7,929 research outputs found
Mission: Impossible (Escape from the Lyman Limit)
We investigate the intrinsic opacity of high-redshift galaxies to outgoing
ionising photons using high-quality photometry of a sample of 27
spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep
Field. Our measurement is based on maximum-likelihood fitting of model galaxy
spectral energy distributions-including the effects of intrinsic Lyman-limit
absorption and random realizations of intervening Lyman-series and Lyman-limit
absorption-to photometry of galaxies from space- and ground-based broad-band
images. Our method provides several important advantages over the methods used
by previous groups, including most importantly that two-dimensional sky
subtraction of faint-galaxy images is more robust than one-dimensional sky
subtraction of faint-galaxy spectra. We find at the 3sigma statistical
confidence level that on average no more than 4% of the ionising photons escape
galaxies of redshift 1.9<z<3.5. This result is consistent with observations of
low- and moderate-redshift galaxies but is in direct contradiction to a recent
result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies.
Dividing our sample in subsamples according to luminosity, intrinsic
ultraviolet colour, and redshift, we find no evidence for selection effects
that could explain such discrepancy. Even when all systematic effects are
included, the data could not realistically accomodate any escape fraction value
larger than ~15%.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 8 pages, 4 b/w figures, MNRAS styl
Phenolics, depsides and triterpenes from the chilean lichen pseudocyphellaria nudata (zahlbr.) D.J. Galloway
Indexación: ScieloThe lichen Pseudocyphellaria nudata is a species endemic to southern South América. From the lichen tallus, methyl orsellinate, 2-methoxy-3,6-dimethyl-4-hydroxybenzaldehyde, methyl-evernate, tenuiorin, hopan-6ß,22-diol and hopan-6α,76,22-triol were isolated and identified as the main lichen constituents. This is the first report of the occurrence of 2-methoxy-3,6-dimethyl-4-hydroxybenzaldehyde in lichens.http://www.scielo.cl/scielo.php?script=sci_arttext&pid=s0717-97072008000300017&nrm=is
Diffusion in a Granular Fluid - Simulation
The linear response description for impurity diffusion in a granular fluid
undergoing homogeneous cooling is developed in the preceeding paper. The
formally exact Einstein and Green-Kubo expressions for the self-diffusion
coefficient are evaluated there from an approximation to the velocity
autocorrelation function. These results are compared here to those from
molecular dynamics simulations over a wide range of density and inelasticity,
for the particular case of self-diffusion. It is found that the approximate
theory is in good agreement with simulation data up to moderate densities and
degrees of inelasticity. At higher density, the effects of inelasticity are
stronger, leading to a significant enhancement of the diffusion coefficient
over its value for elastic collisions. Possible explanations associated with an
unstable long wavelength shear mode are explored, including the effects of
strong fluctuations and mode coupling
The LyAlpha Line Profiles of Ultraluminous Infrared Galaxies: Fast Winds and Lyman Continuum Leakage
We present new Hubble Space Telescope Cosmic Origins Spectrograph
far-ultraviolet (far-UV) spectroscopy and Keck Echellete optical spectroscopy
of 11 ultraluminous infrared galaxies (ULIRGs), a rare population of local
galaxies experiencing massive gas inflows, extreme starbursts, and prominent
outflows. We detect H Lyman alpha emission from 8 ULIRGs and the companion to
IRAS09583+4714. In contrast to the P Cygni profiles often seen in galaxy
spectra, the H Lyman alpha profiles exhibit prominent, blueshifted emission out
to Doppler shifts exceeding -1000 km/s in three HII-dominated and two
AGN-dominated ULIRGs. To better understand the role of resonance scattering in
shaping the H Lyman alpha line profiles, we directly compare them to
non-resonant emission lines in optical spectra. We find that the line wings are
already present in the intrinsic nebular spectra, and scattering merely
enhances the wings relative to the line core. The H Lyman alpha attenuation (as
measured in the COS aperture) ranges from that of the far-UV continuum to over
100 times more. A simple radiative transfer model suggests the H Lyman alpha
photons escape through cavities which have low column densities of neutral
hydrogen and become optically thin to the Lyman continuum in the most advanced
mergers. We show that the properties of the highly blueshifted line wings on
the H Lyman alpha and optical emission-line profiles are consistent with
emission from clumps of gas condensing out of a fast, hot wind. The luminosity
of the H Lyman alpha emission increases non-linearly with the ULIRG bolometric
luminosity and represents about 0.1 to 1% of the radiative cooling from the hot
winds in the HII-dominated ULIRGs.Comment: Submitted to Ap
Role of NASA's SeaBASS Repository for the Legacy of the EXPORTS Field Biogeochemical Measurements
Role of NASA's SeaBASS repository for the legacy of the EXPORTS field biogeochemical measurements
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