3,458 research outputs found
Anisotropy probe of galactic and extra-galactic Dark Matter annihilations
We study the flux and the angular power spectrum of gamma-rays produced by
Dark Matter (DM) annihilations in the Milky Way (MW) and in extra-galactic
halos. The annihilation signal receives contributions from: a) the smooth MW
halo, b) resolved and unresolved substructures in the MW, c) external DM halos
at all redshifts, including d) their substructures. Adopting a self-consistent
description of local and extra-galactic substructures, we show that the
annihilation flux from substructures in the MW dominates over all the other
components for angles larger than O(1) degrees from the Galactic Center, unless
an extreme prescription is adopted for the substructures concentration. We also
compute the angular power spectrum of gamma-ray anisotropies and find that, for
an optimistic choice of the particle physics parameters, an interesting
signature of DM annihilations could soon be discovered by the Fermi LAT
satellite at low multipoles, l<100, where the dominant contribution comes from
MW substructures with mass M>10^4 solar masses. For the substructures models we
have adopted, we find that the contribution of extra-galactic annihilations is
instead negligible at all scales.Comment: 14 pages, 7 figure
Modeling the QSO luminosity and spatial clustering at low redshifts
We investigate the ability of hierarchical models of QSO formation and
evolution to match the observed luminosity, number counts and spatial
clustering of quasars at redshift z<2. These models assume that the QSO
emission is triggered by galaxy mergers, that the mass of the central black
hole correlates with halo properties and that quasars shine at their Eddington
luminosity except, perhaps, during the very early stages of evolution. We find
that models based on simple analytic approximations successfully reproduce the
observed B-band QSO luminosity function at all redshifts, provided that some
mechanisms is advocated to quench mass accretion within haloes larger than
about 1e13 Msun that host bright quasars. These models also match the observed
strength of QSO clustering at z~0.8. At larger redshifts, however, they
underpredict the QSO biasing which, instead, is correctly reproduced by
semi-analytic models in which the halo merger history and associated BHs are
followed by Monte Carlo realizations of the merger hierarchy. We show that the
disagreement between the luminosity function predicted by semi-analytic models
and observations can be ascribed to the use of B-band data, which are a biased
tracer of the quasar population, due to obscuration.Comment: 13 pages, 9 figures. Accepted by MNRA
A numerical study of the effects of primordial non-Gaussianities on weak lensing statistics
While usually cosmological initial conditions are assumed to be Gaussian,
inflationary theories can predict a certain amount of primordial
non-Gaussianity which can have an impact on the statistical properties of the
lensing observables. In order to evaluate this effect, we build a large set of
realistic maps of different lensing quantities starting from light-cones
extracted from large dark-matter only N-body simulations with initial
conditions corresponding to different levels of primordial local
non-Gaussianity strength . Considering various statistical
quantities (PDF, power spectrum, shear in aperture, skewness and bispectrum) we
find that the effect produced by the presence of primordial non-Gaussianity is
relatively small, being of the order of few per cent for values of compatible with the present CMB constraints and reaching at most 10-15
per cent for the most extreme cases with . We also discuss
the degeneracy of this effect with the uncertainties due to the power spectrum
normalization and matter density parameter , finding
that an error in the determination of () of about 3
(10) per cent gives differences comparable with non-Gaussian models having
. These results suggest that the possible presence of an
amount of primordial non-Gaussianity corresponding to is not
hampering a robust determination of the main cosmological parameters in present
and future weak lensing surveys, while a positive detection of deviations from
the Gaussian hypothesis is possible only breaking the degeneracy with other
cosmological parameters and using data from deep surveys covering a large
fraction of the sky.Comment: accepted by MNRA
Constraints on a scale-dependent bias from galaxy clustering
We forecast the future constraints on scale-dependent parametrizations of
galaxy bias and their impact on the estimate of cosmological parameters from
the power spectrum of galaxies measured in a spectroscopic redshift survey. For
the latter we assume a wide survey at relatively large redshifts, similar to
the planned Euclid survey, as baseline for future experiments. To assess the
impact of the bias we perform a Fisher matrix analysis and we adopt two
different parametrizations of scale-dependent bias. The fiducial models for
galaxy bias are calibrated using a mock catalogs of H emitting galaxies
mimicking the expected properties of the objects that will be targeted by the
Euclid survey.
