530 research outputs found
Toxicité de l'ion fluorure envers les organismes d'eau douce et effets de la dureté - revue et nouvelle analyse de données existantes
Les plus importantes sources anthropiques de fluorures dans les systèmes d'eau douce comprennent les eaux usées municipales, les industries productrices de fertilisants et les alumineries. Plusieurs études montrent que la toxicité des fluorures est réduite lorsque le test toxicologique est réalisé en eau dure plutôt qu'en eau douce. Trois mécanismes peuvent être invoqués pour expliquer une telle tendance : (I) influence des ions de dureté (Ca2+ ; Mg2+) sur les organismes tests (soit au niveau de la barrière biologique séparant l'organisme de son milieu, soit au niveau de leur métabolisme interne); (II) complexation entre le fluorure et les ions de dureté dans le milieu d'exposition, menant à une réduction de la concentration en fluorure libre (F-); (III) précipitation de fluorite (CaF2) dans les milieux d'exposition, menant à une réduction de la concentration effective en fluorures. Pour identifier le ou les mécanisme(s) responsables de l'effet protecteur de la dureté, nous avons réalisé une revue de la littérature existante sur les poissons, les invertébrés et les insectes aquatiques d'eau douce. Parmi ces études, les plus complètes ont été sélectionnées et la spéciation des fluorures modélisée pour chaque cas. Les modélisations réalisées indiquent que la spéciation physique du fluorure (distinction entre les espèces dissoutes et particulaires) a beaucoup plus d'importance que sa spéciation chimique en solution dans les systèmes étudiés.Important anthropogenic sources of fluoride to the aquatic environment include municipal waste waters and effluents from fertilizer producing plants and aluminum refineries. Many studies have demonstrated that fluoride toxicity to aquatic organisms is reduced when the toxicological test is performed in hard water rather than soft water. In principle at least three mechanisms could explain this trend: (I) a direct beneficial influence of the hardness cations (Ca2+ ; Mg2+) on the test organism (either externally, at an epithelial membrane, or internally); (II) complexation between fluoride and the hardness cations, leading to a reduction in free the fluoride concentration (F-) in the exposure media; (III) fluorite precipitation (CaF2) in the exposure media, leading to a reduction in the effective fluoride concentration. The present literature review was designed to identify which of these mechanisms might be responsible for the apparent protective effect of hardness on fluoride toxicity.An inventory of the existing literature on the toxicity of fluoride to freshwater fish, invertebrates and aquatic insects was prepared. The most complete studies were selected and the chemical data needed to model cation and anion speciation in the exposure media were extracted from the papers. Speciation at equilibrium was then modelled using as input data the total concentrations of the key constituents (calcium, magnesium, fluoride and chloride), together with the temperature and the pH.The initial speciation calculations revealed a particularity of the chemical systems studied: frequently precipitation of fluorite (CaF2) was predicted by the speciation model (MINEQL+), but the article from which the data had been extracted did not mention the appearance of a precipitate. Fluorite solubility, at a pH of 7 and at an ionic strength of 2.7 mM, is approximately 17 mg CaF2 /L (0.22 mM). When high fluoride concentrations are used in hard water, both fluoride and calcium concentrations are predicted to decrease markedly as a result of fluorite precipitation. This analysis of the published results thus suggests that the reported lower toxicity of fluoride in hard waters is likely due to the chemical precipitation of CaF2 and MgF2, resulting in a decrease in the effective fluoride concentration to which the test organisms are exposed. In other words, changes in the physical speciation of fluoride (i.e., its distribution among dissolved and particulate species) are much more important than changes in its chemical speciation in solution.Given the low solubility of fluoride in hard waters, it would seem difficult if not impossible to carry out fluoride toxicity tests in hard water. However, in a few fluoride toxicity studies the researchers checked for precipitation by monitoring fluoride and calcium concentrations throughout the toxicity test. In some of these cases, even though speciation calculations predicted fluorite formation at the exposure concentrations used, the authors did not detect any precipitation; these