2,517 research outputs found
The Spitzer View of Low-Metallicity Star Formation: II. Mrk 996, a Blue Compact Dwarf Galaxy with an Extremely Dense Nucleus
(abridged) We present new Spitzer, UKIRT and MMT observations of the blue
compact dwarf galaxy (BCD) Mrk 996, with an oxygen abundance of
12+log(O/H)=8.0. This galaxy has the peculiarity of possessing an
extraordinarily dense nuclear star-forming region, with a central density of
~10^6 cm^{-3}. The nuclear region of Mrk 996 is characterized by several
unusual properties: a very red color J-K = 1.8, broad and narrow emission-line
components, and ionizing radiation as hard as 54.9 eV, as implied by the
presence of the OIV 25.89 micron line. The nucleus is located within an
exponential disk with colors consistent with a single stellar population of age
>1 Gyr. The infrared morphology of Mrk 996 changes with wavelength. The IRS
spectrum shows strong narrow Polycyclic Aromatic Hydrocarbon (PAH) emission,
with narrow line widths and equivalent widths that are high for the metallicity
of Mrk 996. Gaseous nebular fine-structure lines are also seen. A CLOUDY model
requires that they originate in two distinct HII regions: a very dense HII
region of radius ~580 pc with densities declining from ~10^6 at the center to a
few hundreds cm^{-3} at the outer radius, where most of the optical lines
arise; and a HII region with a density of ~300 cm^{-3} that is hidden in the
optical but seen in the MIR. We suggest that the infrared lines arise mainly in
the optically obscured HII region while they are strongly suppressed by
collisional deexcitation in the optically visible one. The hard ionizing
radiation needed to account for the OIV 25.89 micron line is most likely due to
fast radiative shocks propagating in an interstellar medium. A hidden
population of Wolf-Rayet stars of type WNE-w or a hidden AGN as sources of hard
ionizing radiation are less likely possibilities.Comment: 48 pages, 13 figures, accepted for publication in the Astrophysical
Journa
Three-component U-Pu-Th fuel for plutonium irradiation in heavy water reactors
This paper discusses concepts for three-component fuel bundles containing plutonium, uranium and thorium for use in pressurised heavy water reactors, and cases for and against implementation of such a nuclear energy system in the United Kingdom. Heavy water reactors are used extensively in Canada, and are deploying within India and China, whilst the UK is considering the use of heavy water reactors to manage its plutonium inventory of 140 tonnes. The UK heavy water reactor proposal uses a mixed oxide (MOX) fuel of plutonium in depleted uranium, within the enhanced CANDU-6 (EC-6) reactor. This work proposes an alternative heterogeneous fuel concept based on the same reactor and CANFLEX fuel bundle, with eight large-diameter fuel elements loaded with natural thorium oxide and 35 small-diameter fuel elements loaded with a MOX of plutonium and reprocessed uranium stocks from UK MAGNOX and AGR reactors. Indicative neutronic calculations suggest that such a fuel would be neutronically feasible. A similar MOX may alternatively be fabricated from reprocessed <5% enriched light water reactor fuel, such as the fuel of the AREVA EPR reactor, to consume newly produced plutonium from reprocessing, similar to the DUPIC (direct use of PWR fuel in CANDU) process
Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
We have carried out an analysis of the radio and optical properties of a
statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey
of nearby galaxies. We find that the space density of bright galaxies (-22 mag
<= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3}
Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy
types, radio spectra, and radio source sizes are uncorrelated with Seyfert
type, as predicted by the unified schemes for active galaxies. Approximately
half of the detected galaxies have flat or inverted radio spectra, more than
expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found
to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm,
particularly among the galaxies with the weakest nuclear activity. We suggest
that this difference can be accommodated in the unified schemes if a minimum
level of Seyfert activity is required for a radio source to emerge from the
vicinity of the active nucleus. Below this level, Seyfert radio sources might
be suppressed by free-free absorption associated with the nuclear torus or a
compact narrow-line region, thus accounting for both the weakness of the radio
emission and the preponderance of flat spectra. Alternatively, the flat spectra
and weak radio sources might indicate that the weak active nuclei are fed by
advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap
Frequency-Dependent Squeezing for Advanced LIGO
The first detection of gravitational waves by the Laser Interferometer
Gravitational-wave Observatory (LIGO) in 2015 launched the era of gravitational
wave astronomy. The quest for gravitational wave signals from objects that are
fainter or farther away impels technological advances to realize ever more
sensitive detectors. Since 2019, one advanced technique, the injection of
squeezed states of light is being used to improve the shot noise limit to the
sensitivity of the Advanced LIGO detectors, at frequencies above Hz.
