436 research outputs found
The Space Density of Compton-thick AGN
We constrain the number density and evolution of Compton-thick Active
Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray
background. In the local Universe we use the wide area surveys from the Swift
and INTEGRAL satellites, while for high redshifts we explore candidate
selections based on mid-IR parameters. We present the properties of a sample of
211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South
(ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR
ratios for these sources are significantly larger than that of star-forming
galaxies and ~2 orders of magnitude smaller than for the general AGN
population, suggesting column densities of NH>5x10^24 cm^-2. The space density
of CT AGN at z~2 derived from these observations is ~10^-5 Mpc^{-3}, finding a
strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.Comment: 4 pages, 3 figures, to appear in proceedings for 'X-ray Astronomy
2009', Bologna 09/2009, AIP Conference Series, Eds. A. Comastri, M. Cappi, L.
Angelini; author list fixe
How to measure the spreading width for decay of superdeformed nuclei
A new expression for the branching ratio for the decay via the E1 process in
the normal-deformed band of superdeformed nuclei is given within a simple
two-level model. Using this expression, the spreading or tunneling width
Gamma^downarrow for superdeformed decay can be expressed entirely in terms of
experimentally known quantities. We show how to determine the tunneling matrix
element V from the measured value of Gamma^downarrow and a statistical model of
the energy levels. The accuracy of the two-level approximation is verified by
considering the effects of the other normal-deformed states.Comment: 4 pages, 4 figure
Development of a Mechanics Reasoning Inventory
Strategic knowledge is required to appropriately organize procedures and concepts to solve problems. We are developing a standardized instrument assessing strategic knowledge in the domain of introductory mechanics. This instrument is inspired in part by Lawson's Classroom Test of Scientific Reasoning and Van Domelen's Problem Decomposition Diagnostic. The predictive validity of the instrument has been suggested by preliminary studies showing significant correlation with performance on final exams administered in introductory mechanics courses at the Massachusetts Institute of Technology and the Georgia Institute of Technology. In order to study the validity of the content from the student's perspective, we have administered the instrument in free-response format to 40 students enrolled in calculus-based introductory mechanics at the University of Wisconsin-Platteville. This procedure has the additional advantage of improving the construct validity of the inventory, since student responses suggest effective distractors for the multiple-choice form of the inventory.National Science Foundation (U.S.) (PHY-0757931)National Science Foundation (U.S.) (DUE-1044294)National Institutes of Health (U.S.) (1-RC1-RR028302-01
Item response theory analysis of the mechanics baseline test
Item response theory is useful in both the development and evaluation of assessments and in computing standardized measures of student performance. In item response theory, individual parameters (difficulty, discrimination) for each item or question are fit by item response models. These parameters provide a means for evaluating a test and offer a better measure of student skill than a raw test score, because each skill calculation considers not only the number of questions answered correctly, but the individual properties of all questions answered. Here, we present the results from an analysis of the Mechanics Baseline Test given at MIT during 2005-2010. Using the item parameters, we identify questions on the Mechanics Baseline Test that are not effective in discriminating between MIT students of different abilities. We show that a limited subset of the highest quality questions on the Mechanics Baseline Test returns accurate measures of student skill. We compare student skills as determined by item response theory to the more traditional measurement of the raw score and show that a comparable measure of learning gain can be computed
Numerical experiments in 2D variational fracture
In the present work we present some results of numerical experiments obtained with a variationalmodel for quasi-static Griffith-type brittle fracture. Essentially the analysis is based on a recent formulation byFrancfort and Marigo the main difference being the fact that we rely on local rather than on globalminimization. Propagation of fracture is obtained by minimizing, in a step by step process, a form of energythat is the sum of bulk and interface terms. To solve the problem numerically we adopt discontinuous finiteelements based on variable meshes and search for the minima of the energy through descent methods. We use asort of mesh dependent relaxation of the interface energy to get out of small energy wells. The relaxationconsists in the adoption of a carefully tailored cohesive type interface energy, tending to the Griffith limit as themesh size tends to zero
When students can choose easy, medium, or hard homework problems
