26,547 research outputs found
Spectroscopic Measurements of the Far-Ultraviolet Dust Attenuation Curve at z~3
We present the first measurements of the shape of the far-ultraviolet
(far-UV; lambda=950-1500 A) dust attenuation curve at high redshift (z~3). Our
analysis employs rest-frame UV spectra of 933 galaxies at z~3, 121 of which
have very deep spectroscopic observations (>7 hrs) at lambda=850-1300 A, with
the Low Resolution Imaging Spectrograph on the Keck Telescope. By using an
iterative approach in which we calculate the ratios of composite spectra in
different bins of continuum color excess, E(B-V), we derive a dust curve that
implies a lower attenuation in the far-UV for a given E(B-V) than those
obtained with standard attenuation curves. We demonstrate that the UV composite
spectra of z~3 galaxies can be modeled well by assuming our new attenuation
curve, a high covering fraction of HI, and absorption from the Lyman-Werner
bands of H2 with a small (<20%) covering fraction. The low covering fraction of
H2 relative to that of the HI and dust suggests that most of the dust in the
ISM of typical galaxies at z~3 is unrelated to the catalysis of H2, and is
associated with other phases of the ISM (i.e., the ionized and neutral gas).
The far-UV dust curve implies a factor of ~2 lower dust attenuation of Lyman
continuum (ionizing) photons relative to those inferred from the most commonly
assumed attenuation curves for L* galaxies at z~3. Our results may be utilized
to assess the degree to which ionizing photons are attenuated in HII regions
or, more generally, in the ionized or low column density (N(HI)<10^17.2 cm^-2)
neutral ISM of high-redshift galaxies.Comment: 12 pages, 1 table, 8 figures, accepted to the Astrophysical Journa
Dispersion and decay of collective modes in neutron star cores
We calculate the frequencies of collective modes of neutrons, protons and
electrons in the outer core of neutron stars. The neutrons and protons are
treated in a hydrodynamic approximation and the electrons are regarded as
collisionless. The coupling of the nucleons to the electrons leads to Landau
damping of the collective modes and to significant dispersion of the low-lying
modes. We investigate the sensitivity of the mode frequencies to the strength
of entrainment between neutrons and protons, which is not well characterized.
The contribution of collective modes to the thermal conductivity is evaluated.Comment: 10 pages, 4 figure
X-ray properties of UV-selected star forming galaxies at z~1 in the Hubble Deep Field North
We present an analysis of the X-ray emission from a large sample of
ultraviolet (UV) selected, star forming galaxies with 0.74<z<1.32 in the Hubble
Deep Field North (HDF-N) region. By excluding all sources with significant
detected X-ray emission in the 2 Ms Chandra observation we are able to examine
the properties of galaxies for which the emission in both UV and X-ray is
expected to be predominantly due to star formation. Stacking the X-ray flux
from 216 galaxies in the soft and hard bands produces significant detections.
The derived mean 2-10 keV rest-frame luminosity is 2.97+/-0.26x10^(40) erg/s,
corresponding to an X-ray derived star formation rate (SFR) of 6.0+/-0.6
Msolar/yr. Comparing the X-ray value with the mean UV derived SFR, uncorrected
for attenuation, we find that the average UV attenuation correction factor is
\~3. By binning the galaxy sample according to UV magnitude and colour,
correlations between UV and X-ray emission are also examined. We find a strong
positive correlation between X-ray emission and rest-frame UV emission. A
correlation between the ratio of X-ray-to-UV emission and UV colour is also
seen, such that L(X)/L(UV) increases for redder galaxies. Given that X-ray
emission offers a view of star formation regions that is relatively unaffected
by extinction, results such as these can be used to evaluate the effects of
dust on the UV emission from high-z galaxies. For instance we derive a
relationship for estimating UV attenuation corrections as a function of colour
excess. The observed relation is inconsistent with the Calzetti et al. (2000)
reddening law which over predicts the range in UV attenuation corrections by a
factor of ~100 for the UV selected z~1 galaxies in this sample (abridged).Comment: 10 pages, 7 figures, accepted for publication in MNRA
The Connection Between Reddening, Gas Covering Fraction, and the Escape of Ionizing Radiation at High Redshift
We use a large sample of galaxies at z~3 to establish a relationship between
reddening, neutral gas covering fraction (fcov(HI)), and the escape of ionizing
photons at high redshift. Our sample includes 933 galaxies at z~3, 121 of which
have very deep spectroscopic observations (>7 hrs) in the rest-UV
(lambda=850-1300 A) with Keck/LRIS. Based on the high covering fraction of
outflowing optically-thick HI indicated by the composite spectra of these
galaxies, we conclude that photoelectric absorption, rather than dust
attenuation, dominates the depletion of ionizing photons. By modeling the
composite spectra as the combination of an unattenuated stellar spectrum
including nebular continuum emission with one that is absorbed by HI and
reddened by a line-of-sight extinction, we derive an empirical relationship
between E(B-V) and fcov(HI). Galaxies with redder UV continua have larger
covering fractions of HI characterized by higher line-of-sight extinctions. Our
results are consistent with the escape of Lya through gas-free lines-of-sight.
