8,365 research outputs found
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
External field control of donor electron exchange at the Si/SiO2 interface
We analyze several important issues for the single- and two-qubit operations
in Si quantum computer architectures involving P donors close to a SiO2
interface. For a single donor, we investigate the donor-bound electron
manipulation (i.e. 1-qubit operation) between the donor and the interface by
electric and magnetic fields. We establish conditions to keep a donor-bound
state at the interface in the absence of local surface gates, and estimate the
maximum planar density of donors allowed to avoid the formation of a
2-dimensional electron gas at the interface. We also calculate the times
involved in single electron shuttling between the donor and the interface. For
a donor pair, we find that under certain conditions the exchange coupling (i.e.
2-qubit operation) between the respective electron pair at the interface may be
of the same order of magnitude as the coupling in GaAs-based two-electron
double quantum dots where coherent spin manipulation and control has been
recently demonstrated (for example for donors ~10 nm below the interface and
\~40 nm apart, J~10^{-4} meV), opening the perspective for similar experiments
to be performed in Si.Comment: 11 pages, 15 figures. Changes in Eq. 24 plus minor typo
Quantum transport in carbon nanotubes
Carbon nanotubes are a versatile material in which many aspects of condensed
matter physics come together. Recent discoveries, enabled by sophisticated
fabrication, have uncovered new phenomena that completely change our
understanding of transport in these devices, especially the role of the spin
and valley degrees of freedom. This review describes the modern understanding
of transport through nanotube devices.
Unlike conventional semiconductors, electrons in nanotubes have two angular
momentum quantum numbers, arising from spin and from valley freedom. We focus
on the interplay between the two. In single quantum dots defined in short
lengths of nanotube, the energy levels associated with each degree of freedom,
and the spin-orbit coupling between them, are revealed by Coulomb blockade
spectroscopy. In double quantum dots, the combination of quantum numbers
modifies the selection rules of Pauli blockade. This can be exploited to read
out spin and valley qubits, and to measure the decay of these states through
coupling to nuclear spins and phonons. A second unique property of carbon
nanotubes is that the combination of valley freedom and electron-electron
interactions in one dimension strongly modifies their transport behaviour.
Interaction between electrons inside and outside a quantum dot is manifested in
SU(4) Kondo behavior and level renormalization. Interaction within a dot leads
to Wigner molecules and more complex correlated states.
This review takes an experimental perspective informed by recent advances in
theory. As well as the well-understood overall picture, we also state clearly
open questions for the field. These advances position nanotubes as a leading
system for the study of spin and valley physics in one dimension where
electronic disorder and hyperfine interaction can both be reduced to a very low
level.Comment: In press at Reviews of Modern Physics. 68 pages, 55 figure
Middle Atlantic Outer Continental Shelf Environmental Studies Volume II-B: Chemical and Biological Benchmark Studies
The Middle Atlantic Outer Continental Shelf Environmental Studies is comprised of three volumes. Volume I. Executive Summary. Volume IIA, IIB, IIC and IID. Chemical and Biological Benchmark Studies. Volume III. Geologic Studies.
This is the second of four sections of the Chemical and Biological Benchmark Studies CHAPTER 5. BOTTOM SEDIMENTS AND SEDIMENTARY FRAMEWORK by Donald .F. Boesch CHAPTER 6. BENTHIC ECOLOGICAL STUDIES: MACROBENTHOS by Donald F. Boesch CHAPTER 7. BENTHIC ECOLOGICAL STUDIES: MEIOBENTHOS by D.J. Hartzband and Donald F. Boesch CHAPTER 8. BENTHIC ECOLOGICAL STUDIES: FORAMINIFERA by Robert L. Ellison
Chapters of this report contain the Institutes\u27 Special Reports in Applied Marine Science and Ocean Engineering No.193,194,195,196
Social Assessment of Section 3 of the A465 Heads of the Valleys Road: Brynmawr to Tredegar
The aim of this report is to provide a social assessment of the impacts of Section 3 of the A465 Heads of the Valleys Road: Brynmawr to Tredegar, using a mixed methods approach which adapts and builds on the UK WebTAG appraisal guidance units 4.1 and 4.2. We define social assessment in this document as a study of the social and distributional impacts which estimate the impacts of the implemented scheme at the point of opening rather than a detailed ex-ante appraisal. In the absence of detailed ex-ante appraisal, this report sets a baseline from which future evaluation may be conducted. This is the first application of a new mixed methods approach to social assessment of the impacts of transport infrastructure investment in the UK. It was commissioned by the Welsh Government in specific recognition of the need for improved guidance in this area of project delivery. The results reported here, along with its accompanying annexes, also contribute to greater understanding of social and distributional impacts, which builds upon and extends the current quantitative approach in WelTAG / WebTAG. This will hopefully lead to better understanding of the wider social effects of transport projects in order to inform future considerations as to how new transport schemes affect wellbeing
Interplay between distribution of live cells and growth dynamics of solid tumours
Experiments show that simple diffusion of nutrients and waste molecules is not sufficient to explain the typical multilayered structure of solid tumours, where an outer rim of proliferating cells surrounds a layer of quiescent but viable cells and a central necrotic region. These experiments challenge models of tumour growth based exclusively on diffusion. Here we propose a model of tumour growth that incorporates the volume dynamics and the distribution of cells within the viable cell rim. The model is suggested by in silico experiments and is validated using in vitro data. The results correlate with in vivo data as well, and the model can be used to support experimental and clinical oncology
Popularity-Driven Networking
We investigate the growth of connectivity in a network. In our model,
starting with a set of disjoint nodes, links are added sequentially. Each link
connects two nodes, and the connection rate governing this random process is
proportional to the degrees of the two nodes. Interestingly, this network
exhibits two abrupt transitions, both occurring at finite times. The first is a
percolation transition in which a giant component, containing a finite fraction
of all nodes, is born. The second is a condensation transition in which the
entire system condenses into a single, fully connected, component. We derive
the size distribution of connected components as well as the degree
distribution, which is purely exponential throughout the evolution.