In our analysis we have obtained two main results. First of all, allowing for
a scale-dependent bias does not significantly increase the errors on the other
cosmological parameters apart from the rms amplitude of density fluctuations,
, and the growth index , whose uncertainties increase by a
factor up to two, depending on the bias model adopted. Second, we find that the
accuracy in the linear bias parameter can be estimated to within 1-2\%
at various redshifts regardless of the fiducial model. The non-linear bias
parameters have significantly large errors that depend on the model adopted.
Despite of this, in the more realistic scenarios departures from the simple
linear bias prescription can be detected with a significance at
each redshift explored.
Finally, we use the Fisher Matrix formalism to assess the impact of assuming
an incorrect bias model and found that the systematic errors induced on the
cosmological parameters are similar or even larger than the statistical ones.Comment: new section added; conclusions unchanged; accepted for publication in
PR
Performance Increase of a Small-scale Liquefied Natural Gas Production Process by Means of Turbo-expanderâ
Abstract In the last years, the growing demand of the energy market has led to the increasing penetration of renewable energy sources in order to achieve the primary energy supply. However, in the next years fossil fuels are expected to remain the dominant energy source, due to the forecasted increase of global energy consumption. In particular, the natural gas is predicted to still play a key role in the energy market, on account of its lower environmental impact than other fossil fuels. Natural gas is currently employed mainly as gaseous fuel for stationary energy generation, but also as liquefied fuel, as an alternative to the diesel fuel, in vehicular applications. Liquefied Natural Gas (LNG) is currently produced in large plants directly located at the extraction sites. The aim of the study is the definition of an optimal small-scale production process for LNG, to be realized â in opposition to the current habit â directly at filling stations. With this purpose, two different LNG production layouts have been proposed and investigated within a thermodynamic analysis: starting from a Joule-Thompson LNG expansion process, a new layout with a turbo-expander has been proposed for the natural gas liquefaction. The carried-out simulations show that the new proposed solution allow to optimize the LNG production process and to minimize the process' energy consumption
Storage Solutions for Renewable Production in Household Sector
Abstract The penetration of renewable sources, particularly wind and solar, into the grid has been increasing in recent years. As a consequence, there have been serious concerns over reliable and safety operation of power systems. One possible solution, to improve grid stability, is to integrate energy storage devices into power system network: storing energy produced in periods of low demand to later use, ensuring full exploitation of intermittent available sources. Focusing on stand-alone photovoltaic (PV) energy system, energy storage is needed with the purpose of ensuring continuous power flow, to minimize or, if anything, to neglect electrical grid supply. A comprehensive study on a hybrid stand-alone photovoltaic power system using two different energy storage technologies has been performed. This study examines the feasibility of replacing electricity provided by the grid with hybrid system to meet household demand. This paper is a part of an experimental and a theoretical study which is currently under development at University of Bologna. A test facility is under construction, at the University of Bologna, for the experimental characterization of the cogenerative performance of small scale hybrid power systems, composed of micro-CHP systems of different technologies : a Micro Rankine Cycles (MRC), a Proton Exchange Membrane Fuel Cells (PEM-FC), a battery, an electrolyzer and a heat recovery subsystem. The test set-up is also integrated with an external load simulator, in order to generate variable load profiles. This paper presents the theoretical results of the performance simulations developed considering an hybrid system consisting on a photovoltaic array (PV), electrochemical batteries (B) and electrolyzer (HY) with a H2 tank and a Proton Exchange Membrane Fuel Cell (PEM-FC) stack, in case of a household electrical demand. The performance of this system have been evaluated by the use of a calculation code, in-house developed by University of Bologna; future activities will be the tuning of the software with the experimental results, in order to realize a code able to define the correct size of each sub-system, ones the load profile of the utility is known or estimated
Studying the WHIM with Gamma Ray Bursts
We assess the possibility to detect and characterize the physical state of
the missing baryons at low redshift by analyzing the X-ray absorption spectra
of the Gamma Ray Burst [GRB] afterglows, measured by a micro calorimeters-based
detector with 3 eV resolution and 1000 cm2 effective area and capable of fast
