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Background Radiation (background + radiation)

Distribution by Scientific Domains

Physics and Astronomy	85%

Selected Abstracts

Background radiation from sterile neutrino decay and reionization

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005
M. Mapelli
ABSTRACT Sterile neutrinos are one of the most promising warm dark matter (WDM) candidates. By considering their radiative- and pion-decay channels, we derive the allowed contribution of sterile neutrinos to the X-ray, optical and near-infrared (NIR) cosmic backgrounds. The X-ray background puts a strong constraint on the mass of radiatively decaying neutrinos (m,s, 14 keV), whereas the allowed mass range for pion-decay neutrinos (for a particle lifetime > 4 × 1017 s) is 150 ,m,s/MeV , 500. Taking into account these constraints, we find that sterile neutrinos do not significantly contribute to the optical and NIR background. We further consider the impact of sterile neutrinos on reionization. We find that the Thomson optical depth due to sterile neutrinos is ,e= (0.4,3) × 10,2 in the case of radiative decays and it is ,10,3 for the pion-decay channel. We conclude that these particles must have played only a minor role in cosmic reionization history. [source]

Spectral estimation on a sphere in geophysics and cosmology

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2008
F. A. Dahlen
SUMMARY We address the problem of estimating the spherical-harmonic power spectrum of a statistically isotropic scalar signal from noise-contaminated data on a region of the unit sphere. Three different methods of spectral estimation are considered: (i) the spherical analogue of the one-dimensional (1-D) periodogram, (ii) the maximum-likelihood method and (iii) a spherical analogue of the 1-D multitaper method. The periodogram exhibits strong spectral leakage, especially for small regions of area A, 4,, and is generally unsuitable for spherical spectral analysis applications, just as it is in 1-D. The maximum-likelihood method is particularly useful in the case of nearly-whole-sphere coverage, A, 4,, and has been widely used in cosmology to estimate the spectrum of the cosmic microwave background radiation from spacecraft observations. The spherical multitaper method affords easy control over the fundamental trade-off between spectral resolution and variance, and is easily implemented regardless of the region size, requiring neither non-linear iteration nor large-scale matrix inversion. As a result, the method is ideally suited for most applications in geophysics, geodesy or planetary science, where the objective is to obtain a spatially localized estimate of the spectrum of a signal from noisy data within a pre-selected and typically small region. [source]

Fate of clumps in damped Ly, systems

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Ilian T. Iliev
ABSTRACT Recent observations have revealed that damped Ly, clouds (DLAs) host star formation activity. In order to examine if such star formation activity can be triggered by ionization fronts, we perform high-resolution hydrodynamics and radiative transfer simulations of the effect of radiative feedback from propagating ionization fronts on high-density clumps. We examine two sources of ultraviolet (UV) radiation field to which high-redshift (z, 3) galaxies could be exposed: one corresponding to the UV radiation originating from stars within the DLA, itself, and the other corresponding to the UV background radiation. We find that, for larger clouds, the propagating I-fronts created by local stellar sources can trigger cooling instability and collapse of significant part, up to 85 per cent, of the cloud, creating conditions for star formation in a time-scale of a few Myr. The passage of the I-front also triggers collapse of smaller clumps (with radii below ,4 pc), but in these cases the resulting cold and dense gas does not reach conditions conducive to star formation. Assuming that 85 per cent of the gas initially in the clump is converted into stars, we obtain a star formation rate of ,0.25 M, yr,1 kpc,2. This is somewhat higher than the value derived from recent observations. On the other hand, the background UV radiation which has harder spectrum fails to trigger cooling and collapse. Instead, the hard photons which have long mean free-path heat the dense clumps, which as a result expand and essentially dissolve in the ambient medium. Therefore, the star formation activity in DLAs is strongly regulated by the radiative feedback, both from the external UV background and internal stellar sources and we predict quiescent evolution of DLAs (not starburst-like evolution). [source]

Constraints on the cosmic neutrino background

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2003
Elena Pierpaoli
ABSTRACT The radiative component of the Universe has a characteristic impact on both large-scale structure (LSS) and the cosmic microwave background radiation (CMB). We use the recent WMAP data, together with previous Cosmic Background Imager (CBI) data and 2dF matter power spectrum, to constrain the effective number of neutrino species Neff in a general cosmology. We find that Neff= 4.31 with a 95 per cent confidence limit 1.6 ,Neff, 7.1. If we include the H0 prior from the HST project we find the best fit Neff= 4.08 and 1.90 ,Neff, 6.62 for a 95 per cent confidence limit. The curvature we derive is still consistent with flat, but assuming a flat Universe from the beginning implies a bias toward lower Neff, as well as artificially smaller error bars. Adding the supernova constraint does not improve the result. We analyse and discuss the degeneracies with other parameters, and point out that probes of the matter power spectrum on smaller scales and accurate independent ,8 measurements, together with better independent measurement of H0, would help in breaking the degeneracies. [source]

Galaxies as fluctuations in the ionizing background radiation at low redshift

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2003
Suzanne M. Linder
ABSTRACT Some Lyman continuum photons are likely to escape from most galaxies, and these can play an important role in ionizing gas around and between galaxies, including gas that gives rise to Lyman-alpha absorption. Thus the gas surrounding galaxies and in the intergalactic medium will be exposed to varying amounts of ionizing radiation depending upon the distances, orientations and luminosities of any nearby galaxies. The ionizing background can be recalculated at any point within a simulation by adding the flux from the galaxies to a uniform quasar contribution. Normal galaxies are found to almost always make some contribution to the ionizing background radiation at a redshift of zero, as seen by absorbers and at random points in space. Assuming that ,2 per cent of ionizing photons escape from a galaxy such as the Milky Way, we find that normal galaxies make a contribution of at least 30,40 per cent of the assumed quasar background. Lyman-alpha absorbers with a wide range of neutral column densities are found to be exposed to a wide range of ionization rates, although the distribution of photoionization rates for absorbers is found to be strongly peaked. On average, fewer highly ionized absorbers are found to arise further from luminous galaxies, while local fluctuations in the ionization rate are seen around galaxies having a wide range of properties. [source]

From primordial quantum fluctuations to the anisotropies of the cosmic microwave background radiation ,

ANNALEN DER PHYSIK, Issue 10-11 2006
N. Straumann
Abstract These lecture notes cover mainly three connected topics. In the first part we give a detailed treatment of cosmological perturbation theory. The second part is devoted to cosmological inflation and the generation of primordial fluctuations. In part three it will be shown how these initial perturbation evolve and produce the temperature anisotropies of the cosmic microwave background radiation. Comparing the theoretical prediction for the angular power spectrum with the increasingly accurate observations provides important cosmological information (cosmological parameters, initial conditions). [source]