Below this frequency, quantum back action, in the form of radiation pressure
induced motion of the mirrors, degrades the sensitivity. To simultaneously
reduce shot noise at high frequencies and quantum radiation pressure noise at
low frequencies requires a quantum noise filter cavity with low optical losses
to rotate the squeezed quadrature as a function of frequency. We report on the
observation of frequency-dependent squeezed quadrature rotation with rotation
frequency of 30Hz, using a 16m long filter cavity. A novel control scheme is
developed for this frequency-dependent squeezed vacuum source, and the results
presented here demonstrate that a low-loss filter cavity can achieve the
squeezed quadrature rotation necessary for the next planned upgrade to Advanced
LIGO, known as "A+."Comment: 6 pages, 2 figures, to be published in Phys. Rev. Let
Granger causality and transfer entropy are equivalent for Gaussian variables
Granger causality is a statistical notion of causal influence based on
prediction via vector autoregression. Developed originally in the field of
econometrics, it has since found application in a broader arena, particularly
in neuroscience. More recently transfer entropy, an information-theoretic
measure of time-directed information transfer between jointly dependent
processes, has gained traction in a similarly wide field. While it has been
recognized that the two concepts must be related, the exact relationship has
until now not been formally described. Here we show that for Gaussian
variables, Granger causality and transfer entropy are entirely equivalent, thus
bridging autoregressive and information-theoretic approaches to data-driven
causal inference.Comment: In review, Phys. Rev. Lett., Nov. 200
Systematic review and meta-analysis of temozolomide in animal models of glioma:was clinical efficacy predicted?
Background:Malignant glioma is an aggressive tumour commonly associated with a dismal outcome despite optimal surgical and radio-chemotherapy. Since 2005 temozolomide has been established as first-line chemotherapy. We investigate the role of in vivo glioma models in predicting clinical efficacy.Methods:We searched three online databases to systematically identify publications testing temozolomide in animal models of glioma. Median survival and number of animals treated were extracted and quality was assessed using a 12-point scale; random effects meta-analysis was used to estimate efficacy. We analysed the impact of study design and quality and looked for evidence of publication bias.Results:We identified 60 publications using temozolomide in models of glioma, comprising 2443 animals. Temozolomide prolonged survival by a factor of 1.88 (95% CI 1.74-2.03) and reduced tumour volume by 50.4% (41.8-58.9) compared with untreated controls. Study design characteristics accounted for a significant proportion of between-study heterogeneity, and there was evidence of a significant publication bias.Conclusion:These data reflect those from clinical trials in that temozolomide improves survival and reduces tumour volume, even after accounting for publication bias. Experimental in vivo glioma studies of temozolomide differ from those of other glioma therapies in their consistent efficacy and successful translation into clinical medicine
Multivariate Granger Causality and Generalized Variance
Granger causality analysis is a popular method for inference on directed
interactions in complex systems of many variables. A shortcoming of the
standard framework for Granger causality is that it only allows for examination
of interactions between single (univariate) variables within a system, perhaps
conditioned on other variables. However, interactions do not necessarily take
place between single variables, but may occur among groups, or "ensembles", of
variables. In this study we establish a principled framework for Granger
causality in the context of causal interactions among two or more multivariate
sets of variables. Building on Geweke's seminal 1982 work, we offer new
justifications for one particular form of multivariate Granger causality based
on the generalized variances of residual errors. Taken together, our results
support a comprehensive and theoretically consistent extension of Granger
causality to the multivariate case. Treated individually, they highlight
several specific advantages of the generalized variance measure, which we
illustrate using applications in neuroscience as an example. We further show
how the measure can be used to define "partial" Granger causality in the
multivariate context and we also motivate reformulations of "causal density"
and "Granger autonomy". Our results are directly applicable to experimental
data and promise to reveal new types of functional relations in complex
systems, neural and otherwise.Comment: added 1 reference, minor change to discussion, typos corrected; 28
pages, 3 figures, 1 table, LaTe
Financial rules of thumb: a review of the evidence and its implications
Cognitive biases are difficult to avoid and emotions are difficult to ignore. They can lead to erroneous decision-making in general and this can be particularly costly when it comes to financial decisions. The busy lives we lead and the plethora of financial products available to us only combine to make decision-making ever more difficult, with precious little time for alternatives to be evaluated and fully-informed, optimal decisions to be made. Financial rules of thumb can help us to avoid some cognitive biases, dampen emotions and allow decisions to be made even in complex scenarios and when time is scarce
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