Covering fractions based on low-ionization interstellar absorption lines
systematically underpredict those deduced from the HI lines, suggesting that
much of the outflowing gas may be metal-poor. We develop a model which connects
the ionizing escape fraction with E(B-V), and which may be used to estimate the
escape fraction for an ensemble of high-redshift galaxies. Alternatively,
direct measurements of the escape fraction for our data allow us to constrain
the intrinsic 900-to-1500 A flux density ratio to be >0.20, a value that favors
stellar population models that include weaker stellar winds, a flatter initial
mass function, and/or binary evolution. Lastly, we demonstrate how the
framework discussed here may be used to assess the pathways by which ionizing
radiation escapes from high-redshift galaxies. [Abridged]Comment: 22 pages, 3 tables, 14 figures, accepted to the Astrophysical Journa
The Keck Lyman Continuum Spectroscopic Survey (KLCS): The Emergent Ionizing Spectrum of Galaxies at
We present results of a deep spectroscopic survey designed to quantify the
statistics of the escape of ionizing photons from star-forming galaxies at z~3.
We measure the ratio of ionizing to non-ionizing UV flux density
_obs, where f900 is the mean flux density evaluated over the range
[880,910] A. We quantify the emergent ratio of ionizing to non-ionizing UV flux
density by analyzing high-S/N composite spectra formed from sub-samples with
common observed properties and numbers sufficient to reduce the statistical
uncertainty in the modeled IGM+CGM correction to obtain precise values of
_out, including a full-sample average
_out=. We further show that _out
increases monotonically with Ly rest equivalent width, inducing an
inverse correlation with UV luminosity as a by-product. We fit the composite
spectra using stellar spectral synthesis together with models of the ISM in
which a fraction f_c of the stellar continuum is covered by gas with column
density N(HI). We show that the composite spectra simultaneously constrain the
intrinsic properties of the stars (L900/L1500)_int along with f_c, N(HI),
E(B-V), and , the absolute escape fraction of ionizing photons. We
find a sample-averaged , and that subsamples fall
along a linear relation . We use the FUV luminosity function, the distribution function
, and the relationship between and
_out to estimate the total ionizing emissivity of
star-forming galaxies with Muv < -19.5:
ergs/s/Hz/Mpc, exceeding the contribution of QSOs by a factor of ,
and accounting for % of the total at
estimated using indirect methods.Comment: 45 pages, 31 figures, ApJ, in pres
Computational and theoretical aspects of a grain-boundary model that accounts for grain misorientation and grain-boundary orientation
A detailed theoretical and numerical investigation of the infinitesimal
single-crystal gradient plasticity and grain-boundary theory of Gurtin (2008)
"A theory of grain boundaries that accounts automatically for grain
misorientation and grain-boundary orientation". Journal of the Mechanics and
Physics of Solids 56 (2), 640-662, is performed. The governing equations and
flow laws are recast in variational form. The associated incremental problem is
formulated in minimization form and provides the basis for the subsequent
finite element formulation. Various choices of the kinematic measure used to
characterize the ability of the grain boundary to impede the flow of
dislocations are compared. An alternative measure is also suggested. A series
of three-dimensional numerical examples serve to elucidate the theory
Integrated care and the working record
By default, many discussions and specifications of electronic health records or integrated care records often conceptualize the record as a passive information repository. This article presents data from a case study of work in a medical unit in a major metropolitan hospital. It shows how the clinicians tailored, re-presented and augmented clinical information to support their own roles in the delivery of care for individual patients. This is referred to as the working record: a set of complexly interrelated clinician-centred documents that are locally evolved, maintained and used to support delivery of care in conjunction with the more patient-centred chart that will be stored in the medical records department on the patient’s discharge. Implications are drawn for how an integrated care record could support the local tailorability and flexibility that underpin this working record and hence underpin practice
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