2,425 research outputs found
The power spectrum of the flux distribution in the Lyman-alpha forest of a Large sample of UVES QSO Absorption Spectra (LUQAS)
The flux power spectra of the Lyman-alpha forest from a sample of 27 QSOs
taken with the high resolution echelle spectrograph UVES on VLT are presented.
We find a similar fluctuation amplitude at the peak of the ``3D'' flux power
spectrum at k ~ 0.03 (km/sec)^(-1) as the study by Croft et al. (2002), in the
same redshift range. The amplitude of the flux power spectrum increases with
decreasing redshift if corrected for the increase in the mean flux level as
expected if the evolution of the flux power spectrum is sensitive to the
gravitational growth of matter density fluctuations. This is in agreement with
the findings of McDonald et al. (2000) at larger redshift. The logarithmic
slope of the "3D" flux power spectrum, P_F(k), at large scales k < 0.03
(km/sec)^(-1), is 1.4 +- 0.3, i.e. 0.3 shallower than that found by Croft et
al. (2002) but consistent within the errors.Comment: 18 pages, 9 PS figures, 6 tables. Note that the k-values of the 1D
flux power spectrum had been erroneously shifted by half a bin size (in log
k) in the previous version. All the other results are unaffected. New tables
can be found at http://www.ast.cam.ac.uk/~rtnigm/luqas.ht
Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest
We compare the improved measurement of the Lya forest flux probability
distribution at 1.7<z<3.2 presented by Kim et al. (2007) to a large set of
hydrodynamical simulations of the Lya forest with different cosmological
parameters and thermal histories. The simulations are in good agreement with
the observational data if the temperature-density relation for the low density
intergalactic medium (IGM), T=T_0 Delta^{gamma-1}, is either close to
isothermal or inverted (gamma<1). Our results suggest that the voids in the IGM
may be significantly hotter and the thermal state of the low density IGM may be
substantially more complex than is usually assumed at these redshifts. We
discuss radiative transfer effects which alter the spectral shape of ionising
radiation during the epoch of HeII reionisation as a possible physical
mechanism for achieving an inverted temperature-density relation at z~3.Comment: 16 pages, 6 figures, accepted for publication in MNRAS following
minor revision. The accepted version includes an expanded discussion of the
flux power spectru
Compact strain-sensitive flexible photonic crystals for sensors
A promising fabrication route to produce absorbing flexible photonic crystals is presented, which exploits self-assembly during the shear processing of multi-shelled polymer spheres. When absorbing material is incorporated in the interstitial space surrounding high-refractive-index spheres, a dramatic enhancement in the transmission edge on the short-wavelength side of the band gap is observed. This effect originates from the shifting optical field spatial distribution as the incident wavelength is tuned around the band gap, and results in a contrast up to 100 times better than similar but nonabsorbing photonic crystals. An order-of-magnitude improvement in strain sensitivity is shown, suggesting the use of these thin films in photonic sensors
The effect of (strong) discrete absorption systems on the Lyman α forest flux power spectrum
We demonstrate that the Lyman alpha forest flux power spectrum of 'randomized' quasi-stellar object (QSO) absorption spectra is comparable in shape and amplitude to the flux power spectrum of the original observed spectra. In the randomized spectra a random shift in wave-length has been added to the observed absorption lines as identified and fitted with VPFIT. At 0.03 s km(-1) 15 contribute at large scales, k < 0.03 s km(-1). We further show that a fraction of &GSIM; 15 per cent of the mean flux decrement is contributed by strong absorbers at z &GSIM; 2.1. Analysis of the flux power spectrum which use numerical simulations with too few strong absorption systems calibrated with the observed mean flux may underestimate the inferred rms fluctuation amplitude and the slope of the initial dark matter power spectrum
The intergalactic medium thermal history at redshift z=1.7--3.2 from the Lyman alpha forest: a comparison of measurements using wavelets and the flux distribution
We investigate the thermal history of the intergalactic medium (IGM) in the
redshift interval z=1.7--3.2 by studying the small-scale fluctuations in the
Lyman alpha forest transmitted flux. We apply a wavelet filtering technique to
eighteen high resolution quasar spectra obtained with the Ultraviolet and
Visual Echelle Spectrograph (UVES), and compare these data to synthetic spectra
drawn from a suite of hydrodynamical simulations in which the IGM thermal state
and cosmological parameters are varied. From the wavelet analysis we obtain
estimates of the IGM thermal state that are in good agreement with other
recent, independent wavelet-based measurements. We also perform a reanalysis of
the same data set using the Lyman alpha forest flux probability distribution
function (PDF), which has previously been used to measure the IGM
temperature-density relation. This provides an important consistency test for
measurements of the IGM thermal state, as it enables a direct comparison of the
constraints obtained using these two different methodologies. We find the
constraints obtained from wavelets and the flux PDF are formally consistent
with each other, although in agreement with previous studies, the flux PDF
constraints favour an isothermal or inverted IGM temperature-density relation.
We also perform a joint analysis by combining our wavelet and flux PDF
measurements, constraining the IGM thermal state at z=2.1 to have a temperature
at mean density of T0/[10^3 K]=17.3 +/- 1.9 and a power-law temperature-density
relation exponent gamma=1.1 +/- 0.1 (1 sigma). Our results are consistent with
previous observations that indicate there may be additional sources of heating
in the IGM at z<4.Comment: 15 pages, 14 figures, matches version accepted for publication on
MNRA
Is radiographic measurement of distal femoral torsion reliable?