systems were thus apparently in a metastable, over-saturated state, where the kinetics of precipitation were slow relative to the duration of the toxicity test. The chemical equilibrium software was used to simulate fluoride speciation in these systems, by allowing the over-saturated solid phases to remain in solution. In particular, we looked for evidence that under such circumstances the hardness cations exerted a beneficial effect. However, no clear picture emerged from this second analysis: two of the studies designed particularly to test the effect of calcium on fluoride toxicity showed a protective effect, whereas one study indicated the opposite effect, i.e. an increase in fluoride toxicity as the calcium concentration was raised. All fish studies for which calcium concentrations were available (N=20 studies; 58 toxicity tests) were pooled and tested for a possible calcium effect on fluoride toxicity. No relationship was observed between fluoride ion toxicity (LC50, expressed as calculated free [F-]) and calculated dissolved calcium concentrations for these pooled data (Fig. 2). Fluorite solubility was the most important factor influencing the data point distribution in the relationship. The same exercise was performed for all the invertebrate studies (N=11 studies; 22 toxicity tests) but again no relationship was found (Fig. 3).Several factors other than hardness affect fluoride toxicity to aquatic organisms. Fluoride toxicity to fish increased with exposure duration up to 200 h, where it reached a threshold LC50 level around 100 mg/L (5.3 mM) expressed as free fluoride (Fig. 4). Fish life stage (Fig. 5), the temperature of the exposure media (Fig. 6) and the chloride concentration also affected fluoride toxicity in fish. For invertebrates, fluoride toxicity increased with exposure duration but to a lesser extent than for fish.In summary, water hardness clearly reduces fluoride toxicity to aquatic organisms by limiting the equilibrium solubility of the fluoride ion. However, in those cases where the precipitation of CaF2 (s) and MgF2 (s) is slow, and where the hardness cations and fluoride co-exist in the dissolved state in the exposure medium, the experimental evidence for a protective effect of hardness on fluoride toxicity is equivocal. To answer the question, new experiments should be performed under conditions that take into account the behaviour of calcium and fluoride in the natural environment. Metastable environments where fluoride concentrations exceed the solubility limit imposed by CaF2 or MgF2 could be reproduced in laboratory toxicity tests by using continuous flow systems. For tests below the solubility limit, toxicity tests with varying levels of Ca or Mg could be designed to stay within the solubility range of CaF2 or MgF2. In both cases, dissolved calcium, magnesium and fluoride concentrations should be monitored throughout the toxicity tests
The origin of ultra diffuse galaxies: stellar feedback and quenching
We test if the cosmological zoom-in simulations of isolated galaxies from the
FIRE project reproduce the properties of ultra diffuse galaxies. We show that
stellar feedback-generated outflows that dynamically heat galactic stars,
together with a passively aging stellar population after imposed quenching
(from e.g. infall into a galaxy cluster), naturally reproduce the observed
population of red UDGs, without the need for high spin halos or dynamical
influence from their host cluster. We reproduce the range of surface
brightness, radius and absolute magnitude of the observed z=0 red UDGs by
quenching simulated galaxies at a range of different times. They represent a
mostly uniform population of dark matter-dominated galaxies with M_star ~1e8
Msun, low metallicity and a broad range of ages. The most massive simulated
UDGs require earliest quenching and are therefore the oldest. Our simulations
provide a good match to the central enclosed masses and the velocity
dispersions of the observed UDGs (20-50 km/s). The enclosed masses of the
simulated UDGs remain largely fixed across a broad range of quenching times
because the central regions of their dark matter halos complete their growth
early. A typical UDG forms in a dwarf halo mass range of Mh~4e10-1e11 Msun. The
most massive red UDG in our sample requires quenching at z~3 when its halo
reached Mh ~ 1e11 Msun. If it, instead, continues growing in the field, by z=0
its halo mass reaches > 5e11 Msun, comparable to the halo of an L* galaxy. If
our simulated dwarfs are not quenched, they evolve into bluer low-surface
brightness galaxies with mass-to-light ratios similar to observed field dwarfs.
