427 research outputs found
Phenomenological models of Cosmic Ray transport in Galaxies
No full textLecture slides for the course "Phenomenological models of Cosmic Ray transport in Galaxies" delivered by Carmelo Evoli at the International School of Physics Enrico Fermi - Course 208 Foundations of Cosmic Ray Astrophysics
Turbulence in the intergalactic medium
No full textWe study supernova-driven galactic outflows as a mechanism for injecting turbulence in the intergalactic medium (IGM) far from galaxies. To this aim, we follow the evolution of a 1013-Msun galaxy along its merger tree, with carefully calibrated prescriptions for star formation and wind efficiencies. At z ≈ 3, the majority of the bubbles around galaxies are old (ages > 1 Gyr), that is, they contain metals expelled by their progenitors at earlier times; their filling factor increases with time, reaching about 10 per cent at z < 2. The energy deposited by these expanding shocks in the IGM is predominantly in kinetic form (mean energy density of 1 μeV cm-3, about two to three times the thermal one), which is rapidly converted in disordered motions by instabilities, finally resulting in a fully developed turbulent spectrum whose evolution is followed through a spectral transfer function approach. The derived mean IGM turbulent Doppler parameter, bt, peaks at z ≈ 1 at about 1.5 km s-1 with the maximum bt= 25 km s-1. The shape of the bt distribution does not significantly evolve with redshift but undergoes a continuous shift towards lower bt values with time, as a result of bubble ageing. We also find a clear trend of decreasing bt with ? and a more complex dependence on Rs resulting from the age spread of the bubbles. We have attempted a preliminary comparison with the data, hampered by the scarcity of the latter and by the challenge provided by the subtraction of peculiar and thermal motions. Finally, we comment on the implications of turbulence for various cosmological studies.We study supernova-driven galactic outflows as a mechanism for injecting turbulence in the intergalactic medium (IGM) far from galaxies. To this aim, we follow the evolution of a 1013-M⊙ galaxy along its merger tree, with carefully calibrated prescriptions for star formation and wind efficiencies. At z ≈ 3, the majority of the bubbles around galaxies are old (ages > 1 Gyr), that is, they contain metals expelled by their progenitors at earlier times; their filling factor increases with time, reaching about 10 per cent at z < 2. The energy deposited by these expanding shocks in the IGM is predominantly in kinetic form (mean energy density of 1 μeV cm-3, about two to three times the thermal one), which is rapidly converted in disordered motions by instabilities, finally resulting in a fully developed turbulent spectrum whose evolution is followed through a spectral transfer function approach. The derived mean IGM turbulent Doppler parameter, bt, peaks at z ≈ 1 at about 1.5 km s-1 with the maximum bt= 25 km s-1. The shape of the bt distribution does not significantly evolve with redshift but undergoes a continuous shift towards lower bt values with time, as a result of bubble ageing. We also find a clear trend of decreasing bt with ? and a more complex dependence on Rs resulting from the age spread of the bubbles. We have attempted a preliminary comparison with the data, hampered by the scarcity of the latter and by the challenge provided by the subtraction of peculiar and thermal motions. Finally, we comment on the implications of turbulence for various cosmological studies
The role of turbulence in interstellar and intergalactic environments
No full textThis thesis aim at studying the role of turbulence in different astrophysical environments. In fact, turbulence has been observed in a large variety of astrophysical sites, making turbulence one of the most important physical processes under investigation in astrophysics thanks to the contribution of either huge computer simulations and tough analytical treatments
The puzzling origin of the 6Li plateau
No full textWe discuss the 6Li abundance evolution within a hierarchical model of Galaxy formation which correctly reproduces the [Fe/H] distribution of metal-poor halo stars. Contrary to previous findings, we find that neither the level (6Li/H = 6 × 10-12) nor the flatness of the 6Li distribution with [Fe/H] can be reproduced under the most favourable conditions by any model in which 6Li production is tied to a (data-constrained) Galactic star formation rate via cosmic ray spallation. Thus, the origin of the plateau might be due to some other early mechanism unrelated to star formation
Expectations for high energy diffuse galactic neutrinos for different cosmic ray distributions
No full textThe interaction of cosmic rays with the gas contained in our Galaxy is a guaranteed source of diffuse high energy neutrinos. We provide expectations for this component by considering different assumptions for the cosmic ray distribution in the Galaxy which are intended to cover the large uncertainty in cosmic ray propagation models. We calculate the angular dependence of the diffuse galactic neutrino flux and the corresponding rate of High Energy Starting Events in IceCube by including the effect of detector angular resolution. Moreover we discuss the possibility to discriminate the galactic component from an isotropic astrophysical flux. We show that a statistically significant excess of events from the galactic plane in present IceCube data would disfavour models in which the cosmic ray density is uniform, thus bringing relevant information on the cosmic ray radial distribution
Reionization during the dark ages from a cosmic axion background