Furthermore, we present a criterion for the emergence of sudden condensation
for general homogeneous connection rates.Comment: 5 pages, 2 figure
The Wide Brown Dwarf Binary Oph 1622-2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623-2402): A New Class of Young Evaporating Wide Binaries?
We imaged five objects near the star forming clouds of Ophiuchus with the
Keck Laser Guide Star AO system. We resolved Allers et al. (2006)'s #11 (Oph
16222-2405) and #16 (Oph 16233-2402) into binary systems. The #11 object is
resolved into a 243 AU binary, the widest known for a very low mass (VLM)
binary. The binary nature of #11 was discovered first by Allers (2005) and
independently here during which we obtained the first spatially resolved R~2000
near-infrared (J & K) spectra, mid-IR photometry, and orbital motion estimates.
We estimate for 11A and 11B gravities (log(g)>3.75), ages (5+/-2 Myr),
luminosities (log(L/Lsun)=-2.77+/-0.10 and -2.96+/-0.10), and temperatures
(Teff=2375+/-175 and 2175+/-175 K). We find self-consistent DUSTY evolutionary
model (Chabrier et al. 2000) masses of 17+4-5 MJup and 14+6-5 MJup, for 11A and
11B respectively. Our masses are higher than those previously reported (13-15
MJup and 7-8 MJup) by Jayawardhana & Ivanov (2006b). Hence, we find the system
is unlikely a ``planetary mass binary'', (in agreement with Luhman et al. 2007)
but it has the second lowest mass and lowest binding energy of any known
binary. Oph #11 and Oph #16 belong to a newly recognized population of wide
(>100 AU), young (<10 Myr), roughly equal mass, VLM stellar and brown dwarf
binaries. We deduce that ~6+/-3% of young (<10 Myr) VLM objects are in such
wide systems. However, only 0.3+/-0.1% of old field VLM objects are found in
such wide systems. Thus, young, wide, VLM binary populations may be
evaporating, due to stellar encounters in their natal clusters, leading to a
field population depleted in wide VLM systems.Comment: Accepted version V2. Now 13 pages longer (45 total) due to a new
discussion of the stability of the wide brown dwarf binary population, new
summary Figure 17 now included, Astrophysical Journal 2007 in pres
Cognitive architectures as Lakatosian research programmes: two case studies
Cognitive architectures - task-general theories of the structure and function of the complete cognitive system - are sometimes argued to be more akin to frameworks or belief systems than scientific theories. The argument stems from the apparent non-falsifiability of existing cognitive architectures. Newell was aware of this criticism and argued that architectures should be viewed not as theories subject to Popperian falsification, but rather as Lakatosian research programs based on cumulative growth. Newell's argument is undermined because he failed to demonstrate that the development of Soar, his own candidate architecture, adhered to Lakatosian principles. This paper presents detailed case studies of the development of two cognitive architectures, Soar and ACT-R, from a Lakatosian perspective. It is demonstrated that both are broadly Lakatosian, but that in both cases there have been theoretical progressions that, according to Lakatosian criteria, are pseudo-scientific. Thus, Newell's defense of Soar as a scientific rather than pseudo-scientific theory is not supported in practice. The ACT series of architectures has fewer pseudo-scientific progressions than Soar, but it too is vulnerable to accusations of pseudo-science. From this analysis, it is argued that successive versions of theories of the human cognitive architecture must explicitly address five questions to maintain scientific credibility
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