While our simulation sample covers a limited range of formation histories and
halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy
type around Ms~1e8 Msun, both in the field and in clusters.Comment: 20 pages, 13 figures; match the MNRAS accepted versio
The Great Observatories Origins Deep Survey: Constraints on the Lyman Continuum Escape Fraction Distribution of Lyman--Break Galaxies at 3.4<z<4.5
We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1
narrow-band imaging in the GOODS Southern field to derive limits on the
distribution of the escape fraction (f_esc) of ionizing radiation for L >~
L*(z=3) Lyman Break Galaxies (LBGs) at redshift 3.4--4.5. Only one LBG, at
redshift z=3.795, is detected in its Lyman continuum (LyC; S/N~5.5), the
highest redshift galaxy currently known with a direct detection. Its
ultraviolet morphology is quite compact (R_eff=0.8, kpc physical). Three out of
seven AGN are also detected in their LyC, including one at redshift z=3.951 and
z850 = 26.1. From stacked data (LBGs) we set an upper limit to the average
f_esc in the range 5%--20%, depending on the how the data are selected (e.g.,
by magnitude and/or redshift). We undertake extensive Monte Carlo simulations
that take into account intergalactic attenuation, stellar population synthesis
models, dust extinction and photometric noise in order to explore the moments
of the distribution of the escaping radiation. Various distributions
(exponential, log-normal and Gaussian) are explored. We find that the median
f_esc is lower than ~6% with an 84% percentile limit not larger than 20%. If
this result remains valid for fainter LBGs down to current observational
limits, then the LBG population might be not sufficient to account for the
entire photoionization budget at the redshifts considered here, with the exact
details dependent upon the assumed ionizing background and QSO contribution
thereto. It is possible that f_esc depends on the UV luminosity of the
galaxies, with fainter galaxies having higher f_esc, and estimates of f_esc
from a sample of faint LBG from the HUDF (i775<28.5) are in broad quantitative
agreement with such a scenario.Comment: 58 pages, 23 figures; submitted to ApJ, revised version in response
to referee's comment
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Variations in the slope of the resolved star-forming main sequence: A tool for constraining the mass of star-forming regions
The correlation between galaxies' integrated stellar masses and star formation rates (the 'star formation main sequence', SFMS) is a well-established scaling relation. Recently, surveys have found a relationship between the star formation rate (SFR) and stellar mass surface densities on kpc and sub-kpc scales (the 'resolved SFMS', rSFMS). In this work, we demonstrate that the rSFMS emerges naturally in Feedback In Realistic Environments 2 (FIRE-2) zoom-in simulations of Milky Way-mass galaxies. We make SFR and stellar mass maps of the simulated galaxies at a variety of spatial resolutions and star formation averaging time-scales and fit the rSFMS using multiple methods from the literature. While the absolute value of the SFMS slope (αMS) depends on the fitting method, the slope is steeper for longer star formation time-scales and lower spatial resolutions regardless of the fitting method employed. We present a toy model that quantitatively captures the dependence of the simulated galaxies' αMS on spatial resolution and use it to illustrate how this dependence can be used to constrain the characteristic mass of star-forming clumps
A High-Frequency Search for Pulsars Within the Central Parsec of SgrA*
We report results from a deep high-frequency search for pulsars within the
central parsec of Sgr A* using the Green Bank Telescope. The observing
frequency of 15 GHz was chosen to maximize the likelihood of detecting normal
pulsars (i.e. with periods of \,ms and spectral indices of ) close to Sgr A*, that might be used as probes of gravity in the
strong-field regime; this is the highest frequency used for such pulsar
searches of the Galactic Center to date. No convincing candidate was detected
in the survey, with a detection threshold of Jy