No full textRecently it has been pointed out that a cosmic background of relativistic axion-like particles (ALPs) would be produced by the primordial decays of heavy fields in the post-inflation epoch, contributing to the extra-radiation content in the Universe today. Primordial magnetic fields would trigger conversions of these ALPs into sub-MeV photons during the dark ages. This photon flux would produce an early reionization of the Universe, leaving a significant imprint on the total optical depth to recombination τ. Using the current measurement of τ and the limit on the extra-radiation content Δ Neff by the Planck experiment we put a strong bound on the ALP-photon conversions. Namely we obtain upper limits on the product of the photon-ALP coupling constant gaγ times the magnetic field strength B down to gaγ B 6 × 10-18 GeV-1 nG for ultralight ALPs
Energy deposition by weakly interacting massive particles: a comprehensive study
No full textWe present results obtained with the updated version of our code MEDEA2, which includes
all physical processes necessary to study the energy deposition in the surrounding environment
from primary photons and fast leptons produced by dark matter (DM) particle decay/
annihilation. Such interactions now include also Compton scattering of primary photons
off electrons and pair creation of photons on atoms. Our ultimate aim is a thorough study
of the impact of DM annihilations on the thermal and ionization history of the high-redshift
intergalactic medium (IGM) during the dark ages. In addition, a precise determination of the
effects of DM decays/annihilations can help constrain its nature. We present the results for
some selected DM candidates: (i) a 10-GeV bino-like neutralino; (ii) a heavy DM candidate of
rest mass 1 TeV that pair annihilates into muons; and (iii) a 200-GeV wino-like neutralino with
a pair annihilation into W+W− pairs. An interface to DARKSUSY allows us to use the computed
annihilation spectra in input for our code and follow the complete secondary cascade. The
fractional energy depositions into the IGM depend strongly on the DM particle rest mass:
whereas for the 10-GeV particle the absorbed energy fraction, in the redshift range 10 < z < 1000, is ∼ 50 per cent, higher mass candidates deposit their energy less efficiently (∼1–10 per
cent), making their impact on the high-z IGM considerably weaker. Noticeably, our approach
allows us to consistently follow the low-energy deposition of the cascade products, which
can be of interest for a broad range of applications. Finally, we provide both tabulated results
and analytical fits that can be readily implemented in theoretical studies of the effects and
detectability of the most popular DM candidates
Constraints on dark matter annihilations from diffuse gamma-ray emission in the Galaxy
No full textRecent advances in gamma-ray cosmic ray, infrared and radio astronomy have allowed us to develop a significantly better understanding of the galactic medium properties in the last few years. In this work using the DRAGON code, that numerically solves the GR propagation equation and calculating gamma-ray emissivities in a 2-dimensional grid enclosing the Galaxy, we study in a self consistent manner models for the galactic diffuse gamma-ray emission. Our models are cross-checked to both the available CR and gamma-ray data We address the extend to which dark matter annihilations in the Galaxy can contribute to the diffuse gamma-ray flux towards different directions on the sky. Moreover we discuss the impact that astrophysical uncertainties of non DM nature. have on the derived gamma-ray limits. Such uncertainties are related to the diffusion properties on the Galaxy, the interstellar gas and the interstellar radiation field energy densities. Light similar to 10 GeV dark matter annihilating dominantly to hadrons is more strongly constrained by gamma-ray observations towards the inner parts of the Galaxy and influenced the most by assumptions of the gas distribution; while TeV scale DM annihilating dominantly to leptons has its tightest constraints from observations towards the galactic center avoiding the galactic disk plane, with the main astrophysical uncertainty being the radiation field energy density. In addition, we present a method of deriving constraints on the dark matter distribution profile from the diffuse gamma-ray spectra. These results critically depend on the assumed mass of the dark matter particles and the type of its end annihilation products
Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles
No full textpeer reviewedWe revise the bound from the supernova SN1987A on the coupling of ultralight axion-like particles (ALPs) to photons. In a core-collapse supernova, ALPs would be emitted via the Primakoff process, and eventually convert into gamma rays in the magnetic field of the Milky Way. The lack of a gamma-ray signal in the GRS instrument of the SMM satellite in coincidence with the observation of the neutrinos emitted from SN1987A therefore provides a strong bound on their coupling to photons. Due to the large uncertainty associated with the current bound, we revise this argument, based on state-of-the-art physical inputs both for the supernova models and for the Milky-Way magnetic field. Furthermore, we provide major amendments, such as the consistent treatment of nucleon-degeneracy effects and of the reduction of the nuclear masses in the hot and dense nuclear medium of the supernova. With these improvements, we obtain a new upper limit on the photon-ALP coupling: g_{a\gamma} < 5.3 x 10^{-12} GeV^{-1}, for m_a < 4.4 x 10^{-10} eV, and we also give its dependence at larger ALP masses. Moreover, we discuss how much the Fermi-LAT satellite experiment could improve this bound, should a close-enough supernova explode in the near future
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