6,254 research outputs found
An extreme [OIII] emitter at : a low metallicity Lyman continuum source
[Abridged] We investigate the physical properties of a Lyman continuum
emitter candidate at with photometric coverage from to MIPS
24m band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. Investigation of the
UV spectrum confirms a direct spectroscopic detection of the Lyman continuum
emission with . Non-zero Ly flux at the systemic redshift and
high Lyman- escape fraction suggest a low HI column density. The weak C
and Si low-ionization absorption lines are also consistent with a low covering
fraction along the line of sight. The
[OIII] equivalent width is one of the
largest reported for a galaxy at
(, rest-frame) and the NIR spectrum shows that this is mainly due to an
extremely strong [OIII] emission. The large observed [OIII]/[OII] ratio ()
and high ionization parameter are consistent with prediction from
photoionization models in case of a density-bounded nebula scenario.
Furthermore, the
is
comparable to recent measurements reported at , in the reionization
epoch. We also investigate the possibility of an AGN contribution to explain
the ionizing emission but most of the AGN identification diagnostics suggest
that stellar emission dominates instead. This source is currently the first
high- example of a Lyman continuum emitter exhibiting indirect and direct
evidences of a Lyman continuum leakage and having physical properties
consistent with theoretical expectation from Lyman continuum emission from a
density-bounded nebula.Comment: 14 pages, 11 figures, accepted for publication in A&A. Minor
modifications, Figure 2 updated, Figure 9 adde
Nyquist method for Wigner-Poisson quantum plasmas
By means of the Nyquist method, we investigate the linear stability of
electrostatic waves in homogeneous equilibria of quantum plasmas described by
the Wigner-Poisson system. We show that, unlike the classical Vlasov-Poisson
system, the Wigner-Poisson case does not necessarily possess a Penrose
functional determining its linear stability properties. The Nyquist method is
then applied to a two-stream distribution, for which we obtain an exact,
necessary and sufficient condition for linear stability, as well as to a
bump-in-tail equilibrium.Comment: 6 figure
Cold ideal equation of state for strongly magnetized neutron-star matter: effects on muon production and pion condensationn
Neutron stars with very strong surface magnetic fields have been suggested as
the site for the origin of observed soft gamma repeaters (SGRs). In this paper
we investigate the influence of such strong magnetic fields on the properties
and internal structure of these magnetized neutron stars (magnetars). We study
properties of a degenerate equilibrium ideal neutron-proton-electron (npe) gas
with and without the effects of the anomalous nucleon magnetic moments in a
magnetic field. The presence of a sufficiently strong magnetic field changes
the ratio of protons to neutrons as well as the neutron drip density. We also
study the appearance of muons as well as pion condensation in strong magnetic
fields. We discuss the possibility that boson condensation in the interior of
magnetars might be a source of SGRs.Comment: 10 pages included 9 figures, ApJ in pres
^{17}O and ^{51}V NMR for the zigzag spin-1 chain compound CaV2O4
V NMR studies on CaV2O4 single crystals and O NMR studies on
O-enriched powder samples are reported. The temperature dependences of
the O NMR line width and nuclear spin-lattice relaxation rate give
strong evidence for a long-range antiferromagnetic transition at Tn = 78 K in
the powder. Magnetic susceptibility measurements show that Tn = 69 K in the
crystals. A zero-field V NMR signal was observed at low temperatures (f
237 MHz at 4.2 K) in the crystals. The field swept spectra with the
field in different directions suggest the presence of two antiferromagnetic
substructures. Each substructure is collinear, with the easy axes of the two
substructures separated by an angle of 19(1) degree, and with their average
direction pointing approximately along the b-axis of the crystal structure. The
two spin substructures contain equal number of spins. The temperature
dependence of the ordered moment, measured up to 45 K, shows the presence of an
energy gap Eg in the antiferromagnetic spin wave excitation spectrum.
Antiferromagnetic spin wave theory suggests that Eg lies between 64 and 98 K.Comment: 11 pages, 14 figures. v2: 2 new figures; version published in Phys.
Rev.