We investigate student-chosen, multi-level homework in our Integrated Learning Environment for Mechanics [1] built using the LON-CAPA [2] open-source learning system. Multi-level refers to problems categorized as easy, medium, and hard. Problem levels were determined a priori based on the knowledge needed to solve them [3]. We analyze these problems using three measures: time-per-problem, LON-CAPA difficulty, and item difficulty measured by item response theory. Our analysis of student behavior in this environment suggests that time-per-problem is strongly dependent on problem category, unlike either score-based measures. We also found trends in student choice of problems, overall effort, and efficiency across the student population. Allowing students choice in problem solving seems to improve their motivation; 70% of students worked additional problems for which no credit was given.National Science Foundation (U.S.) (Grant PHY-0757931)National Science Foundation (U.S.) (Grant DUE-1044294
Numerical experiments in 2D variational fracture
In the present work we present some results of numerical experiments obtained with a variationalmodel for quasi-static Griffith-type brittle fracture. Essentially the analysis is based on a recent formulation byFrancfort and Marigo the main difference being the fact that we rely on local rather than on globalminimization. Propagation of fracture is obtained by minimizing, in a step by step process, a form of energythat is the sum of bulk and interface terms. To solve the problem numerically we adopt discontinuous finiteelements based on variable meshes and search for the minima of the energy through descent methods. We use asort of mesh dependent relaxation of the interface energy to get out of small energy wells. The relaxationconsists in the adoption of a carefully tailored cohesive type interface energy, tending to the Griffith limit as themesh size tends to zero
The Multiwavelength Survey by Yale-Chile (MUSYC): Deep Medium-Band optical imaging and high quality 32-band photometric redshifts in the ECDF-S
We present deep optical 18-medium-band photometry from the Subaru telescope
over the ~30' x 30' Extended Chandra Deep Field-South (ECDF-S), as part of the
Multiwavelength Survey by Yale-Chile (MUSYC). This field has a wealth of
ground- and space-based ancillary data, and contains the GOODS-South field and
the Hubble Ultra Deep Field. We combine the Subaru imaging with existing
UBVRIzJHK and Spitzer IRAC images to create a uniform catalog. Detecting
sources in the MUSYC BVR image we find ~40,000 galaxies with R_AB<25.3, the
median 5 sigma limit of the 18 medium bands. Photometric redshifts are
determined using the EAZY code and compared to ~2000 spectroscopic redshifts in
this field. The medium band filters provide very accurate redshifts for the
(bright) subset of galaxies with spectroscopic redshifts, particularly at 0.1 <
z 3.5. For 0.1 < z < 1.2, we find a 1 sigma scatter in \Delta
z/(1+z) of 0.007, similar to results obtained with a similar filter set in the
COSMOS field. As a demonstration of the data quality, we show that the red
sequence and blue cloud can be cleanly identified in rest-frame color-magnitude
diagrams at 0.1 < z < 1.2. We find that ~20% of the red-sequence-galaxies show
evidence of dust-emission at longer rest-frame wavelengths. The reduced images,
photometric catalog, and photometric redshifts are provided through the public
MUSYC website.Comment: 19 pages, 14 image
Irradiation detection of herbal ingredients used in plant food supplements by Electron Spin Resonance on samples pre-treated with alcoholic extraction
This study aimed to verify the applicability of the EN 1787 method for the detection of irradiation in herbal ingredients used in Plant Food Supplements (PFSs). In matrices such as herbs and spices the main limit of the method is the presence of intrinsic radicals responsible for spurious signals leading to complex ESR spectra. To overcome this limit, before ESR measurement a treatment with alcohol has been proposed (Delincée and Soika, 2002; Ahn et al., 2012, 2014). As reported in the literature, this treatment is expected to reduce/eliminate the confounding signals so that the samples may be correctly classified. In this study the efficacy of the pre-treatment was tested on raw herbal ingredients largely used for PFSs, namely Camellia sinensis, Cinnamomum verum, Curcuma longa, Ginkgo biloba, Silybum marianum, Vaccinium myrtillus and Zingiber officinale. Non-irradiated and irradiated (5, 10 kGy) samples were analysed before and after pre-treatment. The results showed a general decrement of signal intensity. In some cases, this was associated with the elimination of some spurious signals, which, however, did not always ensue in an easier interpretation of the ESR spectra. Only for two matrices (Camellia sinensis and Vaccinium myrtillus) was alcoholic extraction crucial for the correct classification of the samples
Unveiling the nature of the "Green Pea" galaxies
We review recent results on the oxygen and nitrogen chemical abundances in
extremely compact, low-mass starburst galaxies at redshifts between 0.1-0.3
recently named to as "Green Pea" galaxies. These galaxies are genuine
metal-poor galaxies ( one fifth solar) with N/O ratios unusually high for
galaxies of the same metallicity. In combination with their known general
properties, i.e., size, stellar mass and star-formation rate, these findings
suggest that these objects could be experiencing a short and extreme phase in
their evolution. The possible action of both recent and massive inflow of gas,
as well as stellar feedback mechanisms are discussed here as main drivers of
the starburst activity and their oxygen and nitrogen abundances.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon,
September 2010, Springer Verlag, in pres
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