re-pointing, similar to that on board of the recently proposed X-ray satellites
EDGE and XENIA. For this purpose we have analyzed mock absorption spectra
extracted from different hydrodynamical simulations used to model the
properties of the Warm Hot Intergalactic Medium [WHIM]. These models predict
the correct abundance of OVI absorption lines observed in UV and satisfy
current X-ray constraints. According to these models space missions like EDGE
and XENIA should be able to detect about 60 WHIM absorbers per year through the
OVII line. About 45 % of these have at least two more detectable lines in
addition to OVII that can be used to determine the density and the temperature
of the gas. Systematic errors in the estimates of the gas density and
temperature can be corrected for in a robust, largely model-independent
fashion. The analysis of the GRB absorption spectra collected in three years
would also allow to measure the cosmic mass density of the WHIM with about 15 %
accuracy, although this estimate depends on the WHIM model. Our results suggest
that GRBs represent a valid, if not preferable, alternative to Active Galactic
Nuclei to study the WHIM in absorption. The analysis of the absorption spectra
nicely complements the study of the WHIM in emission that the spectrometer
proposed for EDGE and XENIA would be able to carry out thanks to its high
sensitivity and large field of view.Comment: 16 pages, 16 figures, accepted for publication by Ap
Complex energy networks: Energy-ecological efficiency based evaluations towards the sustainability in energy sector
In the last years, international programs in diverse sectors and national frameworks have been driven by the need of a sustainable growth, in a green economy perspective. In order to reduce the energy losses/dissipations, as well as the fossil fuels employment and related pollutant emissions, indeed, the spread of combined heat and power units and/or renewable sources generators is promoted into both the electrical grids and the thermal networks but are often in conflict with the economic aspects. In this context, the optimal management of complex energy networks - including, in particular, smart district heating - may lead to the achievement of important goals from the environmental and sustainability viewpoints. The aim of this paper is to develop a preliminary methodology for the complete evaluation of complex energy networks, considering energy, economic and environmental aspects. With this purpose, a case study consisting in a network for the fulfillment of electrical and thermal needs of the connected users will be analyzed, considering different scenarios in terms of energy generation mix and operation and applying different optimization software. In addition, the carried out evaluations will allow to set the basis for the discussion about the future of energy policies and possible incentives towards the sustainable development of the energy sector
Comparison of the ENEAR Peculiar Velocities with the PSCz Gravity Field
We present a comparison between the peculiar velocity field measured from the ENEAR all-sky DnâÏ catalogue and that derived from the galaxy distribution of the IRAS Point Source Catalog Redshift Survey (PSCz). The analysis is based on a modal expansion of these data in redshift space by means of spherical harmonics and Bessel functions. The effective smoothing scale of the expansion is almost linear with redshift reaching 1500kmsâ1 at 3000kmsâ1. The general flow patterns in the filtered ENEAR and PSCz velocity fields agree well within 6000kmsâ1, assuming a linear biasing relation between the mass and the PSCz galaxies. The comparison allows us to determine the parameter ÎČ = Ω0.6 / b, where Ω is the cosmological density parameter and b is the linear biasing factor. A likelihood analysis of the ENEAR and PSCz modes yields ÎČ = 0.5 ± 0.1, in good agreement with values obtained from Tully-Fisher surveys
From solar to hydrogen: Preliminary experimental investigation on a small scale facility
Issues of exhaustible natural resources, fluctuating fossil fuel prices and improvements in electric power systems motivated governments to behave positively toward the development of distributed generation. In addition, progresses in small size generation technologies and storage systems give rise to a significant diffusion in microgrids, working together with conventional power grid. Indeed, in the next future, domestic microgrids are expected to play a fundamental role in electric power networks, driving both the academic and industrial research interests in developing high efficient and reliable conversion and storage technologies. In this context, this study presents a feasible configuration of a solar-hydrogen integrated microgrid and documents the procedure to characterize the overall efficiency of a laboratory scale test facility. Experimental results highlight that the most significant inefficiencies in the solar to hydrogen conversion process are mainly attributed to the solar to electrical energy conversion process, being responsible for about 89% of losses. The overall laboratory scale solar to hydrogen chain can reach conversion efficiency up to 5.3%
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