Near-infrared thermal emissivity from ground based atmospheric dust measurements at ORM
We present an analysis of the atmospheric content of aerosols measured at
Observatorio del Roque de los Muchachos (ORM; Canary Islands). Using a laser
diode particle counter located at the Telescopio Nazionale Galileo (TNG) we
have detected particles of 0.3, 0.5, 1.0, 3.0, 5.0 and 10.0 um size. The
seasonal behavior of the dust content in the atmosphere is calculated. The
Spring has been found to be dustier than the Summer, but dusty conditions may
also occur in Winter. A method to estimate the contribution of the aerosols
emissivity to the sky brightness in the near-infrared (NIR) is presented. The
contribution of dust emission to the sky background in the NIR has been found
to be negligible comparable to the airglow, with a maximum contribution of
about 8-10% in the Ks band in the dusty days.Comment: 6 pages, 3 figures, 6 tables, accepted for publication in MNRA
Hubble imaging of the ionizing radiation from a star-forming galaxy at z=3.2 with fesc>50%
Star-forming galaxies are considered to be the leading candidate sources that
dominate the cosmic reionization at z>7, and the search for analogs at moderate
redshift showing Lyman continuum (LyC) leakage is currently a active line of
research. We have observed a star-forming galaxy at z=3.2 with Hubble/WFC3 in
the F336W filter, corresponding to the 730-890A rest-frame, and detect LyC
emission. This galaxy is very compact and also has large Oxygen ratio
[OIII]5007/[OII]3727 (>=10). No nuclear activity is revealed from
optical/near-infrared spectroscopy and deep multi-band photometry (including
the 6Ms X-ray, Chandra). The measured escape fraction of ionizing radiation
spans the range 50-100\%, depending on the IGM attenuation. The LyC emission is
detected at S/N=10 with m(F336W)=27.57+/-0.11 and it is spatially unresolved,
with effective radius R_e<200pc. Predictions from photoionization and radiative
transfer models are in line with the properties reported here, indicating that
stellar winds and supernova explosions in a nucleated star-forming region can
blow cavities generating density-bounded conditions compatible with optically
thin media. Irrespective to the nature of the ionizing radiation, spectral
signatures of these sources over the entire electromagnetic spectrum are of
central importance for their identification during the epoch of reionization,
when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometric
signature of intense rest-frame optical emissions ([OIII]+Hbeta) observed
recently at z~7.5-8.5 is similar to what is observed in this galaxy. Only the
James Webb Space Telescope will measure optical line ratios at z>7 allowing a
direct comparison with lower redshift LyC emitters, as reported here.Comment: 6 pages, 5 figures, ApJ submitted (comments welcome
Shear stress fluctuations in the granular liquid and solid phases
We report on experimentally observed shear stress fluctuations in both
granular solid and fluid states, showing that they are non-Gaussian at low
shear rates, reflecting the predominance of correlated structures (force
chains) in the solidlike phase, which also exhibit finite rigidity to shear.
Peaks in the rigidity and the stress distribution's skewness indicate that a
change to the force-bearing mechanism occurs at the transition to fluid
behaviour, which, it is shown, can be predicted from the behaviour of the
stress at lower shear rates. In the fluid state stress is Gaussian distributed,
suggesting that the central limit theorem holds. The fibre bundle model with
random load sharing effectively reproduces the stress distribution at the yield
point and also exhibits the exponential stress distribution anticipated from
extant work on stress propagation in granular materials.Comment: 11 pages, 3 figures, latex. Replacement adds journal reference and
addresses referee comment
Interplay between carrier and impurity concentrations in annealed GaMnAs intrinsic anomalous Hall Effect
Investigating the scaling behavior of annealed GaMnAs anomalous
Hall coefficients, we note a universal crossover regime where the scaling
behavior changes from quadratic to linear, attributed to the anomalous Hall
Effect intrinsic and extrinsic origins, respectively. Furthermore, measured
anomalous Hall conductivities when properly scaled by carrier concentration
remain constant, equal to theoretically predicated values, spanning nearly a
decade in conductivity as well as over 100 K in T. Both the qualitative
and quantitative agreement confirms the validity of new equations of motion
including the Berry phase contributions as well as tunablility of the intrinsic
anomalous Hall Effect.Comment: 4 pages, 5 figure
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