1,721,141 research outputs found
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
Interpreting the transmission windows of distant quasars
No full textWe propose the Apparent Shrinking Criterion (ASC) to interpret the spatial
extent, R_w, of transmitted flux windows in the absorption spectra of high-z
quasars. The ASC can discriminate between the two regimes in which R_w
corresponds either to the physical size, R_HII, of the quasar HII region, or to
the distance, R^max_w, at which the transmitted flux drops to =0.1 and a
Gunn-Peterson (GP) trough appears. In the first case (HR regime), one can
determine the IGM mean HI fraction, x_HI; in the second (PR regime), the value
of R_w allows to measure the local photoionization rate and the local
enhancement of the photoionization rate, Gamma_G, due to nearby/intervening
galaxies. The ASC has been tested against radiative transfer+SPH numerical
simulations, and applied to 15 high-z (z>5.8) quasars sample from Fan et al.
(2006). All sample quasars are found to be in the PR regime; hence, their
observed spectral properties (inner flux profile, extent of transmission
window) cannot reliably constrain the value of x_HI. Four sample quasars show
evidence for a local enhancement (up to 50%) in the local photoionization rate
possibly produced by a galaxy overdensity. We discuss the possible
interpretations and uncertainties of this result
The proximity effect around high-redshift galaxies
No full textRecent observations have shown that the intergalactic medium (IGM) is more transparent to Lyα photons close to Lyman break galaxies (LBGs) than at large distance from them, i.e. there is a proximity effect. Cosmological simulations including winds from LBGs have been so far unable to explain this trend. By coupling such simulations with the radiative transfer code CRASH, we investigate whether the addition of the ionizing radiation emitted by LBGs can increase the transmissivity by decreasing the neutral hydrogen fraction in the inner Mpc of the galaxy halo. The transmissivity as a function of distance is roughly reproduced only if LBGs are identified with dwarf galaxies (with masses <~ 109 Msolar) which are undergoing a vigorous (50 Msolar yr-1) burst of star formation. Similar star formation rates in larger galaxies are not sufficient to overwhelm the large recombination rates associated with their denser environment. If so, photoionization partly reconciles theory with observations, although we discuss a number of uncertainties affecting both approaches
Orthodontic correction of an ectopic mandibular first permanent molar: A case report
No full textEctopic eruption of mandibular first molar is a rare clinical problem. It is one result of an improper balance in the forces of growth and eruption. A case of ectopic eruption of the mandibular first permanent molar is described and the technique for repositioning it is showed
CRASH : a radiative transfer scheme
No full textWe present a largely improved version of crash, a 3D radiative transfer code that treats the effects of ionizing radiation propagating through a given inhomogeneous H/He cosmological density field on the physical conditions of the gas. The code, based on a Monte Carlo technique, self-consistently calculates the time evolution of gas temperature and ionization fractions due to an arbitrary number of point/extended sources and/or diffuse background radiation with given spectra. In addition, the effects of diffuse ionizing radiation following recombinations of ionized atoms have been included. After a complete description of the numerical scheme, to demonstrate the performance, accuracy, convergence and robustness of the code, we present four different test cases designed to investigate specific aspects of radiative transfer: (i) a pure-hydrogen isothermal Strömgren sphere; (ii) realistic Strömgren spheres; (iii) multiple overlapping point sources; and (iv) shadowing of background radiation by an intervening optically thick layer. When possible, detailed quantitative comparison of the results against either analytical solutions or 1D standard photoionization codes has been made, and shows a good level of agreement. For more complicated tests the code yields physically plausible results, which could be eventually checked only by comparison with other similar codes. Finally, we briefly discuss future possible developments and cosmological applications of the code
UV background fluctuations traced by metal ions at z ≈ 3
No full textHere we investigate how LyC-opaque systems present in the intergalactic medium at z ≈ 3 can distort the spectral shape of a uniform UV background (UVB) through radiative transfer (RT) effects. With this aim in mind, we perform a multifrequency RT simulation through a cosmic volume of 10 h-1 cMpc scale polluted by metals, and self-consistently derive the ions of all the species. The UVB spatial fluctuations are traced by the ratio of He II and H I column density, η, and the ratio of C IV and Si IV optical depths, ξ . We find that: (i) η spatially fluctuates through overdense systems (Δ) with statistically significant deviations δη > 25 per cent in 18 per cent of the volume; (ii) same fluctuations in ξ are also present in 34 per cent of the enriched domain (only 8 per cent of the total volume) and derive from a combination of RT induced effects and in-homogeneous metal enrichment, both effective in systems with Δ > 1.5
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Relativistic tidal effects in nonstandard Kerr spacetime
Full text linkAstrophysical phenomena involving massive black holes (BHs) in close binaries are expected to leave detectable signatures in the electromagnetic and gravitational-wave spectrum. Such imprints may provide precious information to probe the space-time around rotating BHs and to reveal new insights on the nature of gravity in the strong-field regime. To support this observational window, it is crucial to develop suitable tests to verify the predictions of General Relativity. In this framework, the metric recently proposed by Johannsen and Psaltis parametrizes strong-field deviations from a Kerr space-time in a theory-independent way. In the following, we make use of this approach to describe the tidal field produced by spinning BHs. We compute the gravito-magnetic and gravito-electric tidal tensors for particles moving on equatorial circular geodesics, comparing our results with those obtained in the standard General Relativity scenario. Our calculations show significant differences even for distances far form the last stable orbit, which may affect the evolution of the binary and leave detectable signatures. We test our framework computing quasiequilibrium sequences of BH-white dwarf systems by means of the affine model, for different binary configurations
Observing binary black hole ringdowns by advanced gravitational wave detectors
No full textThe direct discovery of gravitational waves from compact binary systems leads for the first time to explore the possibility of black hole spectroscopy. Newly formed black holes produced by coalescing events are copious emitters of gravitational radiation, in the form of damped sinusoids, the quasinormal modes. The latter provides a precious source of information on the nature of gravity in the strong field regime, as they represent a powerful tool to investigate the validity of the no-hair theorem. In this work we perform a systematic study on the accuracy with which current and future interferometers will measure the fundamental parameters of ringdown events, such as frequencies and damping times. We analyze how these errors affect the estimate of the mass and the angular momentum of the final black hole, constraining the parameter space which will lead to the most precise measurements. We explore both single and multimode events, showing how the uncertainties evolve when multiple detectors are available. We also prove that, for the second generation of interferometers, a network of instruments is a crucial and necessary ingredient to perform strong-gravity tests of the no-hair theorem. Finally, we analyze the constraints that a third generation of detectors may be able to set on the mode's parameters, comparing the projected bounds against those obtained for current facilities
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