achieved in two separate observing sessions. This survey represents a
significant improvement over previous searches for pulsars at the Galactic
Center and would have detected a significant fraction ($\gtrsim 5%) of the
pulsars around Sgr A*, if they had properties similar to those of the known
population. Using our best current knowledge of the properties of the Galactic
pulsar population and the scattering material toward Sgr A*, we estimate an
upper limit of 90 normal pulsars in orbit within the central parsec of Sgr A*.Comment: 10 pages, 7 figures, accepted for publication in the ApJ
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
G-protein Signaling Modulator-3 Regulates Heterotrimeric G-protein Dynamics through Dual Association with Gβ and Gα i Protein Subunits
Regulation of the assembly and function of G-protein heterotrimers (Gα·GDP/Gβγ) is a complex process involving the participation of many accessory proteins. One of these regulators, GPSM3, is a member of a family of proteins containing one or more copies of a small regulatory motif known as the GoLoco (or GPR) motif. Although GPSM3 is known to bind Gαi·GDP subunits via its GoLoco motifs, here we report that GPSM3 also interacts with the Gβ subunits Gβ1 to Gβ4, independent of Gγ or Gα·GDP subunit interactions. Bimolecular fluorescence complementation studies suggest that the Gβ-GPSM3 complex is formed at, and transits through, the Golgi apparatus and also exists as a soluble complex in the cytoplasm. GPSM3 and Gβ co-localize endogenously in THP-1 cells at the plasma membrane and in a juxtanuclear compartment. We provide evidence that GPSM3 increases Gβ stability until formation of the Gβγ dimer, including association of the Gβ-GPSM3 complex with phosducin-like protein PhLP and T-complex protein 1 subunit eta (CCT7), two known chaperones of neosynthesized Gβ subunits. The Gβ interaction site within GPSM3 was mapped to a leucine-rich region proximal to the N-terminal side of its first GoLoco motif. Both Gβ and Gαi·GDP binding events are required for GPSM3 activity in inhibiting phospholipase-Cβ activation. GPSM3 is also shown in THP-1 cells to be important for Akt activation, a known Gβγ-dependent pathway. Discovery of a Gβ/GPSM3 interaction, independent of Gα·GDP and Gγ involvement, adds to the combinatorial complexity of the role of GPSM3 in heterotrimeric G-protein regulation
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. II: anisotropy
We investigate the effects of stochasticity in the spatial and temporal
distribution of supernova remnants on the anisotropy of cosmic rays observed at
Earth. The calculations are carried out for different choices of the diffusion
coefficient D(E) for propagation in the Galaxy. The propagation and spallation
of nuclei are taken into account. At high energies we assume that
, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars with and without
accounting for the spiral structure of the Galaxy. Our calculations allow us to
determine the contribution to anisotropy resulting from both the large scale
distribution of SNRs in the Galaxy and the random distribution of the nearest
remnants. The naive expectation that the anisotropy amplitude scales as D(E) is
shown to be an oversimplification which does not reflect in the predicted
anisotropy for any realistic distribution of the sources. The fluctuations in
the anisotropy pattern are dominated by nearby sources, so that predicting or
explaining the observed anisotropy amplitude and phase becomes close to
impossible. We find however that the very weak energy dependence of the
anisotropy amplitude below GeV and the rise at higher energies, can
best be explained if the diffusion coefficient is . Faster
diffusion, for instance with , leads in general to an exceedingly
large anisotropy amplitude. The spiral structure introduces interesting trends
in the energy dependence of the anisotropy pattern, which qualitatively reflect
the trend seen in the data. For large values of the halo size we find that the
anisotropy becomes dominated by the large scale regular structure of the source
distribution, leading indeed to a monotonic increase of with energy.Comment: 21 Pages, to appear in JCA
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