BACKGROUND: Distal femur torsion (DFT) is a crucial parameter in knee replacement surgery. The reference standard for measuring DFT is posterior condylar angle (PCA) measurement using computed tomography (CT). The objective of this study was to assess the feasibility and reliability of a radiographic PCA measurement method.
MATERIALS AND METHODS: We studied 125 osteoarthritic knees in 79 patients (42 women and 37 men) with a mean age of 71.6 ± 8.8 years (range 47 to 86 years); 32 knees were aligned, 85 in varus, and eight in valgus. DFT was measured on an antero-posterior (AP) radiograph of the knee in 90° of flexion (known as the seated AP view). The PCA was defined as the angle subtended by the tangent to the posterior condyles and the transepicondylar axis (anatomic PCA [aPCA]) or the line connecting the lateral epicondyle to the medial sulcus (surgical PCA [sPCA]). The PCA was conventionally recorded as positive in the event of external torsion and negative in the event of internal torsion. PCA measurements were performed three times by each of five observers to allow assessments of inter-observer and test-retest reliabilities.
RESULTS: aPCA was consistently negative (mean, -6.1 ± 1.6°) (range, 0 to -10°); inter-observer and test-retest reliability were satisfactory (0.54
CONCLUSION: Radiographic measurement of DFT is simple and non-invasive. Measurement reproducibility was satisfactory for aPCA but not for sPCA. aPCA showed marked inter-individual variability and tended to increase when the knee was in valgus. Mean aPCA values were comparable to those reported using CT. In contrast to CT, radiographic DFT measurement can easily be incorporated into the pre- and postoperative work-ups for knee replacement surgery, provided the patient can achieve 90° of knee flexion.
LEVEL OF EVIDENCE: Level IV, prospective cohort study
Biodegradation of Plastics Induced by Marine Organisms: Future Perspectives for Bioremediation Approaches
Plastic pollution is a distinctive element of the globalized world. In fact, since the 1970s the expansion and use of plastics, particularly in the consumer and commercial sectors, has given this material a permanent place in our lives. The increasing use of plastic products and the wrong management of end-of-life plastic products have contributed to increasing environmental pollution, with negative impacts on our ecosystems and the ecological functions of natural habitats. Nowadays, plastic pollution is pervasive in all environmental compartments. As aquatic environments are the dumping points for poorly managed plastics, biofouling and biodegradation have been proposed as promising approaches for plastic bioremediation. Known for the high stability of plastics in the marine environment, this represents a very important issue to preserve marine biodiversity. In this review, we have summarized the main cases reported in the literature on the degradation of plastics by bacteria, fungi, and microalgae and the degradation mechanisms involved, to highlight the potential of bioremediation approaches to reduce macro and microplastic pollution
An improved measurement of the flux distribution of the Ly\u3b1 forest in QSO absorption spectra: the effect of continuum fitting, metal contamination and noise properties
We have performed an extensive Voigt profile analysis of the neutral hydrogen (HI) and metal absorption present in a sample of 18 high resolution, high signal-to-noise QSO spectra observed with VLT/UVES. We use this analysis to separate the metal contribution from the HI absorption and present an improved measurement of the flux probability distribution function (PDF) due to HI absorption alone at = 2.07, 2.52, and 2.94. The flux PDF is sensitive to the continuum fit in the normalised flux range 0.8 < F < 1.0 and to metal absorption at 0.2 < F < 0.8. Our new measurements of the flux PDF due to HI absorption alone are systematically lower at 0.2 < F < 0.8 by up to 30% compared to the widely used measurement of McDonald et al. (2000), based on a significantly smaller sample of Keck/HIRES data. This discrepancy is probably due to a combination of our improved removal of the metal absorption and cosmic variance, since variations in the flux PDF between different lines-of-sight are large. The HI effective optical depth at 1.7 < z < 4 is best fit with a single power law, (0.0023 +-0.0007) (1+z)^(3.65 +- 0.21), in good agreement with previous measurements from comparable data. As also found previously, the effect of noise on the flux distribution is not significant in high resolution, high signal-to-noise data
Studying the WHIM with Gamma Ray Bursts
We assess the possibility to detect and characterize the physical state of
the missing baryons at low redshift by analyzing the X-ray absorption spectra
of the Gamma Ray Burst [GRB] afterglows, measured by a micro calorimeters-based
detector with 3 eV resolution and 1000 cm2 effective area and capable of fast
re-pointing, similar to that on board of the recently proposed X-ray satellites
EDGE and XENIA. For this purpose we have analyzed mock absorption spectra
extracted from different hydrodynamical simulations used to model the
properties of the Warm Hot Intergalactic Medium [WHIM]. These models predict
the correct abundance of OVI absorption lines observed in UV and satisfy
current X-ray constraints. According to these models space missions like EDGE
and XENIA should be able to detect about 60 WHIM absorbers per year through the
OVII line. About 45 % of these have at least two more detectable lines in
addition to OVII that can be used to determine the density and the temperature
of the gas. Systematic errors in the estimates of the gas density and
temperature can be corrected for in a robust, largely model-independent
fashion. The analysis of the GRB absorption spectra collected in three years
would also allow to measure the cosmic mass density of the WHIM with about 15 %
accuracy, although this estimate depends on the WHIM model. Our results suggest
that GRBs represent a valid, if not preferable, alternative to Active Galactic
Nuclei to study the WHIM in absorption. The analysis of the absorption spectra
nicely complements the study of the WHIM in emission that the spectrometer
proposed for EDGE and XENIA would be able to carry out thanks to its high
sensitivity and large field of view.Comment: 16 pages, 16 figures, accepted for publication by Ap
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