GRB afterglows showing more than one Mg II system are a factor of 1.7 brighter than the others, suggesting a lensing origin of the observed discrepancy; (4) gravitational lensing (in different forms, from galaxy lensing to microlensing) can bias high the counts of Mg II systems along GRBs if the luminosity functions of the prompt gamma-ray emission and of the optical afterglows have a mean faint-end slope approaching -5/3 to -2; and (5) some of the absorbers can be associated with the circumburst environment or produced by supernova remnants unrelated to the GRB event itself but lying in the same star-forming region. With the possible exception of magnification bias, it is unlikely that one of these effects on its own can fully account for the observed counts. However, the combined action of some of them can substantially reduce the statistical significance of the discrepancy
Cosmic and Galactic neutrino backgrounds from thermonuclear sources
No full textWe estimate energy spectra and fluxes at the Earth’s surface of the cosmic and
Galactic neutrino backgrounds produced by thermonuclear reactions in stars. The
extra-galactic component is obtained by combining the most recent estimates of the
cosmic star formation history and the stellar initial mass function with accurate
theoretical predictions of the neutrino yields all over the thermonuclear lifetime of
stars of different masses. Models of the structure and evolution of the Milky Way
are used to derive maps of the expected flux generated by Galactic sources as a
function of sky direction. The predicted neutrino backgrounds depend only slightly
on model parameters.
In the relevant 50 keV-10 MeV window, the total flux of
cosmic neutrinos ranges between 20 and 65 cm−2 s−1. Neutrinos reaching the Earth
today have been typically emitted at redshift z = 2. Their energy spectrum peaks at
E = 0.1−0.3 MeV. The energy and entropy densities of the cosmic background are
negligible with respect to the thermal contribution of relic neutrinos originated in
the early universe. In every sky direction, the cosmic background is outnumbered by
the Galactic one, whose integrated flux amounts to 300-1000 cm−2 s−1. The emission
from stars in the Galactic disk contributes more than 95 per cent of the signal
On the Spatial Distribution of Dark Matter Halos
No full textWe 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
The large-scale monopole of the power spectrum in a Euclid-like survey: Wide-angle effects, lensing, and the 'finger of the observer'
Full text linkRadial redshift-space distortions due to peculiar velocities and other light-cone effects shape the maps we build of the Universe. We address the open question of their impact onto the monopole moment of the galaxy power spectrum, P0(k). Specifically, we use an upgraded numerical implementation of the liger method to generate 140 mock galaxy density fields for a full Euclid-like survey and we measure P0(k) in each of them utilizing a standard estimator. We compare the spectra obtained by turning on and off different effects. Our results show that wide-angle effects due to radial peculiar velocities generate excess power above the level expected within the plane-parallel approximation. They are detectable with a signal-to-noise ratio of 2.7 for Mpc-1. Weak-lensing magnification also produces additional power on large scales which, if the current favourite model for the luminosity function of Hα emitters turns out to be realistic, can only be detected with a signal-to-noise ratio of 1.3 at best. Finally, we demonstrate that measuring P0(k) in the standard of rest of the observer generates an additive component reflecting the kinematic dipole overdensity caused by the peculiar velocity. This component is characterized by a damped oscillatory pattern on large scales. We show that this 'finger of the observer' effect is detectable in some redshift bins and suggest that its measurement could possibly open new research directions in connection with the deteination of the cosmological parameters, the properties of the galaxy population under study, and the dipole itself
Physical constraints on the halo mass function
No full textWe analyse the effect of two relevant physical constraints on the mass multiplicity function of dark matter haloes in a Press-Schechter type algorithm. Considering the random walk of linear Gaussian density fluctuations as a function of the smoothing scale, we simultaneously (i) account for mass semi-positivity and (ii) avoid the `cloud-in-cloud' problem. It is shown that the former constraint implies a severe cut-off of low-mass objects, balanced by an increase on larger mass scales. The analysis is performed both for scale-free power spectra and for the standard cold dark matter (CDM) model. Our approach shows that the well-known `infrared' divergence of the standard Press-Schechter mass function is caused by unphysical, negative mass events which inevitably occur in a Gaussian distribution of density fluctuations
Excursion set approach to the clustering of dark matter haloes in Lagrangian space
No full textWe present a stochastic approach to the spatial clustering of dark matter haloes in Lagrangian space. Our formalism is based on a local formulation of the `excursion set' approach by Bond et al., which automatically accounts for the `cloud-in-cloud' problem in the identification of bound systems. Our method allows us to calculate correlation functions of haloes in Lagrangian space using either a multidimensional Fokker-Planck equation with suitable boundary conditions, or an array of Langevin equations with spatially correlated random forces. We compare the results of our method with theoretical predictions for the halo autocorrelation function considered in the literature, and find good agreement with the results recently obtained within a treatment of halo clustering in terms of `counting fields' by Catelan et al. Finally, the possible effect of spatial correlations on numerical simulations of halo merger trees is discussed
Cosmological parameters from the likelihood analysis of the galaxy power spectrum and bispectrum in real space
Full text linkWe present a joint likelihood analysis of the halo power spectrum and bispectrum in real space. We take advantage of a large set of numerical simulations and of an even larger set of halo mock catalogs to provide a robust estimate of the covariance properties. We derive constraints on bias and cosmological parameters assuming a theoretical model from perturbation theory at one-loop for the power spectrum and tree-level for the bispectrum. By means of the Deviance Information Criterion, we select a reference bias model dependent on seven parameters that can describe the data up to k max,P = 0.3 h Mpc-1 for the power spectrum and k max,B = 0.09 h Mpc-1 for the bispectrum at redshift z = 1. This model is able to accurately recover three selected cosmological parameters even for the rather extreme total simulation volume of 1000h -3 Gpc3. With the same tools, we study how relations among bias parameters can improve the fit while reducing the parameter space. In addition, we compare common approximations to the covariance matrix against the full covariance estimated from the mocks, and quantify the (non-negligible) effect of ignoring the cross-covariance between the two statistics. Finally, we explore different selection criteria for the triangular configurations to include in the analysis, showing that excluding nearly equilateral triangles rather than simply imposing a fixed maximum k max,B on all triangle sides can lead to a better exploitation of the information contained in the bispectrum
Diagnosi eziologica della broncopatia croncia nel cavallo: valutazione di un protocollo per la stadiazione clinica della malattia e limiti dell'uso del test intradermico
No full textThe bias field of dark matter haloes
No full textThis paper presents a stochastic approach to the clustering evolution of dark matter haloes in the Universe. Haloes, identified by a Press-Schechter-type algorithm in Lagrangian space, are described in terms of `counting fields', acting as non-linear operators on the underlying Gaussian density fluctuations. By ensemble-averaging these counting fields, the standard Press-Schechter mass function as well as analytic expressions for the halo correlation function and corresponding bias factors of linear theory are obtained, extending the recent results by Mo & White. The non-linear evolution of our halo population is then followed by solving the continuity equation, under the sole hypothesis that haloes move by the action of gravity. This leads to an exact and general formula for the bias field of dark matter haloes, defined as the local ratio between their number density contrast and the mass density fluctuation. Besides being a function of position and `observation' redshift, this random field depends upon the mass and formation epoch of the objects and is both non-linear and non-local. The latter features are expected to leave a detectable imprint on the spatial clustering of galaxies, as described, for instance, by statistics like the bispectrum and the skewness. Our algorithm may have several interesting applications, among which is the possibility of generating mock halo catalogues from low-resolution N-body simulations
- …
