1,721,321 research outputs found
Infrared light from wandering stars
No full textAn explanation has been proposed for the observed excess of cosmic light at infrared wavelengths. It invokes stars that are cast into the dark-matter haloes of their parent galaxies during powerful galaxy collision
The energy cascade from warm dark matter decays
Full text linkWe use a set of Monte Carlo simulations to follow the cascade produced by a primary electron of energy Ein in the intergalactic medium. We choose Ein= 3–10 keV as expected from the decay of one of the most popular warm dark matter (WDM) candidates, sterile neutrinos. Our simulation takes into account processes previously neglected such as free—free interactions with ions and recombinations, and uses the best available cross-sections for collisional ionizations and excitations with H and He and for electron—electron collisions. We precisely derive the fraction of the primary electron energy that heats the gas, ionizes atoms and produces line and continuum photons as a function of the ionization fraction. Handy fitting formulae for all the above energy depositions are provided. By keeping track of the individual photons, we can distinguish between photons in the Lyα resonance and those with energy E < 10.2 eV that do not interact further with gas. This separation is important because a Lyα background can heat or cool the gas depending on the nature of the photons, and can have effects on the 21-cm radiation emitted by neutral H, which will probably become detectable at z > 6 in the near future by the next generation radio interferometers
The Hubble Deep Field North reveals a supernova at z~0.95
Full text linkWe report the discovery of a variable object in the Hubble Deep Field North (HDF-N) which brightened, during the 8.5 d sampled by the data, by more than 0.9 mag in I814 and about 0.7 mag in V606, remaining stable in B450. Subsequent observations of the HDF-N show that two years later this object has dimmed to about its original brightness in I814. The colours of this object, its brightness, its time behaviour in the various filters and the evolution of its morphology are consistent with it being a Type Ib supernova in a faint galaxy at z
Empowering line intensity mapping to study early galaxies
No full textLine intensity mapping is a superb tool to study the collective radiation from early galaxies. However, the method is hampered by the presence of strong foregrounds, mostly produced by low-redshift interloping lines. We present here a general method to overcome this problem which is robust against foreground residual noise and based on the cross-correlation function ψαL(r) between diffuse line emission and Lyα emitters (LAE). We compute the diffuse line (Lyα is used as an example) emission from galaxies in a (800 Mpc)3 box at z = 5.7 and 6.6. We divide the box in slices and populate them with 14 000(5500) LAEs at z = 5.7(6.6), considering duty cycles from 10−3 to 1. Both the LAE number density and slice volume are consistent with the expected outcome of the Subaru Hyper Suprime Cam survey. We add Gaussian random noise with variance σN up to 100 times the variance of the Lyα emission, σα, to simulate residual foregrounds and compute ψαL(r). We find that the signal-to-noise ratio of the observed ψαL(r) does not change significantly if σN ≤ 10σα and show that in these conditions the mean line intensity, ILyα, can be precisely recovered independently of the LAE duty cycle. Even if σN = 100σα, Iα can be constrained within a factor 2. The method works equally well for any other line (e.g. [C ii], He ii) used for the intensity-mapping experiment
Gravitational Magnification of Population III Supernovae in Hierarchical Cosmological Models: Next Generation Space Telescope Perspectives
No full textSviluppo di tecnologie e tecniche innovative per il dimensionamento ottimale dei sistemi logistici industriali
No full textLa progettazione dei sistemi logistici industriali può definirsi un processo complesso a causa del gran numero di fattori che caratterizzano il comportamento del sistema e la loro complessità di modellazione. Anche se nel corso degli anni sono stati sviluppati diversi approcci per la progettazione di sistemi logistici industriali, un'efficace implementazione nell'industria non è stata ancora completamente raggiunta. Questo è causato principalmente da: (i) i modelli e i metodi proposti sono spesso basati su ipotesi non così vicine al processo che caratterizza i sistemi reali, (ii) la cultura ingegneristica, tipicamente orientate alla "pratica", rende l'introduzione di metodologie complesse più difficoltoso.
Il progetto di ricerca è stato strutturato al fine di recuperare questo gap. Da un lato, sono stati sviluppati modelli e tecniche basate su ipotesi di progettazione più realistiche, con particolare attenzione alla modellazione dei fenomeni stocastici; dall'altro lato, tali tecniche, sono state sviluppate al fine di rendere più facile l'usabilità agli gli utenti finali adottando appropriate tecnologie e interfacce.The dimensioning of industrial logistics systems is a complex process, due to the huge number of factors that characterized the system behavior and their modelling complexity. Even though over the years various approaches have been developed for the design of industrial logistics systems, their effective application in industry has not been fully achieved yet. This is mainly caused by: (i) models and the proposed methods are often based on assumptions not so close to the actual process of real systems, (ii) the culture of industrial engineers, typically oriented to the "practical" one, makes the introduction of complex methodological approaches more challenging.
The research project has been oriented to recover this gap. From one hand, design models and techniques based on more realistic assumptions has been developed, with particular attention to model stochastic phenomena; on the other hand, these techniques has been developed with the purpose to make them more user friendly for final users adopting appropriate technologies and interfaces
On the Formation of Molecular Clumps in QSO Outflows
Full text linkWe study the origin of the cold molecular clumps in quasar outflows, recently detected in CO and HCN emission. We first describe the physical properties of such radiation-driven outflows and show that a transition from a momentum- to an energy-driven flow must occur at a radial distance of
. During this transition, the shell of swept-up material fragments due to Rayleigh–Taylor instabilities, but these clumps contain little mass and are likely to be rapidly ablated by the hot gas in which they are immersed. We then explore an alternative scenario in which clumps form from thermal instabilities at , possibly containing enough dust to catalyze molecule formation. We investigate this process with 3D two-fluid (gas+dust) numerical simulations of a kpc3 patch of the outflow, including atomic and dust cooling, thermal conduction, dust sputtering, and photoionization from the QSO radiation field. In all cases, dust grains are rapidly destroyed in years; and while some cold clumps form at later times, they are present only as transient features, which disappear as cooling becomes more widespread. In fact, we only find a stable two-phase medium with dense clumps if we artificially enhance the QSO radiation field by a factor of 100. This result, together with the complete destruction of dust grains, renders the interpretation of molecular outflows a very challenging problem
Radiative transfer effects on the Lya forest
Full text linkStrong observational evidence for a fluctuating ultraviolet background (UVB) has been accumulating through a number of studies of the HI and HeII Lyα forest as well as accurate intergalactic medium (IGM) metallicity measurements. UVB fluctuations could arise both from the inhomogeneous distribution of the ionizing sources and/or from radiative transfer (RT) through the filamentary IGM. In this study we investigate, via numerical simulations, the role of RT effects, such as shadowing, self-shielding and filtering of the ionizing radiation, in giving rise to a fluctuating UVB. We focus on possible detectable signatures of these effects on quantities derived from Lyα forest spectra, as photoionization rate fluctuations, η(≡NHeII/NHI) parameter distributions and the IGM temperature at z~ 3. We find that RT induces fluctuations up to 60 per cent in the UVB, which are tightly correlated to the density field. The UVB mean intensity is progressively suppressed toward higher densities and photon energies above 4 Ryd, due to the high HeII opacity. Shielding of overdense regions (Δ>~ 5) from cosmic HeII ionizing radiation produces a decreasing trend of η with overdensity. Furthermore, we find that the mean η value inferred from HI-HeII Lyα forest observations can be explained only by properly accounting for the actual IGM opacity. We outline and discuss several implications of our findings
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